Unit 3 by kailash sir cell structure & function kp

Unit III: Cell structure and function By: Mr. Kailas Vilegave

Navdhare Academy Shahapur.

Unit III

Cell Structure and Function

1. Introduction:

Organisms are composed of cells, and these cells have specific structures within in them that allow

them to carry out their functions. They are building blocks of life. These structures are called Organelles.

The fine detail of the cell (which may be revealed by an electron microscope) is called the

cell's ultrastructure. Organelles perform different functions within a cell, and this is called the Division

of Labour.

2. Important Events in the Discovery of Cells

· 1665 - Robert Hooke English scientist looks at cork under a microscope. Calls the chambers he see

"cells" (cellula- little room/ compartment)& wrote his findings in a book titled ‘Micrographia’

· 1665 - 75 Anton van Leeuwenhoek, the person incorrectly given credit for the invention of the

microscope (actually, he was just damn good at making and using them, and his scopes soon became

the standard, and history has just given him credit as the inventor of the microscope), studies

organisms living in pond water (like you did in lab). He calls them "Animalcules."

· 1672- M. Malpighi, an Italian anatomist and N. Grew an English physician reported that certain

parts of plant were made up of minute elementary organisms- utricles, sacs, and vesicles.

· 1757- Haller and Bonnet- fibers

· 1781- Fontana- Cylinders

· 1828- cell division by Turpine

· 1839 - German scientists Schleiden and Schawann summarize the findings of many scientists and

conclude that all living organisms are made of cells. This forms the basis of the Cell Theory of

Biology

· 1840- J. E. Purkinje coined the term protoplasm for the cell content.

· 1893- The term cytology was coined by Hertwig.

3. The Cell Theory of Biology

· All organisms are composed of cells

· The cell is the structural unit of life - units smaller than cells are not alive

· Cells arise by division of preexisting cells - spontaneous generation does not exist.



3.1 Objections of cell theory:

· Viruses don’t have cellular structure

· Monera and protistans are acellular

· Certain organisms have multinucleated eg. Plant like Volvax and condition called Coenocyte and

animal like Ascaris (epidermal cells) condition called Syncytium.

· A typical nucleus is absent in prokaryotic cell.

· Certain cell lose their nuclei in the mature state (RBC and sieve tube cells)

4. Properties of Cells

4.1 Cells are complex and highly organized

· They contain numerous internal structures

· Some are membrane bound (organelles) while others do not

4.2 Cells contain a genetic blueprint and machinery to use it

· Genes are instructions for cells to create specific proteins

· All cells use the same types of information

o The genetic code is universal

o The machinery used for synthesis is interchangeable

· However, for this to function properly, information transfer must be error free

o Errors are called mutations

4.3 Cells arise from the division of other cells

· Daughter cells inherit the genes from the mother cells

· Binary fission - cell division in bacteria

· Mitosis - the genetic complement of each daughter cell is identical to the other and to the mother

cell. This is asexual reproduction

· Meiosis - the genetic complement of each daughter cell is reduced by half and each daughter cell

is genetically unique. This is used in sexual reproduction

· Daughter cells inherit cytoplasm and organelles from the mother cells

o Asexual - organelles from mother cell

o Sexual - organelles predominately from one parent

§ In eukaryotes, the chloroplasts and mitochondria come from the egg cell

§ This can be used to trace the evolutionary origin of the organism

4.4 Cells acquire and utilize energy

· Plant cells undergo photosynthesis



o convert light energy and CO2 to chemical energy (ATP and glucose)

· Most cells respire

o release energy found in organic compounds

o convert organic compounds to CO2 and O2

o make ATP

4.5 Cells can perform a variety of chemical reactions

· Transform simple organic molecules into complex molecules (anabolism)

· Breakdown complex molecules to release energy (catabolism)

· Metabolism - all reactions performed by cells

4.6 Cells can engage in mechanical activities

· Cells can move

· Organelles can move

· Cells can respond to stimuli

o chemotaxis - movement towards chemicals

o phototaxis - movement towards light

o hormone responses

o touch responses

4.7 Cells can regulate activities

· Cells control DNA synthesis and cell division

· Gene regulation - cells make specific proteins only when needed

· Turn on and off metabolic pathways

4.8 Cells all contain the following structures:

· Plasma membrane - separates the cell from the external environment

· Cytoplasm – fluid filled cell interior

· Nuclear material - genetic information stored as DNA

· Ribosomes



5. Types of Cells

There are two types of cells, eukaryotes, which contain a nucleus, and prokaryotes, which do not.

Prokaryotic cells are usually single-celled organisms, while eukaryotic cells can be either single-celled or

part of multicellular organisms.

Prokaryotes

· Pro = before; karyon = nucleus

· relatively small - 5 to 10 um

· lack membrane-bound organelles

· earliest cell type

Archaea

· Originally thought to be

prokaryotes

· relatively small - 5 to 10 um

· lack membrane-bound organelles

· Usually live in extreme

environments (thermophiles,

halophiles, etc)

Eukaryotes

· Eu = true; karyon = nucleus

· contain membrane-bound

organelles

· Evolved from prokaryotes by

endosymbiotic association of two

or more prokaryotes

· Include Protists, Fungi, Animals,

and Plants



5.1 Features of Prokaryotic Cells:

-The first cells - Smallest cells and simplest type of cells, - Cell lacks nucleus and lack membrane bound organells. - DNA is not enclosed in membrane



There are two major kinds of prokaryotes:

· Bacteria

· Archaea (single-celled organisms)

As you may have read earlier in this unit, biologists now estimate that each human being carries

nearly 20 times more bacterial, or prokaryotic, cells in his or her body than human, or eukaryotic, cells. If

that statistic overwhelms you, rest assured that most of these bacteria are trying to help, and not hurt, you.

Numerically, at minimum, there are 20 times more prokaryotic cells on Earth than there are eukaryotic cells.

This is only a minimum estimate because there are trillions of trillions of bacterial cells that are not

associated with eukaryotic organisms. In addition, all Archaea are also prokaryotic. As is the case for

bacteria, it is unknown how many Archaean cells are on Earth, but the number is sure to be astronomical. In

all, eukaryotic cells make up only a very small fraction of the total number of cells on Earth.

There are four main structures shared by all prokaryotic cells, bacterial or Archaean:

1. The plasma membrane.

2. Cytoplasm

3. Ribosomes

4. Genetic material (DNA and RNA)

Some prokaryotic cells also have other structures like the cell wall, pili (singular pillus),

and flagella (singular flagellum). Each of these structures and cellular components plays a critical role in the

growth, survival, and reproduction of prokaryotic cells.

5.1.1 Prokaryotic Plasma Membrane

Prokaryotic cells can have multiple plasma membranes. Prokaryotes known as "gram-negative bacteria,"

often have two plasma membranes with a space between them known as the periplasm. As in all cells, the

plasma membrane in prokaryotic cells is responsible for controlling what gets into and out of the cell. A

series of proteins stuck in the membrane (poor fellas) also aid prokaryotic cells in communicating with the

surrounding environment. Among other things, this communication can include sending and receiving

chemical signals from other bacteria and interacting with the cells of eukaryotic organisms during the

process of infection. Infection is the kind of thing that you don't want prokaryotes doing to you. Keep in

mind that the plasma membrane is universal to all cells, prokaryotic and eukaryotic. Because this cellular

component is so important and so common.



5.1.3 Prokaryotic Ribosomes

Prokaryotic ribosomes are smaller and have a slightly different shape and composition than those found in

eukaryotic cells. Bacterial ribosomes, for instance, have about half of the amount of ribosomal RNA

(rRNA) and one third fewer ribosomal proteins (53 vs. ~83) than eukaryotic ribosomes have.3 Despite

these differences, the function of the prokaryotic ribosome is virtually identical to the eukaryotic version.

Just like in eukaryotic cells, prokaryotic ribosomes build proteins by translating messages sent from DNA.

5.1.4 Prokaryotic Genetic Material

All prokaryotic cells contain large quantities of genetic material in the form of DNA and RNA. Because

prokaryotic cells, by definition, do not have a nucleus, the single large circular strand of DNA containing

most of the genes needed for cell growth, survival, and reproduction is found in the cytoplasm.

The DNA tends to look like a mess of string in the middle of the cell:

Usually, the DNA is spread throughout the entire cell, where it is readily accessible to be transcribed

into messenger RNA (mRNA) that is immediately translated by ribosomes into protein. Sometimes, when

biologists prepare prokaryotic cells for viewing under a microscope, the DNA will condense in one part of

the cell producing a darkened area called a nucleoid.

As in eukaryotic cells, the prokaryotic chromosome is intimately associated with special proteins involved

in maintaining the chromosomal structure and regulating gene expression. In addition to a single large

piece of chromosomal DNA, many prokaryotic cells also contain small pieces of DNA called plasmids.

5.1.2 Prokaryotic Cytoplasm

The cytoplasm in prokaryotic cells is a

gel-like, yet fluid, substance in which all of the

other cellular components are suspended. Jello

for cells. It is very similar to the eukaryotic

cytoplasm, except that it does not contain

organelles. Recently, biologists have discovered

that prokaryotic cells have a complex and

functional cytoskeleton similar to that seen in

eukaryotic cells. The cytoskeleton helps

prokaryotic cells divide and helps the cell

maintain its plump, round shape. As is the case

in eukaryotic cells, the cytoskeleton is the

framework along which particles in the cell,

including proteins, ribosomes, and small rings

of DNA called plasmids, move around.



Plasmid: Bacteria may contains one or more plasmids, is a non essential piese of DNA that confers

an advantages to the bacteria such as antibiotic resistance, virulence (ability to cause disease) and

conjugation (ability of bacteria to share plasmid to another bacteria).

Plasmids also found in some eukaryotic cells like Yeast.

These circular rings of DNA are replicated independently of the chromosome and can be transferred from

one prokaryotic cell to another through pili, which are small projections of the cell membrane that can form

physical channels with the pili of adjacent cells.

5.2 Properties of Eukaryotic Cells:

- True nucleus

- Larger and more complex

- Includes protists, fungi, plants and animals

- Having nucleus and membrane bound organelles.

A cell is defined as eukaryotic if it has a membrane-bound nucleus. Any organism composed of

eukaryotic cells is also considered a eukaryotic organism. Biologists do not know of any single organism

on Earth that is composed of both eukaryotic and prokaryotic cells. However, many different types of

prokaryotic cells, usually bacteria, can live inside larger eukaryotic organisms. We humans, for example,

have trillions of bacteria living in our colons, not to mention in our mouths and stomachs and small

intestines.

All of the organisms we can see with the naked eye are composed of one or more eukaryotic cells, with

most having many more than one. This means that most of the organisms we are familiar with are

eukaryotic. However, most of the organisms on Earth, by number, are actually prokaryotic.



Most plants, animals, and fungi are composed of many cells and are, for that reason, aptly classified as multicellular, while most protists consist of a single cell and are classified as unicellular. All eukaryotic cells have

1. A nucleus 2. Genetic material

3. A plasma membrane 4. Ribosomes 5. Cytoplasm, including the cytoskeleton

Most eukaryotic cells also have other membrane-bound internal structures called organelles. Organelles include

· Mitochondria · Golgi bodies

· Lysosomes · Endoplasmic reticulum · Vesicles

Table 1: Comparison of features of prokaryotic and eukaryotic cells

Prokaryotes Eukaryotes

Typical organisms bacteria, archaea protists, fungi, plants, animals

Typical size ~ 1–5 µm[9] ~ 10–100 µm[9]

Type of nucleus nucleoid region; no true nucleus true nucleus with double membrane

DNA circular (usually) linear molecules (chromosomes) with histone proteins

RNA/protein

synthesis coupled in the cytoplasm

RNA synthesis in the nucleus protein synthesis in the cytoplasm

Ribosomes 50S and 30S 60S and 40S

Cytoplasmic

structurevery few structures highly structured by endomembranes and a cytoskeleton

Cell movement flagella made of flagellin

flagella and cilia containing microtubules; lamellipodia and filopodia containing actin

Mitochondria none one to several thousand (though some lack mitochondria)

Chloroplasts none in algae and plants

Organization usually single cells single cells, colonies, higher multicellular organisms with specialized cells

Cell division Binary fission (simple division)

Mitosis (fission or budding) Meiosis



Eukaryotic Cell Prokaryotic Cell

Nucleus Present Absent

Number of chromosomes More than one One--but not true chromosome:

Plasmids

Cell Type Usually multicellular Usually unicellular (some

cyanobacteria may be multicellular)

True Membrane bound

Nucleus

Present Absent

Genetic Recombination Meiosis and fusion of gametes Partial, undirectional transfers DNA

Lysosomes and peroxisomes Present Absent

Microtubules Present Absent or rare

Endoplasmic reticulum Present Absent

Mitochondria Present Absent

Cytoskeleton Present May be absent

DNA wrapping on proteins.

Eukaryotes wrap their DNA around proteins called histones.

Multiple proteins act together to fold and condense prokaryotic DNA. Folded DNA is then organized into a variety of conformations that are supercoiled and wound around tetramers of the HU protein.

Ribosomes larger smaller

Vesicles Present Present

Golgi apparatus Present Absent

ChloroplastsPresent (in plants) Absent; chlorophyll scattered in the

cytoplasm

Flagella

Microscopic in size; membrane bound; usually arranged as nine doublets surrounding two singlets

Submicroscopic in size, composed of only one fiber

Permeability of Nuclear

Membrane

Selective not present

Plasma membrane with

steroid

Yes Usually no

Cell wall Only in plant cells and fungi (chemically simpler)

Usually chemically complexed

Vacuoles Present Present

Cell size 10-100um 1-10um



66.. SSttrruuccttuurreess FFoouunndd iinn AAllll EEuukkaarryyoottiicc CCeellllss

66..AA)) PPllaass mmaa mmeemmbbrraa nnee

66..BB)) CCyyttoo ppllaass mm

a) Endoplasmic reticulum

b) Golgi complex,

c) Lysosomes,

d) Ribosomes,

e) Microbodies,

f) Plastids, and chloroplast

g) Cilia and flagella,

h) Centriole (centrosomes),

i) Cytoskeleton

j) Mitochondria

66..CC)) NNuucc lleeuuss

a) Nucleolus

b) Nucleoplasm

c) Chromatine.

66..DD)) VVaaccuuoo llee

66..AA)) PPll aassmmaa mmee mmbbrr aannee::

Cell wall is first discovered by Robert Hook in 1665.

The cell membranes separate cell from its environment and form distinct functional nucleus and organelles in the cell. The outer cell membrane is called plasma membrane.or plasmalemma.



In the cell of bacteria cyanobacteria, protest, fungi and plants a thick rigid protective but porous coat the cell

wall outside the plasma membrane is found.

Composition of cell wall:

In plants cell wall is made up of cellulose, hemicellulose (arabinose manose galactose) and pectine. In bacteria cell wall is composed of protein-lipid polysaccharides. Having two important chemical components- N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM)

In fungi cell wall is composed of chitin. Algal cell wall contains cellulose and variety of glycoproteins.

The most important deference between plant cell and animal cell is presence of cell wall (non-living protective layer) in plant cell. In animal cell and many protist, cell membrane are covered by filamentous layer (also called cell coat) of an oligosaccharide sialic acid (called glycocalyx) which protects the underlying membrane and helps in helps

in recognition of the cells by Wilson 1907.

Recognition ability is mainly due to Ca++ and Mg2+ absorbed over glycocalyx.

A typical cell wall is made up of 4 layers middle lamella, Primary, Secondary, and Tertiary wall.

Middle lamella is the cementing layer between the cells. It is made up of Ca and Mg pectates.

Function of cell wall.

- It maintain shape of the cell - It protects the cell from mechanical injury. - It protect from the attack of pathogens ( viruses, bacteria, fungi,). - It provides mechanical support against gravity.

- The cell wall prevents undue expansion of the cell when the water enters by osmosis to compensate for the lack of contractile vacuoles. This prevents bursting of cell

- It allows the material to pass in and out of the cell. -

66..BB)) CCyyttooppllaassmm::

The cytoplasm surrounds the nucleus and is enclosed by the plasma membrane.

The cytoplasm was discovered in the year 1835 by Robert Brown and other scientists.

Cytoplasm is jelly like substance (called cytosol or cytoplasmic matrix) and composed of mainly water.

Autonomic movement of matrix in the cytoplasm in a cell is called cytoplasmic streaming or cyclosis.

Cytoplasm exist in two states- sol (plasmasol) and

Gel (plasmagel)

-sol or hydrol is a liquid colloidal solution where the colloidal partical are well dispersed in water.



Gel is a thick semisolid colloidal system in which the colloidal particle come in contact and form a sort of

network with water.

Only the sol part of the cytoplasm shows cyclosis.

Cytoplasm contains cell organells and cytoskeleton.

Basically cytoplasm is the substance that fills the cell. it is made up of 80 % water and is usually clear and

colorless. It liquefies when it is agitated or stirred.

The cytoplasm is the substance of life, the cytoplasm serves as a molecular soup, it is in the cytoplasm

where all the cellular organelles are suspended and are bound together by a lipid bilayer membrane.

The cell membrane surrounds the cytoplasm and it also surrounds the nucleus and the cellular organelles.

The cytoskeleton present in the cytoplasm gives the cell its shape. The cytoplasm constitutes of dissolved

nutrients and it aids to dissolve waste products. Cytoplasm also constitutes numerous salts and is a very

good conductor of electricity.

Most of the cellular activities occur in the cytoplasm. Metabolic pathways like glycolysis and

cellular processes like cell division take place in the cytoplasm.

The outer clear and glassy layer of the cytoplasm is called the ectoplasm or the cell cortex and the inner

granular mass is called the endoplasm.

Characteristics of Cytoplasm

· Cytoplasm is the fluid substance that fills the space between the cell membrane and the cellular

organelles.

· Cytoplasm shows differential staining properties, the areas stained with the basic dyes are the

basophilic areas of the cytoplasm and is termed as ergatoplasm for this material.

· It is heterogenous mixture of opaque granules and organic compounds which gives it its colloidal

nature.

· The peripheral zone of cytoplasm is thick and jellylike substance, known as the plasmogel. The

surrounding area of the nuclear zone is thin and liquefied in nature and is known as the plasmosol.

· The physical nature of cytoplasm is colloidal. It has a high percentage of water and particles of

various shapes and sizes are suspended in it.

· It also contains proteins, of which 20-25 percent are soluble proteins including enzymes.



· Also, certain amount of carbohydrates, inorganic salts, lipids and lipoidal substances are found.

· The plasmogel part of the cytoplasm is capable of absorbing water and removing it, according to the

cells need.

· The stomatal guard cell present in the leaves exhibit this property.

· An organized system of fibrers can be observed by specific staining techniques.

· Chemically cytoplasm contains 90% water and 10% include a mixture of organic and inorganic

compounds in various proportions.

Organelles

Organelles mean "little organs", that are membrane bound. They are present inside the cell and perform

specific functions that are necessary for the survival of the cell. Some of the constituents of the cell that are

suspended in the cytosol are cellular organelles like mitochondria, endoplasmic reticulum, Golgi apparatus,

vacuoles, lysosomes and chloroplasts in plant cells.

Cytoplasm Function

1. Cytoplasm is the site of many biochemical reactions that are vital and crucial for maintaining life.

2. The cytoplasm is the place where the cell expands and growth of the cell takes place.

3. The cytoplasm provides a medium for the organelles to remain suspended.

4. The cytoskeleton of the cytoplasm provides shape to the cell and it also facilitates movement.

5. It also aids in the movement of the different cellular elements.

6. The enzymes in the cytoplasm metabolize the macromolecules into small parts, so that it can be

easily available for the other cellular organelles like mitochondria.

7. The cytoplasm is a means of transport for genetic material.

8. It also transports the products of cellular respiration.

9. The cytoplasm acts as a buffer and protects the genetic material of the cell and also the cellular

organelles from damage caused due to movement and collision with other cells.

10. The cytoplasmic organelles are specialized structures that haves its own functions like cellular

respiration, protein synthesis,etc.

11. The cytoplasmic inclusions are non-soluble molecules, they are seen floating in the cytoplasm, they

act as stored fats and sugars that are ready for celllular respiration.

12. The cytoplasm and the proteins prevent the grouping of organelles in place due to gravity, that

would impede their function.



Plant Cell Cytoplasm

Plant cell cytoplasm is similar to animal cell cell cytoplasm. Cytoplasm provides mechanical support to the

internal structures. It is the medium for suspension for the internal organelles of the cell. Cytoplam

maintains the shape and consistency of the cell. It also stores many chemicals that are vital for life.

Important metabolic reactions like the glycolysis and synthesis of proteins takes place in the cytoplasm. In

plants the movements of the cytoplasm around the vacuoles, this is known as cytoplasmic streaming.

Animal Cell Cytoplasm

The cytoplasm of the animal cell is a gel- like material made of water. It fills the cells and contains proteins

and important molecules that are necessary for the cell. The cytoplasm is made of proteins, carbohydrates,

salts, sugars, amino acids and nucleotides. Cytoplasm holds all the cellular organelles. Cytoskeleton present

in the cytoplasm help in the movement of the cell through cytoplasmic streaming.

66..BB..aa)) EEnnddooppllaassmmiicc rreettiiccuulluumm

- The endoplasmic reticulum (ER) is a eukaryotic organelle.

-First noted by Porter, Claude and Fullan in 1945 as a network. And it was names ER by porter in 1953.

- it is absent in prokaryotes but present in all the eukaryotes except germinal cells and mature mammalian

erythrocytes.

- the striated muscle fibers have special type of ER called sarcoplasmic reticulum SR.

-ER is a well developed electron microscopic network of interconnected cisternae, tubules and vesicles present throughout the cytoplasm especially in the endoplasm.



- Cisternae are flattened, unbranched sac like elements. The sacs in the stack are interconnected with one another. They bear ribosomes on the surface that makes the cisternae appears rough.

- in mature cell, ER occurs in 2 forms – rough (RER) and smooth (SER)

- RER is mainly concerned with the synthesis of proteins.

- SER lacks ribosomes and thus appears smooth, and has many enzymes, important in lipid metabolism,

steroid hormone synthesis, glycogen break down and detoxification.

- SER also abundant in liver cells (Hepatocytes) where it is involved in glycogen metabolism and drug detoxification.

Functions of ER:

Ø Facilitates transport of materials from one part of the cell to another, thus forming the cells circulatory system.

Ø Detoxification of drugs. Ø Associated with muscle contraction by release and uptake of Ca2+ ions Ø Helps in formation of primary lysosomes with hydrolytic enzymes. Ø Helps in the synthesis of nuclear envelop during telophase of cell division. Ø Provide space for temporary storage of synthetic product such as glycogen.

66..BB..bb)) GGoollggii ccoommpplleexx..

Golgi apparatus was discovered in the year 1898 by an Italian biologist Camillo Golgi. It was on of the first cellular organelles to be discovered and observed in detail due to its large size. The term Golgi

apparatus was used in 1910 and in 1913 it first appeared in the scientific literature.



Under the electron microscope the Golgi apparatus is seen to be composed of stacks of flattened structures which contains numerous vesicles containing secretory granules.

The newly synthesized proteins, found in the channels of the rough endoplasmic reticulum are

moved to the Golgi body where the carbohydrates are added to them and these molecules are enveloped in a part of the Golgi membrane and then the enveloped molecules leave the cell. The Golgi apparatus hence acts as the assembly factory of the cell where the raw materials are directed to the Golgi apparatus before being passed out from the cell.The Golgi complex is referred to as the manufacturing and the shipping

center of the eukaryoric cell. The Golgi complex is responsible inside the cell for packaging of the protein molecules before they are sent to their destination.

This organelles helps in processing and packaging the macromolecules like proteins and lipids that are synthesized by the cell, It is known as the 'post office' of the cell.

The major function of the Golgi body is to modif , sort and package the macromolecules. It also

helps in transportation of lipids around the cell and the creation of lysosomes.

Golgi Apparatus Structure

· They are membrane bound organelles, which are sac- like. They are found in the cytoplasm of plant

and animal cells.

· The Golgi complex is composed of stacks of membrane-bound structures, these structures are known

as the cisternae. An individual stack of the cisternae is sometimes referred as dictyosome.

· In a typical animal cell, there are about 40 to 100 stacks. In a stack there are about four to eight

cisternae.

· Each cisternae is a disc enclosed in a membrane, it possess special enzymes of the Golgi which help

to modify and transport of the modified proteins to their destination.

· The flat sacs of the cisternae are stacked and are bent and semicircular in shape.

· Each group of stacks is membrane bound and its insides are separated from the cytoplasm of the cell.

· The interaction in the Golgi membrane in responsible for the unique shape of the apparatus.

· The Golgi complex is polar in nature.

· The membranes of one end of the stack is different in composition and thickness to the membranes

at the other end.

· One end of the stack is known as the cis face, it is the 'receiving department" while the other e nd is

the trans face and is the "shipping department". The cis face of the Golgi apparatus is closely

associated with the endoplasmic reticulum.



Golgi Apparatus Function

1. The cell synthesize a huge amount of variety of macromolecules. The main function o f the Golgi

apparatus is to modify, sort and package the macromolecules that are synthesized by the cells for

secretion purposes or for use within the cell.

2. It mainly modifies the proteins that are prepared by the rough endoplasmic reticulum.

3. They are also involved in the transport of lipid molecules around the cell.

4. They also create lysosomes.

5. The Golgi complex is thus referred as post office where the molecules are packaged, labelled and

sent to different parts of the cell.

6. The enzymes in the cisternae have the ability to modify proteins by the addition of carbohydrates

and phosphate by the process of glycosylation and phoshphorylation respectively.

7. In order to modify the proteins the golgi complex imports substances like nucleotides from the

cytosol of the cell. The modifications brought about by the golgi body might form a signal sequence.

This determines the final destination of the protein.

8. The Golgi complex also plays an important role in the production of proteoglycans. The

proteoglycans are molecules that are present in the extracellular matrix of the animal cells.

9. It is also a major site of synthesis of carbohydrates. These carbohydratres includes the synthesis of

glycoasaminoglycans, Golgi attaches to these polysaccharides which then attaches to a protein

produced in the endeoplasmic reticulum to form proteoglycans.

10. The Golgi involves in the sulfation process of certain molecules.

11. The process of phosphorylation of molecules by the Golgi requires the import of ATP into the lumen

of the Golgi.



Types Description Example

Exocytotic

vesicles(continuous)

Vesicle contains proteins destined for extracellular release. After packaging, the vesicles bud off and immediately move towards the plasma membrane, where they fuse and release the contents into the extracellular space in a process known as constitutive secretion.

Antibody release by activated plasma B cells

Secretory

vesicles(regulated)

Vesicle contains proteins destined for extracellular release. After packaging, the vesicles bud off and are stored in the cell until a signal is given for their release. When the appropriate signal is received they move towards the membrane and fuse to release their contents. This process is known as regulated secretion.

Neurotransmitterrelease fromneurons

Lysosomal vesicles

Vesicle contains proteins and ribosomes destined for the lysosome, an organelle of degradation containing many acid hydrolases, or to lysosome-like storage organelles. These proteins include both digestive enzymes and membrane proteins. The vesicle first fuses with the late endosome, and the contents are then transferred to the lysosome via unknown mechanisms.

Digestiveproteasesdestined for thelysosome

Diagram of secretory process from endoplasmic reticulum (orange) to Golgi apparatus (pink).

66..BB..cc)) LLyyssoossoommeess

Derived from the Greek words lysis, meaning "to loosen", and soma, "body" is a membrane-bound cell

organelle found in animal cells (they are absent in red blood cells).



They are structurally and chemically spherical vesicles containing acid hydrolases, which are capable of breaking down virtually all kinds of biomolecules, including proteins, nucleic

acids,carbohydrates, lipids, and cellular debris.

They are known to contain more than 50 different enzymes which are all active at an acidic environment of about pH 5. Thus they act as waste disposal system of the cell by digesting unwanted materials in then cytoplasm, both from outside of the cell and obsolete components inside the cell. For this function they are popularly referred to as "suicide bags" or "suicide sacs" of the cell.

Lysosomes are responsible for cellular homeostasis for their involvements in secretion, plasma

membrane repair, cell signalling and energy metabolism, which are related to health and diseases. Depending on their functional activity their sizes can be very different, as the biggest ones can be more than 10 times bigger than the smallest ones. They were discovered and named by Belgian biologist Christian de

Duve, who eventually received the Nobel Prize in Physiology or Medicine in 1974.

Enzymes of the lysosomes are synthesised in the rough endoplasmic reticulum. The enzymes are

released from Golgi apparatus in small vesicles which ultimately fuse with acidic vesicles called endosomes, thus becoming full lysosomes. In the process the enzymes are spec ifically tagged with mannose 6-phosphate to differentiate them from other enzymes. Lysosomes are interlinked with three intracellular processes namely phagocytosis, endocytosis and autophagy. Extracellular materials such as microorganisms taken up by phagocytosis, macromolecules by endocytosis, and unwanted cell organelles are fused with lysosomes in which they are broken down to their basic molecules. Thus lysosomes are the

recycling units of a cell.

Christian de Duve, then chairman of the Laboratory of Physiological Chemistry at the Catholic University of Louvain in Belgium, had been studying the mechanism of action of a pancreatic hormone insulin in liver cells. By 1949 he and his team had focused on the enzyme called glucose 6-phosphatase, which is the

first crucial enzyme in sugar metabolism and the target of insulin.



66..BB..dd)) RRiibboossoommeess

When the ribosomes are attached to the same mRNA strand, this structure is known as polysome. The existence of ribosomes is temporary, after the synthesis of polypeptide the two sub-units separate and is reused or broken up. Amino acids are joined by the ribosomes at a rate of 200 per minute . Therefore

small proteins can be made quickly but two or three hours are needed for proteins which are as large as 30,000 amino acids.

The ribosomes present in the prokaryotes function differently in protein production than the ribosomes of the eukaryote organisms. The ribosomes of bacteria, archea and eukaryotes differ significantly

from each other in structure and RNA sequences. The differences in the ribosomes allows the antibiotc to kill the bacterial ribosome by inhibiting the activity of the bacterial ribosomes, the human ribosome sramin unaffected. The ribosomes of the eukaryotic cellas are similar to the ribosomes of the bacterial cells, showing the evolutionary origin of the organelle.

Ribosomes are small particles, present in large numbers in all the living cells. They are sites of protein synthesis. The ribosome word is derived - 'ribo' from ribonucleic acid and 'somes' from the Greek word 'soma' which means 'body'. The ribosomes link amino acids together in the order that is specified by the messenger RNA molecules. The ribosomes are made up of two subunits - a small and a large subunit. The small subunit reads the mRNA while the large subunit joins the amino acids to form a chain of polypeptides.

Ribosmal subunits are made of one or more rRNA (ribosomal RNA) molecules and various proteins.

Characteristics of ribosomes:

· Typically ribosomes are composed of two subunits: a large subunit and a small subunit.

· The subunits of the ribosome are synthesized by the nucleolus.

· The subunits of ribosomes join together when the ribosomes attaches to the messenger RNA during

the process of protein synthesis.

· Ribosomes along with a transfer RNA molecule (tRNA), helps to translate the protein-coding genes in mRNA to proteins.

Proteins are necessary for the cells to perform cellular functions. Ribosomes are the cellular component that make

proteins from all amino acids. Ribosomes are made from complexes of RNAs and proteins . The number of ribosomes in a cell depends on the activity of the cell. Ribosomes are freely suspended in the cytoplasm or attached to the endoplasmic

reticulum forming the rough endoplasmic reticulum. On an average in a mammalian cell there can be about 10 million ribosomes.



Ribosome Structure

· Ribosomes in a cell are located in two regions of the cytoplasm.

· They are found scattered in the cytoplasm and some are attached to the endoplasmic reticulum.

· When the ribosomes are bound to the ER there is known as the rough endoplasmic reticulum.

· Ribosomes are composed of both RNA and proteins.

· About 37 - 62% of RNA are made up of RNA and the rest is proteins.

· Ribosome is made up of two subunits. The subunits of ribosomes are named according to their ability of sedimentation on a special gel which the Sevdberg Unit.

· Prokarytotes have 70S ribosomes each subunit consisting of small subunit is of 30S and the large subunit is of 50S. Eukarytotes have 80S ribosomes each consisting of small (40S) and large (60S)

subunit.

· The ribosomes found in the chloroplasts of mitochondria of eukaryotes consists of large and small subunits bound together with proteins into one 70S particle.

· The ribosomes share a core structure which is similar to all ribosomes despite differences in its size.

· The RNA is organized in various tertiary structures. The RNA in the larger ribosomes are into several continuous insertion as they form loops out of the core structure without disrupting or

changing it.

· The catalytic activity of the ribosome is carried out by the RNA, the proteins reside on the surface and stabilize the structure.

· The differences between the ribosomes of bacterial and eukaryotic are used to create antibiotics that can destroy bacterial infection without harming human cells.

Ribosome Function

· They assemble amino acids to form specific proteins, proteins are essential to carry out cellular activities.



· The process of production of proteins, the deoxyribonucleic acid produces mRNA by the process of DNA transcription.

· The genetic message from the mRNA is translated into proteins during DNA translation.

· The sequences of protein assembly during protein synthesis are specified in the mRNA.

· The mRNA is synthesized in the nucleus and is transported to the cytoplasm fo r further process of

protein synthesis.

· In the cytoplasm, the two subunits of ribosomes are bound around the polymers of mRNA; proteins are then synthesized with the help of transfer RNA.

· The proteins that are synthesized by the ribosomes present in the cytoplasm are used in the cytoplasm itself. The proteins produced by the bound ribosomes are transported outside the cell.

Bacterial Ribosome

Prokaryotic ribosomes

In a bacterial cell there are about 10,000 ribosomes which make upto 30% of the weight of the cell. The bacterial ribosomes are present free in the cytoplasm. The bacterial ribosome sediments as 70S particle

which is composed of 30S and a large subunit is of 50S. The small subunit of the prokarytoic ribosome functions in the association with messenger RNA during translation and decoding. The large subunits of the ribosomes function as peptidyl transferase center and it is the site of peptide bond formation. The structure of bacterial ribosome is made up of over 50 proteins and three large domains of RNA molecule. They are the site of protein synthesis.

Plant Cell Ribosome

Plant cell do have ribosomes and they are composed of proteins and ribosomal RNA. The ribosomes in a plant cell are found in the cytoplasm, the surface of the rough endoplasmic reticulum, the mitochondria and

on chloroplasts. There are two types of ribosomes - free ribsomes and attached ribosomes. The attached ribosomes are bound to the surface of the endoplasmic reticulum and they are the site for protein synthesis. Synthesis of proteins also occurs in the free ribosomes. Animal Cell Ribosome

Ribosomes are proein builders of the cll. They are found in many place in the cytoplasm. They might be sseen freely floatin gin the cytoplasm and they are seen attached to the endoplasmic reticulum. Ribsomes of animal cells are also made of two subunits large (60S) and small (40S. When there is need for proteins ina cell, the mRNA is produced in the nucleus and is sent to the cell and the ribosomes. The two subunits of the ribsomes come together at the time of protein formation and combine with the mRNA molecule and hence the proteins are synthesized



66..BB..ee)) MMiiccrroobbooddiieess

A microbody is a type of organelle that is found in the cells of plants, protozoa, and animals. Organelles in the microbody family include peroxisomes, glyoxysomes, glycosomes and hydrogenosomes. In

vertebrates, microbodies are especially prevalent in the liver and kidney organs.

A microbody is usually a vesicle with a spherical shape, ranging from 0.2-1.5 micrometers in diameter. Microbodies are found in the cytoplasm of a cell, but they are only visible with the use of an electron microscope. They are surrounded by a single phospholipid bilayer membrane and they contain a

matrix of intracellular material including enzymes and other proteins, but they do not seem to contain any genetic material to allow them to self-replicate.

Microbodies contain enzymes that participate in the preparatory or intermediate stages of biochemical reactions within the cell. This facilitates the breakdown of fats, alcohols and amino acids. Different types of microbodies have different functions:

Ø Peroxisomes is a type of microbody that functions to help the body break down large molecules and detoxify hazardous substances. It contains enzymes like oxidase, which can create hydrogen peroxide as a



byproduct of its enzymatic reactions. Within the peroxisome, hydrogen peroxide can then be converted to water by enzymes like catalase andperoxidase.

Ø Glyoxysomes

are specialized peroxisomes found in plants and mold, which help to convert stored lipids into carbohydrates so they can be used for plant growth. In glyoxysomes the fatty acids are hydrolyzed to acetyl-CoA by peroxisomal β-oxidation enzymes. Besides peroxisomal functions, glyoxysomes also possess the key enzymes of the Glyoxylate cycle.

Microbodies were first discovered and named in 1954 by Rhodin. Two years later in 1956, Rouiller and

Bernhard presented the first worldwide accepted images of microbodies in liver cells. Then in 1965, Christian de Duve and coworkers isolated microbodies from the liver of a rat. De Duve also believed that the name Microbody was too general and chose the name of Peroxisome because of its relationship with

hydrogen peroxide. In 1967, Breidenbach and Beevers were the first to isolate microbodies from plants, which they named Glyoxysomes because they were found to contain enzymes of the Glyoxylate cycle.

66..BB..ff)) PPllaassttiiddss aanndd CChhlloorrooppllaasstt

Chloroplasts are organelles present in plant cells and some eukaryotic organisms. Chloroplasts are

the most important plastids found in plant cells. It is the structure in a green plant cell in which

photosynthesis occurs.

Chloroplast is one of the three types of plastids. The chloroplasts take part in the process of

photosynthesis and it is of great biological importance. Animal cells do not have chloroplasts. All green

plant take part in the process of photosynthesis which converts energy into sugars and the byproduct of

the process is oxygen that all animals breathe. This process happens in chloroplasts.

The distribution of chloroplasts is homogeneous in the cytoplasm of the cells and in certain cells

chloroplasts become concentrated around the nucleus or just beneath the plasma membrane. A typical plant

cell might contain about 50 chloroplasts per cell.

Chloroplast Definition



The word chloroplast is derived from the Greek word "chloros" meaning "green" and "plastes" meaning "the one who forms ". The chloroplasts are cellular organelles of green plants and some eukaryotic organisms. These organelles conduct photosynthesis. They absorb sunlight and convert it into sugar molecules and also produce free energy stored in the form of ATP and NADPH through photosynthesis. Chloroplasts are unique organelles and are said to have originated as endosymbiotic

bacteria.

Chloroplast Diagram

The chloroplast are double membrane bound organelles and are the site of photosynthesis The chloroplasts have a system of three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer and the inner membrane of the chloroplast enclose a semi-gel- like fluid known as the stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma.

Chloroplast Structure

Chloroplasts found in higher plants are generally biconvex or planoconvex shaped. In different plants chloroplasts have different shapes, they vary from spheroid, filamentous saucer-shaped, discoid or ovoid shaped. They are vesicular and have a colorless center. Some chloroplasts are in shape of club, they have a thin middle

zone and the ends are filled with chlorophyll. In algae a single huge chloroplast is seen that appears as a network, a spiral band or a stellate plate. The size of the chloroplast also varies from species to species and it is constant for a given cell type. In higher plants, the average size of chloroplast is 4-6 Âµ in diameter and 1-3 Âµ in thickness.



Outer membrane - It is a semi-porous membrane and is permeable to small molecules and ions, which diffuses easily. The outer membrane is not permeable to larger proteins.

Intermembrane Space - It is usually a thin intermembrane space about 10-20 nanometers and it is present

between the outer and the inner membrane of the chloroplast.

Inner membrane - The inner membrane of the chloroplast forms a border to the stroma. It regulates passage of materials in and out of the chloroplast. In addition of regulation activity, the fatty acids, lipids and carotenoids are synthesized in the inner chloroplast membrane.

Stroma

Stroma is a alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast. The space outside the thylakoid space is called the stroma. The chloroplast DNA chlroplast

ribosomes and the thylakoid sytem, starch granules and many proteins are found floating around the stroma. Thylakoid System

The thylakoid system is suspended in the stroma. The thylakoid system is a co llection of membranous sacks

called thylakoids. The chlorophyll is found in the thylakoids and is the sight for the process of light reactions of photosynthesis to happen. The thylakoids are arranged in stacks known as grana.

Each granum contains around 10-20 thylakoids.

Thylakoids are interconnected small sacks, the membranes of these thylakoids is the site for the light reactions of the photosynthesis to take place. The word 'thylakoid' is derived from the Greek word "thylakos" which means 'sack'.

Important protein complexes which carry out light reaction of photosynthesis are embedded in the membranes of the thylakoids. The Photosystem I and the Photosystem II are complexes that harvest light with chlorophyll and carotenoids, they absorb the light energy and use it to energize the electrons. The molecules present in the thylakoid membrane use the electrons that are energized to pump hydrogen ions into the thylakoid space, this decrease the pH and become acidic in nature. A large protein complex

known as the ATP synthase controls the concentration gradient of the hydrogen ions in the thylakoid space to generate ATP energy and the hydrogen ions flow back into the stroma.

Thylakoids are of two types - granal thylakoids and stromal thylakoids. Granal thylakoids are arranged in the grana are pancake shaped circular discs, which are about 300-600 nanometers in diameter. The stromal thylakoids are in contact with the stroma and are in the form of helicoid sheets.

The granal thylakoids contain only photosystem II protein complex, this allows them to stack tightly and form many granal layers wiht granal membrane. This structure increases stability and surface area for the capture of light.

The photosystem I and ATP synthase protein complexes are present in the stroma. These protein complexes acts as spacers between the sheets of stromal thylakoids.



Chloroplast Function

· In plants all the cells participate in plant immune response as they lack specialized immune cells. The chloroplasts with the nucleus and cell membrane and ER are the key organelles of pathogen

defense.

· The most important function of chloroplast is to make food by the process of photosynthesis. Food is prepared in the form of sugars. During the process of photosynthesis sugar and oxygen are made using light energy, water, and carbon dioxide.

· Light reactions takes place on the membranes of the thylakoids.

· Chloroplasts, like the mitochondria use the potential energy of the H+ ions or the hydrogen ion gradient to generate energy in the form of ATP.

· The dark reactions also known as the Calvin cycle takes place in the stroma of chloroplast.

· Production of NADPH2 molecules and oxygen as a result of photolysis of water.

· BY the utilization of assimilatory powers the 6-carbon atom is broken into two molecules of

phosphoglyceric acid.

66..BB..gg)) CCiilllliiaa aanndd ffllaaggeellllaa

Ø Cilia are short, more numerous hair like structures made of bundle of microtubules to help cell move.

Ø Cilia occur in group ciliata of protista, flame cells of worms, larval bodies of many invertebrates,

epithelium of respiratory tract, renal tubules, oviducal funnel, etc.

Ø A flagellum is like a cilium, but it is longer and there is usually only one or 2 flagella on a cell.

Ø There are three main varieties of flagellum - the bacterial flagellum (a helical filament that rotates

like a screw), archaeal flagellum (similar but nonhomologous to the bacterial flagellum), and the

eukaryotic flagellum (a whip- like structure that lashes back and forth).

Ø Flagella of bacteria do not show 9 + 2 arrangement.

Ø The principal protein of cilia and flagella is tubulin.

Ø Both cilia and flagella have following parts - basal body, rootlets, basal plate and shaft.

Ø Basal body or kinetosome is also called basal granule or blepharoplast.

Ø The basal bodies of cilia are found embedded in the refractile, gelatinous ectoplasm immediately

beneath the cell surface and are uniformly spaced in straight parallel rows. The basal bodies are said to

be homologous to the centriole.

Ø Rootlets are striated fibrillar outgrowths which develop from the outer lower part of the basal body and

are meant for providing support to the basal body.



Ø The rootlets are made of bundles of microfilaments. They are commonly present in the ciliated

epithelium of lower animals but are absent in the ciliated epithelium of mammals and in the ciliated

protozoa.

Ø Basal plate is an area of high density which lies above the basal body at the level of plasma membrane.

In the region of basal plate, one subfibre of each peripheral fibril disappears. The central fibrils develop

in this area.

Ø Shaft is the hair-like projecting part of flagellum or cilium.

Ø The shaft is covered on the outside by a sheath which is the extension of plasma membrane. Internally, it

contains a semifluid matrix having an axoneme (an essential motile element) of 9 peripheral doublet

fibrils and 2 central singlet fibrils. This arrangement is called 9 + 2 or 11- stranded in comparison to 9

+ 0 arrangement of the centriole or basal body.

Ø Each axoneme is organized by and anchored in a basal body.

Ø The function of cilia and flagella includes locomotion for one celled organism and to move substances

over cell surface in multicelled organism.

Ø The movements of cilia and flagella are brought about by sliding of doublets past each other rather than

by their contraction.

Ø The cilia may beat in metachronous or synchronous (isochronous) rhythm. In metachronous rhythm,

the cilia of a row beat one after the other, whereas in synchronous rhythm all the cilia of a row beat

simultaneously.

Ø Movements of cilia and flagella are of four types - pendulus movement, undulant movement,

unciform movement, and infundibuliform movement.

Functions of cilia and flagella are -

· They help in locomotion in flagellate and ciliated organisms.

· They create current for obtaining food from aquatic medium. It is also called food current.

· In some protists and animals, the organelles take part in capturing food.

· The canal system of porifers operates with the help of flagella present in their collar cells or

choanocytes.

· In coelenterates, they circulate food in the gastrovascular cavity. In tunicates and lancelets, the cilia

help in movement of food and its egestion.

· In land animals the cilia of the respiratory tract help in eliminating dust particles in the incoming air.



· Internal transport of several organs is performed by cilia, e.g., passage of eggs in oviduct, passage of

excretory substances in the kidneys, etc.

· Ciliated larvae take part in dispersal of the species.

66..BB..hh)) CCeennttrroossoommeess ((CCeennttrriioolleess))

All animal cells have two small organelles known as centrioles. The centrioles help the cell to divide. Centrioles are seen the process of mitosis and meiosis. The centrioles together are typically located near

the nucleus in the centrosome. Centrosomes is granular mass that is the organizing center for the microtubules. The position of the centrosome within the centrosome is at right angles to each other. Centrioles are made of nine bundles of microtubules that are arranged in a ring.

In animal cells the centrioles play a major role in cell division but the plant cells have the ability to reproduce even without the centrioles. In certain animal cells, like the female oocytes, some cells have have

shown successful division even in the absence of the centrioles. The absence of centrioles causes divisional errors and delays in the mitotic process. It is consequently suggested that centrioles have evolved as a cell

refinement that makes mitosis a more efficient and lesser error prone process.

Centriole Structure

· At the anaphase and telophase stages the centrioles appear as two cylindrical structures. They are open at both the ends and are located at right angles to each other.

The centrioles are cylindrical shaped cellular organelles. They are found in most of the eukaryotic cells. The

centrioles are made of groups of

microtubules, these microtubules are arranged in a pattern of 9*3. The pattern of the microtubules for a ring of 9 microtubules known as "triplets" and the microtubules are arranged at right angles to one another. In animals cells, the

centrioles help the organizing and assembly of microtubules during

the process of cell division. The replication (duplication) of centrioles happens in the interphase of mitosis and meiosis. The centrioles called basal

bodies form cilia and flagella.



· Each of the centriole is made up of nine triplet fibers and these triplet fibers are arranged in a circular manner that gives it a barrel-shaped appearance. Within the centrosome a mother centriole and the daughter centriole are arranged at perpendicular angles to each other. These two centrioles are tightly attached to each other and are surrounded by dense matrix called pericentriolar material.

· The mother centriole is a mature structure, it has additional appendages and is involved in anchoring and positioning of the microtubules. Comparatively the daughter is a young and immature structure.

· The length of the centriole is about 3,000 to 5,000 A and it is about 1,500 to 1,800 Ã… in diameter.

· The triplets are tilted, they form an angle of 40 degrees to the radius of the cylinder.

· Each triplet fiber is composed of three sub-tubules or sub-fibres. Each sub-tubule is of 250 Ã… in

diameter. These sub-tubules are hollow structures and their walls are made of monomeric units of proteins.The sub-tubules are made up of protein tubulin.

· The centriole internally shows a characteristic cart wheel structure. The cart wheel structure has a prominent central rod, and nine spokes radiating from the central rod.

· In 1958 scientists Bellis et al., said that the centrioles are surrounded by two crowns that lie one above the other. Each crown like structure is comprised of nine spheres that are called as massules or corpuscles.

· All the structures that surrounds the centriole together constitute the centriole satellite. The number of these satellites varies.

· The centrosome structure is made of lipids and proteins. However, it also contains carbohydrates and nucleic acids too.

Centrioles Function



· In higher animal cells the centrioles form the mitotic poles.

· The centrioles function as the microtubule organizing center, it is an important event in major cellular process, that is cell division and flagella formation.

· The centrioles pair duplicates within a cell and the two pairs migrate to the opposite ends of the cell to organize the mitotic spindle. During the transition phase of G1/S of the interphase stage, the existing centrioles disengage from each other.

· Each centriole gives rise to a new centriole. The centrioles that are newly formed remain tightly attached to the parent centriole, and it elongates during the S and G2 phase.

· In the prophase stage the centriole pairs start moving towards the opposite pokes of the cell, and also forming the spindle simultaneously.

· The migration and the positioning of the centrioles determines the orientation of the spindle. It also influences the chromosomes attachment to the spindle fibers.

· The spindle fibers are responsible for the segregation of chromosomes into the daughter cells..

· At end of each cell cycle, the cell has two centrioles - one the mother centriole and the other newly formed centriole which is the daughter centriole.

· After segregation, the centrioles determine the position of the nucleus and also influence the cellular organization in the new formed daughter cells.

· The centrioles may produce flagella or cilia.

· The fiber of the tail of sperms also arises from the centriole.

· The dysfunctioning of the centrosome is also responsible for the development of certain cancers.

Centrioles in Plant Cell



Plant cells do not have centrioles. Hence, the structure of the poles is different to that of the cells tha t possess centrioles. The poles of the cells of plants are broader when compared to the cells of animals. This is probably because of the absence of a cellular organelle that is defined to act as a focal point. In some of the plant spindle, the spindles are delocalized in a such a way that the spindle barely narrower than the rest of the spindle. The poles of hte plant cells are more diffuse and have many fewer astral microtubules.

Centrioles in Animal Cell

Centrosomes of the animal cells contain two barrel shaped structure that are called centrioles. The centrioles

help in the organization of the mitotic spindle and in the completion of cytokinesis. The centrioles are required for the formation of the mitotic spindle. These centrioles are important part of the centrosomes, they are involved in organizing the microtubules in the cytoplasm. The centriole position determines the position of the nucleus and it also plays are crucial role in the spatial arrangement of the cell.

66..BB..ii)) CCyyttoosskkeelleettoonn

· Microtubules are thickest cytoskeleton components with diameters of 24 nm. They are fine tubular

structures of variable length, with dense wall (5 nm thick) and a clear internal space (14 nm across). The

walls are composed of subunits called tubulin heterodimers, each of which consists of one a- tubulin

and one (3-tubulin protein molecule.

· The tubulin heterodimers are arranged in thread like polymers called protofilaments. Microtubules

increase in length by adding new heterodimers to one end, called the nucleation site. This

polymerization can be controlled experimentally by regulating calcium ion concentration or by treating

cells with antimitotic alkaloids.

· The cytoskeleton is a cellular scaffolding or sKeieion contained within the cytoplasm.

· Cytoskeleton consists of a network of long

protein tubes and strands in the cytoplasm to give cells shape and helps move

organelles. · The cytoskeleton is a mesh of filamentous

elements called microtubules,

microfilaments and intermediate

filaments and provide structural stability for the maintainence of cell shape.

· It is important in cell movement and in the rearrangement of cytoplasmic components. Microtubules are larger, hollow tubules of the protein called tubulin that maintain cell

shape, serve a tracks for organelle

movement & help cells divide by forming

spindle fibres that separate chromosome

pairs.



· Colchicine blocks the process by binding to the nucleation site. Vinblastine disrupts microtubules b y

binding to free tubulin.

· Microtubules have roles in the maintenance of cellhape, axoplasmic transport in neurons, melanin

dispersion in pigment cells, chromosome movements during mitosis, organization of the Golgi complex,

and the shuttling of vesicles within the cell.

· Unlike microfilaments, microtubules are unable to contract. Shortening occurs via depolymerization.

· Microtubules are found throughout the cytoplasm of most cells and in highly groupings in centrioies,

cilia, flagella, basal bodies and the mitotic spindle apparatus.

· Microfilaments are rope like structures made of 2 twisted strands of the protein actin capable of

contracting to cause cellular movement (muscle cells have many microfilaments).

· Microfilaments, the thinnest cytoskeletal components (5-7 nm wide) are usually composed of one of

several types of actin protein. Microfilaments are contractile, but to contract they usually must interact

with myosin. Microfilaments occur in eukaryotic plant and animal cells.

· Microfilaments often associate to form hexagonal bundles. They may also occur in parallel bundles or

loose network. Microfilaments generally lie at sol- gel interphase as well as below plasma membrane.

Microfilaments are also connected with spindle fibres, endoplasmic reticulum, chloroplast, etc.

· During mitosis of animal cells, they have been found associated with cleavage furrows.

· Microfilaments form the contractile machinery of the cell, like formation and retraction of

pseudopodia, plasma membrane undulations, microvilli, endocytosis, cytoplasmic streaming and

movement of other cell organelles.

· The microfilaments serve a number of functions - support, intracellular movement, streaming

movement, cleavage, locomotion, change in form, contraction, movement of villi, movement of plasma

membrane, membrane undulations, and formation of spindle.

· Intermediate filaments are intermediate in thickness (10-12 nm) between microtubules and

microfilaments. They are supportive elements in the cytoplasm of the eukaryotic cells except the plant

cells.

· They occur in the cell junctions and in the form of basket around nucleus of animal cells.

· Examples of intermediate filaments are - cytokeratins in epithelial cells, vimentin in

· mesenchymally derived cells (eg, fibroblasts, chondrocytes), desmin in muscle cells, glial fibrillary

acidic protein in glial cells, neurofilaments (intermediate filament bundles) in neurons.

The IFs serve a variety of functions -

1. They form a part of cytoskeleton that supports the fluid cytosol and maintains the shape of the cell.



2. They stabilize the epithelia by binding to the spot desmosomes.

3. They form major structural proteins of skin and hair.

4. They integrate the muscle cell components into a functional unit.

5. They provide strength to the axons.

6. They keep nucleus and other organelles in place.

7. Cardiac muscle cells are interconnected by spot desmosomes. Desmin filaments interconnect these

desmosomes, allowing the stress and strain of the contractile force of one muscle to be transmitted to

the other.

66..BB..jj)) MMiittoocchhoonnddrriiaa

· Long Oval Shaped organelle

· 1 mm to 4 mm in length and 0.2 mm to 1.0 mm in diameter.

· Within the cytoplasm of the cell

· Makes up 15-20% of cell’s total volume.

Structure of Mitochondria

Mitochondria are rod shaped structure found in both animal and plant cells. It is a double membrane bound organelle. It has the outer membrane and the inner membrane. The membranes are made up of phospholipids and proteins.

The components of mitochondria are as follows:

Outer membrane

· It is smooth and is composed of equal amounts of phospholipids and proteins.

· It has a large number of special proteins known as the porins.



· The porins are integral membrane proteins and they allow the movement of molecules that are of

5000 daltons or less in weight to pass through it.

· The outer membrane is freely permeable to nutrient molecules, ions, energy molecules like the

ATP and ADP molecules.

Inner membrane

· The inner membrane of mitochondria is more complex in structure.

· It is folded into a number of folds many times and is known as the cristae.

· This folding help to increase the surface area inside the organelle.

· The cristae and the proteins of the inner membrane aids in the production of ATP molecules.

· Various chemical reactions takes place in the inner membrane of the mitochondria.

· Unlike the outer membrane, the inner membrane is strictly permeable, it is permeable only to

oxygen, ATP and it also helps in regulating transfer of metabolites across the membrane.

Intermembrane space

· It is the space between the outer and inner membrane of the mitochondria, it has the same

composition as that of the cell's cytoplasm.

· There is a difference in the protein content in the intermembrane space.

Matrix

· The matrix of the mitochondria is a complex mixture of proteins and enzymes. These enzymes are

important for the synthesis of ATP molecules, mitochondrial ribosomes, tRNAs and mitochondrial

DNA.

Function of Mitochondria

Functions of mitochondria depends on the cell type in which they are present.

· The most important function of the mitochondria is to produce energy. The simpler molecules of

nutrition are sent to the mitochondria to be processed and to produce charged molecules. These

charged molecules combine with oxygen and produce ATP molecules. This process is known as

oxidative phosphorylation.

· Mitochondria help the cells to maintain proper concentration of calcium ions within the

compartments of the cell.

· The mitochondria also help in building certain parts of blood and hormones like testosterone and

estrogen.

· The liver cells mitochondria have enzymes that detoxify ammonia.



· The mitochondria also play important role in the process of apoptosis or programmed cell death.

Abnormal death of cells due to the dysfunction of mitochondria can affect the function of organ.

66..CC)) NNuucc lleeuuss::

Nucleus the most prominent organelle of the cell. The number of nuclei may vary, they may be uni-

nucleate (single nucleus), bi-nucleate (two nuclei) or even multi-nucleate.

Nucleus is present in all eukaryotic cells, they may be absent in few cells like the mammalian RBCs.

The shape of the nucleus is mostly round, it may be oval, disc shaped depending on the type of cell.

Nucleus was the first cell organelle to be discovered. Antonie von Leeuvenhoek (1632 - 1723) observed

lumen (nucleus) in the red blood cells of salmon. Nucleus was also described by Franz Bauer in 1804 and

by Robert Brown in 1831.

Robert Brown in 1833 named and discovered nucleus in plant cells.



The word nucleus is derived from a Latin word nucleus or nuculeus which means 'kernel'. Nucleus a

double-membrane bound cell organelle present in eukaryotic cells. The nucleus constitutes most of the

genetic material of the cell - the DNA.

The nucleus maintains the integrity of the genes which regulate the gene expression, in-turn regulating the

activities of the cell. Therefore, the nucleus is known as the control center of the cell

.

Nucleus Structure

The nucleus is the largest organelle of the cell. The nucleus appears to be dense, spherical organelle. It

occupies about 10% of the total volume of the cell.

In mammalian cells the average diameter of the nucleus is approximately 6 micrometers. A semi-fluid

matrix nucleoplasm is seen inside the nucleus which is a viscous fluid and is similar to the composition of

the cytoplasm.

Nuclear Envelope

The nuclear envelope is also known as the nuclear membrane.

It is made up of two membranes the outer membrane and the inner membrane.

The outer membrane of the nucleus is continuous with the membrane of the rough endoplasmic reticulum.

The space between these layers is known as the perinuclear space.

The nuclear envelope encloses the nucleus and separates the genetic material of the cell from the cytoplasm

of the cell.

It also serves as a barrier to prevent passage of macro-molecules freely between the nucleoplasm and the

cytoplasm.

Nuclear Pore

The nuclear envelope is perforated with numerous pores called nuclear pores.

The nuclear pores are composed of many proteins known as nucleoproteins.



The nuclear pores regulate the passage of the molecules between the nucleus and cytoplasm.

The pores allow the passage of molecules of only about 9nm wide. The larger molecules are transferred

through active transport.

Molecules like of DNA and RNA are allowed into the nucleus. But energy molecules (ATP), water and ions

are permitted freely.

Chromosomes

The nucleus of the cell contains majority of the cells genetic material in the form of multiple linear DNA

molecules.

These DNA molecules are organized into structures called chromosomes.

The DNA molecules are in complex with a large variety of proteins (histones) which form the chromosome.

In the cell they are organized in a DNA-protein complex known as chromatin.

During cell-division the chromatin forms well-defined chromosomes.

The genes within the chromosomes consists of the cells nuclear genome.

Mitochondria of the cell also contains a small fraction of genes.

Human cells has nearly 6 feet of DNA, which is divided into 46 individual molecules.

Nucleolus

The nucleolus is not surrounded by a membrane, it is a densely stained structure found in the nucleus.

The nucleoli are formed around the nuclear organizer regions.

It synthesizes and assembles ribosomes and rRNA.

The number of nucleoli is different from species to species but within a species the number is fixed.

During cell division, the nucleolus disappears.

Functions of the Nucleus

· It controls the heredity characteristics of an organism.

· It is responsible for protein synthesis, cell division, growth and differentiation.

· Stores heredity material in the form of deoxy-ribonucleic acid (DNA) strands.

· Also stores proteins and ribonucleic acid (RNA) in the nucleolus.

· It is a site for transcription process in which messenger RNA (m RNA) are produced for protein

synthesis.



· Aids in exchange of DNA and RNA (heredity materials) between the nucleus and the rest of the cell.

Nucleolus produces ribosomes and are known as protein factories.

· It also regulates the integrity of genes and gene expression.

Animal Cell Nucleus

Animal cell nucleus is a membrane bound organelle. It is surrounded by double membrane. The nucleus

communicates with the surrounding cell cytoplasm through the nuclear pores.

The DNA in the nucleus is responsible for the hereditary characteristics and protein synthesis.

The active genes on the DNA are similar, but some genes may be turned on or off depend ing on the specific

cell type. This is the reason why a muscle cell is different from a liver cell.

Nucleolus is a prominent structure in the nucleus. This aids in ribosomes production and protein synthesis.

Plant Cell Nucleus

Plant cell nucleus is a double-membrane bound organelle. It controls the activities of the cell and is known

as the master mind or the control center of the cell.

The plant cell wall has two layers - the outer membrane and the inner membrane, which encloses a tiny

space known as perinuclear space.

The nucleus communicates to the cell cytoplasm through the nuclear pores present in the nuclear membrane.

The nuclear membrane is continuous with the endoplasmic reticulum. The DNA is responsible for cell

division, growth and protein synthesis.



Animal Cell Plant Cell

Cell wall Absent Present (formed of cellulose)

Shape Round (irregular shape) Rectangular (fixed shape)

Vacuole One or more small vacuoles (much

smaller than plant cells).

One, large central vacuole taking up 90%

of cell volume.

Centrioles Present in all animal cells Only present in lower plant forms.

Chloroplast Animal cells don't have

chloroplasts

Plant cells have chloroplasts because they

make their own food

Cytoplasm Present Present

Endoplasmic

Reticulum (Smooth

and Rough)

Present Present

Ribosomes Present Present

Mitochondria Present Present

Plastids Absent Present

Golgi Apparatus Present Present

Plasma Membrane only cell membrane cell wall and a cell membrane



Animal Cell Plant Cell

Microtubules/

Microfilaments

Present Present

Flagella May be found in some cells May be found in some cells

Lysosomes Lysosomes occur in cytoplasm. Lysosomes usually not evident.

Nucleus Present Present

Cilia Present It is very rare

CELL STRUCTURE LOCATIO

N DESCRIPTION FUNCTION

Cell Wall

Plant, Fungi, &

Bacteria, but not animal

cells

· Outer layer · Rigid & strong · Made of cellulose

· Support (grow tall) · Protection · allows H2O, O2,

CO2 to diffuse in & out of cell

Cell Membrane

All cells

· Plant - inside cell wall

· Animal - outer layer; cholesterol

· Double layer of phospholipids with proteins

· Selectively permeable

· Support · Protection · Controls movement

of materials in/out of cell

· Barrier between cell and its environment

· Maintains homeostasis

Nucleus

All cells except prokaryotes

· Large, oval · May contain 1 or

more nucleoli · Holds DNA

· Controls cell activities

· Contains the hereditary material of the cell



Nuclear membrane

All

cells except prokaryotes

· Surrounds nucleus · Double membrane · Selectively

permeable

· Controls movement of materials in/out of nucleus

Cytoplasm

All cells

· Clear, thick, jellylike material (cytosol)

· Organelles found inside cell membrane

· Contains the cytoskeleton fibers

· Supports and protects cell organelles

Endoplasmic

reticulum (ER)

All

cells except prokaryotes

· Network of tubes or membranes

· Smooth w/o ribosomes

· Rough with embedded ribosomes

· Connects to nuclear envelope & cell membrane

· Carries materials through cell

· Aids in making proteins

Ribosome

All cells

· Small bodies free or attached to ER

· Made of rRNA & protein

· Synthesizes proteins

Mitochondrion


· Peanut shaped · Double membrane · Outer membrane

smooth · Inner membrane

folded into cristae

· Breaks down sugar (glucose) molecules to release energy

· Site of aerobic cellular respiration



Vacuole

Plant cells have a single, large

vacuole

Animal cells have

small vacuoles

· Fluid-filled sacs · Largest organelle in

plant cells

· Store food, water, metabolic & toxic wastes

· Store large amounts of food or sugars in plants

Lysosome

Plant - uncommon Animal - common

· Small and round with a single membrane

· Breaks down larger food molecules into smaller molecules

· Digests old cell parts

Chloroplast

Plants and algae

· Green, oval containing chlorophyll (green pigment)

· Double membrane with inner membrane modified into sacs called thylakoids

· Stacks of thylakoids called grana & interconnected

· Gel like innermost substance called stroma

· Uses energy from sun to make food (glucose) for the plant

· Process called photosynthesis

· Release oxygen

nucleolus


· Found inside the cell's nucleus

· May have more than one

· Disappear during cell division

· Make ribosomes

Golgi Apparatus


· Stacks of flattened sacs

· Have a cis & trans face

· Modify proteins made by the cells

· Package & export proteins

Cilia Animal cells,

Protozoans

· Have a 9-2 arrangement of microtubules

· Movement



· Short, but numerous

Flagellum

Bacterial cells &

Protozoans

· Have a 9-2 arrangement of microtubules

· Long, but few in number

· Movement

Centrioles

Animal cells

· · Paired structures

near the nucleus · Made of a cylinder

of microtubule pairs

· · Separate

chromosome pairs during mitosis

Cytoskeleton

All cells · Made of

microtubules 7 microfilaments

· Strengthen cell & maintains the shape

· Moves organelles within the cell

7. Transport and the cell.

· All cells are enclosed by a thin, film- like plasma membrane or plasmalemma.

· The term “plasmalemma” was introduced by Seifriz in 1928 later followed by J.Q. Plower (1931).

· This is also known as cell membrane (Nageli and Cramer 1855) which is a component of the cell

surface and form the cell boundary.

· The other cell membranes includes the

• nuclear envelope (encloses the nucleus)

• tonoplast (encloses the vacuole of plant cells), and



• the membranes of the various cell organelles such as ER, Golgi apparatus, mitochondria,

chloroplasts and lysosomes.

· In prokaryotic cells, the membranes forms the boundary of the cytoplasm being guarded from outside by

extracellular matrix and the cell walls. The plasma membrane and other intracellular membranes

surrounding the organelles and vacuoles are collectively known as biological membranes. Historically,

E. Overton (1895) was first to study the structure or composition of plasma membrane.

· Overton postulated that cell membrane is composed of a continuous layer of lipid material.

· E. Gorter and F. Gredel (1925) studied RBCs of a variety of mammals and proposed that the cell

membrane is formed of a bimolecular layer of lipid sheet.

· Cells communicate with their environment by cell membrane.

· Permeability is fundamental to the functioning of the living cell and to the maintenance of

satisfactory intracellular physiological conditions. This function determines which substances can

enter the cell, many of which may be necessary to maintain its vital processes and the synthesis of living

substances. It also regulates the outflow of excretory material and water from the cell.

· The presence of a membrane establishes a net difference between the intracellular fluid and the

extracellular fluid in which the cell is bathed. This may be fresh or salt water in unicellular organisms

grown in ponds or the sea, but in multicellular organisms the internal fluid, i.e., the blood, the lymph,

and especially the interstitial fluid, is in contact with the outer surface of the cell membrane. One of the

functions of the cell membrane is to maintain a balance between the osmotic pressure of the

intracellular fluid and that of the interstitial fluid.

· The plasma membrane is a semi-permeable (not every thing can pass through) boundary between the

cell and its external environment.

· Plasma membrane act as semi-permeable because it has the character of selective permeability means

that the cell membrane has some control over what can cross it, so that only certain molecules either

enter or leave the cell while keeping other constituents from escaping from the cell; and detects and

responds to the changes in the surrounding. The cell membrane is a fluid mosaic of lipids, proteins and

carbohydrates.

· Chemically a biomembrane consists of lipids (20-40%), proteins (50-75%) and carbohydrates (1-5%).

· The main lipid components of the plasma membrane are phospholipids, cholesterol and galactolipids.

· The major proportion of membrane phospholipids is represented by phosphatidylcholine,

phosphatidylethanolamine, and sphingolipids (eg. sphingomyelin and cerebros ides), all of which have



no net charge at neutral pH (i.e., neutral phospholipids) and tend to pack tightly in the bilayer (This

property is also shared by cholesterol).

· The lipid molecules are amphiatic or amphipathic, i.e, they possess both polar hydrophilic (water

loving) and nonpolar hydrophobic (water repelling) ends. The hydrophilic region is in the form of a

head while the hydrophobic part contains two tails of fatty acids.

· Membrane proteins have been classified as integral (intrinsic) or peripheral (extrinsic)

· according to the degree of their association with the membrane and the methods by which they can be

solubilized.

· Integral proteins are generally transmembrane proteins, with hydrophobic regions that completely span

the hydrophobic interior of the membrane. Proteins are much larger than lipids and move more slowly,

but some do drift.

· Peripheral proteins are not embedded in the lipid bilayer at all; they are loosely bound to the surface of

the membrane, often to the exposed parts of integral proteins.

· Proteins can be fibrous or globular, structural, carrier, receptor or enzymatic. About 30 kinds of enzymes

have been recorded in different biomembranes, e.g., phosphatases, ATP-ase, esterases, nucleases, etc.

· Carbohydrates present in the membrane are branched or unbranched oligosaccharides, e.g., hexose,

fructose, hexoamine, sialic acid, etc. Some of these oligosaccharides are covalently bonded to lipids,

forming molecules called glycolipids. Most are covalently bonded to proteins, which are thereby

glycoproteins.

· The biomembranes are asymmetric i.e the two surfaces of biomembrane are not similar because -

• Lipids present in both the layer are different, eg, lecithin on outer side and cephalin on inner side of

erythrocyte membrane.

• Extrinsic proteins are more abundant on inner side than on the outer surface.

• Oligosaccharides, attached to external surface of lipids and proteins, are absent on the inner side.

•

ULTRASTRUCTURE OF PLASMAMEMBRANE

· Under electron microscope the plasma membrane appears three layered, ie., trilamilar. One optically light

layer is of lipids and on both sides of it two optically dense layers of proteins are present.

· Three important models explaning the ultrastructure of plasma or cell membrane are-

• Danielli - Davson model : Bilayer model

• Robertsonian : Unit membrane concept

• Singer and Nicolson : Fluid mosaic model.



Bilayer model by Davson & Dameffi

· Danielli and Davson proposed that the pissi&t membrane is made up of three laver> biomolecular layer

of lipid sandwiched ber* two layers of proteins.

· Danielli and Davson model is the oldest modd :*i the structure of plasma membrane.

· The inner ends and lipid molecules are hydro" roc: and non-polar while the outer ends are h;. dr.rrfbr and

polar.

· Proteins are attached at the outer er.ri cf iipai —7 (hydrophillic ends) by ionic excharze? ir: hydrostatic

forces.

Unit membrane model by Robertson

· Based upon electron microscopy on myelix Robertson in 1959 proposed his famous *uKt membrane

concept’.

· Robertson called it as a ‘unit membrane' becaoe the pattern of molecular organization was sm all

membranes.

· According to Robertson the thickness of l:r.: biomolecular layer is 3.5 nm, each protein lays: 2.0 nm,

making up a total thickness of 7.5 nm (75A.

· One of the major weaknesses of Robertson ? model was its failure to explain permeability and

transport properties of the membrane.

· It has been well established that in plasci- membrane proteins are of two types -

§ Extrinsic proteins are peripheral proteins, associated with the surface.

§ These can be easily removed, eg., spectrin inred blood cells and ATPase in mitochondria

§ Intrinsic proteins or Integral proteins – enter the lipid bilayer and extend all the way through it,

eg., rhodopsin in retinal rod cells.

§ The portions of the polypeptide chains that extend through the lipid bilayer typically occur as a-

helices composed of hydrophobic amino acids.

Fluid mosaic model by Singer & Nicolson

§ The most universally accepted “fluid mosaic model” of structure of plasma membrane was proposed in

1972 by S. Jonathan Singer of university of California and Garth Nicolson of the Salk Institute.

§ Singer and Nicolson took the help of freeze- fracture techniques in electron microscopy.

§ According to fluid mosaic model, the membrane contains a biomolecular lipid (2 dimensional, liquid)

layer, the surface of which is interrupted by proteins.



§ Some proteins are attached at polar surface of lipids called the peripheral or extrinsic proteins and the

other proteins, which penetrate the bilayer or span membrane entirely are called the integral

transmembrane or intrinsic proteins.

§ The integral proteins form about 70% of the total membrane protein.

§ The extrinsic proteins may be covalently attached to fatty acids chains or non-covalently attached to other

transmembrane proteins.

§ The transmembrane proteins extends through the bilipid layer as a single helix (eg, glycophorms).

§ The proteins on the outer side may bear chain of sugar forming glycoproteins.

§ The three major types of membrane lipids are phospholipids, cholesterol and glycolipids. These lipids are

amphiphatic.

§ Cholesterol becomes intercataled between phospholipids in membranes and increases the stability of the

bilayers and prevents the loss of membrane liquidity at low temperature.

§ The proteins may float freely like icebergs.

§ The lipids act as a barrier to the entry or exit of charged polar substances.

§ Some proteins in the plasma membrane act as ‘gatekeepers” that regulate the traffic of molecules and ions

into and out of the cell.

§ Selective permeability of plasma membrane can be explained with the help of fluid mosaic model.

§ Plasma membrane is composed mainly of protein, lipid and a small percentage (1-5) of carbohydrates.

§ The carbohydrates of plasma membrane are hexose, hexosamine, fructose and sialic acid. Carbohydrates

present in plasma membrane are in the form of glycoproteins and glycolipids. Lipids and intrinsic

proteins form a mosaic pattern. The membrane is semifluid in nature and hence the lipid molecules and

intrinsic proteins move freely. Such membranes are also present around different cell organelles, eg.

mitochondria,Golgi bodies, endoplasmic reticulum, etc.



SPECIALISATION OF PLASMA MEMBRANE

§ The term ‘coenocyte’ is used to describe the multinucleate condition in which cell membrane is

lacking between cells.

§ The concept of membrane fluidity refers to the fact that both lipids and proteins may have

considerable freedom of lateral movement within the bilayer.

§ The fluidity of the membrane can be studied with a series of techniques that can be classified as

physical or biological.

§ The physical techniques are of two main types :

o those that involve a minimal perturbation of the membrane, such as X-ray diffraction and

nuclear magnetic resonance (NMR) spectroscopy; and (2) those that use certain added

molecules to monitor specific sites of the membrane. Into this second class fall fluorescence

microscopy, which uses fluorescent probes, and electron spin resonance (ESR) spectroscopy,

which uses paramagnetic probes (e.g., nitroxide-containing amphipathic molecules) that are

introduced into the lipid bilayer.

§ The biological techniques involves light and fluorescence microscopy and electron microscopy,

including freeze fracturing and radioisotope labelling methods.

§ Several factors that influence membrane fluidity are: temperature; percentage of unsaturated tails;

and the presence of cholesterol.

§ Fluidity increases with rise in temperature and decreases with lowering of temperature.

Membrane fluidity is essentially a property of the lipids.

§ Normally these are fluid at body temperature and the main consideration is the degree of saturation

of the hydrocarbon chains.

§ As temperature decreases, a critical temperature is reached at which the membrane solidifies. At this

temperature, the tails of the phospholipids are packed tightly together and movement is inhibited.

§ One factor that tends to increase rigidity is the concentration of cholesterol. The steroid

cholesterol, which is wedged between phospholipid molecules in the plasma membranes of animals,

helps stabilize the membrane. At relatively warm temperatures, for example, 37°C, the body

temperature of humans, cholesterol makes the membrane less fluid by restraining the movement of

phospholipids. However, because cholesterol hinders the close packing o f phospholipids, it also

lowers the temperature required for the membrane to solidify.

§ Rapid changes in fluidity can be produced by methylation of phosphatidylethenalomine by

methyltransferases present in the membrane. These inturn are regulated by receptors. This has

been confirmed by a simple but ingenious experiment devised by Frye and Edidin (1970).



§ L.D. Frye and M. Edidin (1970) provided evidence for the mobility of membrane proteins

obtained by fusing mouse and human cells to form heterokaryons.

§ Before fusion, they labelled mouse cells with green fluorescent antibody dye fluoroscein and human

cells with red fluoroscent antibody dye rhodamine. The frequency of cell fusion could be greatly

increased by adding Sendai virus.

§ Cell membrane modified into foldings, intercellular junctions and extracellular coats.

§ Infolds (invagination) facilitate pinocytosis (cell drinking) and phagocytosis (cell eating) which

together constitute endocytosis.

§ Invagination results in the formation of pores, mesosomes, lomasomes and transfer cells. Cell-cell

junctions play an important role in cellcell adhesion and in intercellular transport.

The most common types of junctions are -

• Tight junctions (zonula occludens)

• Intermediate junctions (belt desmosomes)

• Spot desomosomes (macula adherens)

• Gap junctions (connexons or nexuses)

• Plasmodesmata

Extracellular coats formed in animal cells are of following types - chitin, glycocalyx, basement

membrane, cell wall etc.

TRANSPORT ACROSS BIOMEMBRANE

Membranes are selectively permeable. The different methods of transport across the cell membrane

consists of: passive transport, active transport, bulk transport.

Passive transport

· Passive transport is a mode of membrane transport where the cell does not spend any metabolic

energy.

· This transport is according to the concentration gradient.

· Passive transport is of following types - diffusion (simple diffusion), facilitated diffusion and osmosis.

· Simple diffusion can occur either through lipid matrix of the membrane or with the help of channels.

· In faciliated diffusion some specific solutes diffuse down electrochemical gradients across membrane

more rapidly than might be expected from size or charge.



· Facilitated diffusion occurs through the agency of special membrane prote ins (called carrier proteins

which is also known as permeases), but like simple diffusion, it requires no metabolic energy.

· Osmosis (discovered by Albe Nollet, 1748) is the net movement of a solvent through a selectively

permeable membrane.

· In living systems, the solvent is water, which moves by osmosis across plasma membranes from an area

of higher water concentration to an area of lower water concentration.

· In osmosis, water moves through a selectively permeable membrane from an area of lower solute

concentration to an area of higher solute concentration.

· The water potential of a solution is the term given to the tendency for water molecules to enter or leave

that solution by osmosis.Water potential is a term derived from thermodynamics, and is a measure of the

free kinetic energy of the water molecules in the solutions.

· Two important factors which determine the water potential of solutions in and around living cells

are- the presence of dissolved solutes (giving rise to a solute potential) and the mechanical pressure acting

on water (pressure potential).

· Osmotic pressure is the maximum pressure which can develop in an osmotically active solution when it

is separated from its pure solvent by a semipermeable membrane under ideal conditions.

· The solute potential of a concentrated solution can be demonstrated dramatically in an apparatus known as

an osmometer.

· Tonicity is the amount of tension developed in a system on account of the occurrence of solute particles in

it. It is usually determined in comparison to other systems or solutions.

· When two solutions are compared for their osmotic pressure or concentration, one may have higher

concentration than the other or the two may have the same concentration.

· On the basis of tonicity, solution are of three types - hypertonic, hypotonic and isotonic.

· Tonicity determines exosmosis and endosmosis.

· Hyperosmotic (hypertonic) solutions will cause water to leave cells by osmosis, and cells may shrink (in

animal cells, membrane collapses or becomes crenated and in plant cell, shows plasmolysis).

· Hypertonic solution has higher proportion (concentration) of solutes.

· Hypo-osmotic (hypotonic) solutions will cause water to enter cells by osmosis, causing the cells to swell

and burst in animal cell and becomes turgid in plant cells.

· In hypoionic, the external solutions has a lower solute concentration (more water) than the internal

solution of the cells.

· Isotonic solutions will have equal proportions of solutes to water on both sides of the membrane.



· Iso-osmotic solutions are osmotically balanced and there is no net movement of water. Water will move

through the membrane, but equal amounts of water will be moving in both directions.

· Types of osmosis are- endosmosis and exosmosis.

o Endosmosis is the process of osmotic entry of solvent or water into a cell or system when it is in

contact with hypotonic solution or pure solvent. Endosmosis is best seen by placing raisins in water.

Exosmosis is the outflow or exit of solvent or water from a cell or system when the same is kept in

contact with hypertonic solution. Exosmosis is best seen when sliced cucumber is sprinkled with

salt.

o Exosmosis causes plasmolysis in plant cells.

Active transport

· The active transport is the transport of ions or molecules against their concentration gradient or

electrochemical gradient.

· Active transport differs from diffusion in that molecules are transported away from thermodynamic

equilibrium, hence energy is required.

· This energy can come from the hydrolysis of ATP, from electron movement or from light.

· Two main types of active transport are : primary active transport and secondary active transport.

· In primary active transport, energy is provided by another coupled reaction such as the hydrolysis of

ATP.

· Sodium-potassium exchange pump is an example of primary active transport.

· For each molecule of ATP used, three Na+ ions are pumped out and two K+ ions are pumped in.

· The Na+ gradient established by the Na+ - K+ pump provides a source of energy that is frequently used to

power the active transport of sugars, amino acids and ions in mammalian cells.

· A secondary active transport takes place with the help of ion gradient from the coupled transport of a

second molecule.



Bulk transport

· Bulk transport is the transport of large quantities of macromolecules, micromolecules and food particles

through the membrane.

· Bulk transport occurs by two main methods, endocytosis and exocytosis.

· Bulk transport involves the enclosure of the material under transport in carrier vesicles of the membrane.

· The inward transport of carrier vesicles is called endocytosis and outward transport of carrier vesicles is

called exocytosis.



· The bulk transport is common in excretory and secretory cells.

· When cells engulf extracellular substances and bring them to the cytoplasm in membrane bound vesicles it

is called endocytosis.

· Endocytosis occur by pinocytosis, phagocytosis and receptor mediated endocytosis.

· Phagocytosis (Gr. phagein, to eat, kytos, cell) is a process whereby certain cells and unicellular organisms

are capable of ingesting and digesting soiid material.

· The term phagocytosis has been coined by a Russian scientist Ilie Metchnikoff in 1893.

· For the description of phagocytosis Metchnikoff got Nobel prize of Physiology and Medicine in 1908.



· Phagocytosis is done by the cell extending pseudopodia which encircle and engulf it into membrane

delimited vesicles called phagosomes.

· The term pinocytosis (Gr. pinein, to drink) was coined by Lewis in 1931.

· Lewis was the first to observe pinocytosis in living cells in culture.

· Pinocytosis (also called cell drinking) is quite common in the cell lining the blood capillaries.

· Lewis described the uptake of fluid matter and substances dissolved in it (eg ions, sugars, amino acids) by

an active movement of undulating membrane formed at the periphery of the cell.

Fig: Receptor mediated endocytosis

Phagocytosis



· Mast and Doyle using fluorescence microscopy indicated that pinocytosis may be important for the

cellular uptake of proteins.

· A receptor mediated selective process is known as absorptive pinocytosis.

· The vesicles formed during absorptive pinocytosis are derived from invaginations (pits) that are coated on

the cytoplasmic side with filamentous material like clathrin.

· The signal for exocytosis is often a hormone which, when it binds to a cell surface receptor, induces a

local and transient change in Ca2+ concentration. Ca2+ triggers exocytosis.

· Exocytosis (the reverse of endocytosis) causes expulsion of materials from the cells.

· In plant cells, exocytosis is an important means of exporting the materials needed to construct the cell wall

through the plasma membrane.

· Among protists, contractile vacuole discharge is a ipnn of exocytosis. In animal cells, exocytosis provides

a mechanism for secreting many hormones, neurotransmitters, digestive enzymes, and other substances.

FUNCTIONS OF BIOMEMBRANES

· The most important function of plasma membrane is to provide passage for various substance, into and out

of the cell.

· Plasma membrane is selectively permeable, ie., allows some solute particles (1-15 A) to pass through it

readily along with all solvents.

· It not only provides mechanical strength but also acts as a protective layer.



· Plasma membrane is responsible for the transportation of materials, molecules or ions, etc., through it.

· Anchoring of the cytoskeleton to provide shape to the cell.

· Attaching to the extracellular matrix to help group cells together in the formation of tissues.

· The plasma membrane takes part in cellular locomotion in two ways : pseudopodia and undulation.

- In pseudopodia Amoeba, macrophages and WBCs move with the help of temporary locomotary

organelles like pseudopodia.

- In undulation some mammalian cells such as fibroblasts can move over a solid surface by wave like

undulations of the plasma membrane.

88.. BBiioommooll eeccuull eess:: ((CChheemmii ccaall ccoonnssttiittuueenntt ooff ll ii vvii nngg cceellll ..))

All living systems can grow, sustain and reproduces themselves. The different chemical changes occur in

living body, fall in the domain of biochemistry. There are various molecules which take part in biochemical

reactions of living body. These molecules are termed as Biomolecules.

Biomolecules is defined as "Biomolecules are those molecules that are involved in the maintenance and

metabolic processes of all living organisms."

These molecules interact with each other under optimum conditions and form different products. Hence,

Biomolecules are complex chemical substances which form the basis of life and are responsible for their

growth, maintenance and ability to reproduce.

Types of Biomolecules

The different bio molecules, which take part in maximum biochemical reactions are carbohydrates, proteins,

enzyme, nucleic acid, lipids, hormones and compounds for energy storage like, adenosine diphospahte

(ADP) and adenosine triphospahte (ATP). There are more than thousand bio molecules in different living

systems.

Even though there are few basic classes of bio molecules. The major bio molecules included glycerine,

polysaccharides, polypeptides and nucleic acids. These all bio molecules have giant molecular

structures as they are polymers of certain small monomers.

For example:

Monomer Polymer Fatty acid Diglyceride, triglyceride

Monosaccharide Polysaccharide



Amino acid Polypeptide(protein) Nucleotide Nucleic acid(DNA, RNA)

dd)) CCaarr bboohhyyddrraatteess

Carbohydrate is the preferred energy source for many of the body's functions. As long as carbohydrate is

available the human brain depends exclusively on it as an energy source.

Carbohydrate shares its fuel providing responsibility with fat. Fat however normally is not used as fuel by

the brain and central nervous system and diets high in certain types of fat are associated with chronic

diseases. The other energy sources available to the body protein and alco hol offer no advantage as fuels.

Carbohydrates are one of the essential food ingredients, which we all require. We consume carbohydrates in

one form or the other. We encounter carbohydrates at every turn of our lives. The paper we use for writing,

the cotton clothes we wear and the wooden furniture around us are all made of cellulose.

The sweetening agents in fruits are nothing but simple carbohydrates. Carbohydrates are organic

molecules composed of carbon, hydrogen and oxygen.

Types of Carbohydrates

The dietary carbohydrates include the sugars, starch and fiber. Chemists describe the sugar as

· Mono saccharides (single sugars)

· Di saccharides (double sugars)

Starch and fibers are

· Polysaccharides (compounds composed of chains of mono saccharides units)

All of these carbohydrates are composed of the single sugar glucose and other compo unds that are much

like glucose in composition and structure.

Monosaccharides

Three mono saccharides are important in nutrition glucose, fructose and galactose. All three mono

saccharides have the same number and kinds of atoms, but in different arrangements.

1. Glucose

Most cells depend on glucose for their fuel to some extent, and the cells of the brain and the rest of the

nervous system depends almost exclusively on glucose for their energy. The body can obtain this glucose

from carbohydrates. To function optimally the body must maintain blood glucose within the limit that allow

the calls to nourish themselves. If blood glucose level falls below normal the person may become fatigued.



Blood glucose homeostasis is regulated primarily by two hormones insulin which moves glucose from blood

into the cells and glucagon which brings glucose out of storage when blood glucose falls.

The structure of glucose is shown below.

2. Fructose

Fructose is the sweetest of the sugars. It occurs naturally in fruits, honey and saps. Other sources include

soft drinks, ready to eat cereals and other products sweetened with high fructose corn syrup. Glucose and

fructose are the most common mono saccharides in nature.

3. Galactose

The third single sugar is galactose occurs mostly as a part of lactose, a di saccharides also known as milk

sugar. During digestion galactose is freed as a single sugar.

Disaccharides

In disaccharides pairs of single sugars are linked together. Three disaccharides are important in nutrition: maltose, sucrose and lactose. All three have glucose as one of their single sugars.

1. Sucrose

Sucrose is the most familiar of the three disaccharides and is what people generally mean when they speak

of "sugar". this sugar is usually obtained by refining the juice from sugar beets or sugarcane to provide the

brown, white and powdered sugars available in the supermarket, but it occurs naturally in many fruits and

vegetables. When a person eats a food containing sucrose, enzymes in the digestive tract split the sucrose

into its glucose and fructose components.



2. Lactose

Lactose is the principal carbohydrate of milk. Most human infants are born with the digestive enzymes

necessary to split lactose into its two mono saccharides parts, glucose and galactose. So as to absorb it.

Breast milk thus provide a simple, easily digested carbohydrate that mee ts an infants energy needs; many

formulas do too, because they are made from milk.

3. Maltose The third disaccharide maltose is a plant sugar that consists of two glucose units. maltose is produced whenever starch breaks down - as happens in plants when they break down their stored starch for energy and start to sprout and in human beings during carbohydrate digestion.

Polysaccharides

Unlike other sugars which contain the three mono saccharides-glucose, fructose and galactose in different combinations, the polysaccharides are composed almost entirely of glucose. Three types of polysaccharides are important in nutrition; glycogen, starch and fibers.

1. Glycogen Glycogen molecules which are made of chains of glucose, are most highly branched than starch mo lecules. Glycogen is found in metals only to a limited extent and not at all in plants. For this reason glycogen is not a significant food source of carbohydrate, but it does play an important role in the body. The human body stores much of its glucose in the liver and muscles. The liver stores the glycogen making it available as

blood glucose for the brain to other tissues when the supply runs low.



Starch

Starch is a long straight or branched chain of hundreds or thousands of glucose units linked together. These giant molecules are packed side by side in grains such as rice or wheat, in root crops and tubers such as

yams and potatoes, and in legumes such as peas and beans. When a person eats the plants, the body splits the starch into glucose units and uses the glucose for energy.

Fibers

Dietary fibers are the structural parts of plants and thus are found in all plant derived foods-vegetables, fruits, whole grains and legumes. Most dietary fibers are polysaccharides chains of sugars just as starch is but in fibers the sugar units are held together by bonds that human digestive enzymes cannot break. Consequently most dietary fibers pass through the body providing little or no energy for its use.

e) Amino Acids

All metabolic activities of living bodies are regulated by Biomolecules. There are mainly four types of

Biomolecules which take part in almost all the metabolic activities of living cells. These Biomolecules are

carbohydrates, lipids, proteins and nucleic acid. These are polymeric forms of some monomer units. For

example; proteins are composed of amino acids that are bonded with each other through peptide linkage.

Amino acids are organic molecules with two functional groups which are bonded to the same carbon atom

of the molecule. One is an amino group (-NH2) and another is carboxylic group (-COOH). The carbon

atom, at which both of the functional groups are bonded, is known as the alpha carbon atom and such amino

acids are known as alpha amino acids. Only alpha amino acids involve in the polymerisat ion to form protein

molecule. Beta and gamma amino acids cannot form protein molecules.

The amino group of one amino acid is involved in a condensation reaction with carboxyl group of another

amino acid to form –CO-NH- linkage. This linkage is called as a peptide bond. Do you know both the



functional groups of an amino acid molecule have different nature? One is acidic and another is alkaline in

nature. Then what will be the nature of an amino acid molecule? Let’s discuss the properties of different

amino acid molecules.

Amino acids are generally referred as α amino acids. The general formula for amino acid is

H2NCHRCOOH. Here 'R' is the functional group of the amino acids.

Ball and stick model

The amino group is attached to the carbon atom immediately next to the carboxylate group that is α carbon,

so, it is called α amino acids. Other amino acids also exist called beta and gamma amino acids, in which

the amino group is attached to the carbon which is next to α carbon called α amino acid and the next is

called α amino acid.

Classification of Amino Acids

Amino acids are classified into different ways based on polarity, structure, nutritional requirement,

metabolic fate, etc. Generally used classification is based on polarity. Amino acid polarity chart shows the

polarity of amino acids.

Based on polarity, amino acids are classified into four groups as follows,

1. Non-polar amino acids

2. Polar amino acids with no charge

3. Polar amino acids with positive charge

4. Polar amino acids with negative charge



Classification based on polarity

Amino acid is classified based on those with non-polar R group, uncharged polar R group, charged polar R

group. The non-polar side chain amino acids are called hydrophobic and the amino acid with uncharged

polar side chain is called hydrophilic. It also contains the acid side chains and basic side chains.

Non Polar Amino Acids

Non Polar Amino Acids have equal number of amino and carboxyl groups and are neutral. These amino

acids are hydrophobic and have no charge on the 'R' group. The amino acids in this group are alanine,

valine, leucine, isoleucine, phenyl alanine, glycine, tryptophan, methionine and proline.

Polar Amino Acids with Positive Charge

Polar amino acids with positive charge have more amino groups as compared to carboxyl groups making it

basic. The amino acids which have positive charge on the 'R' group are placed in this category. They are

lysine, arginine and histidine.



Polar Amino Acids with no Charge

These amino acids do not have any charge on the 'R' group. These amino acids participate in hydrogen

bonding of protein structure. The amino acids in this group are - serine, threonine, tyrosine, cysteine,

glutamine and aspargine.

Polar Amino Acids with Negative Charge

Polar amino acids with negative charge have more carboxyl groups than amino groups making them acidic.

The amino acids which have negative charge on the 'R' group are placed in this category. They are called

as dicarboxylic mono-amino acids. They are aspartic acid and glutamic acid.

Classification based on the availability of amino acids. The amino acids which cannot be synthesized in the

body must be supplied to the body through diet are called essential amino acids. The amino acids which can

be synthesized in the body are called non-essential amino acids.



Functions of Amino Acid

Amino acids can be found in most of the nutrients we eat. Amino acids are the building blocks of healthy

protein. If the food consumed is rich in protein, our body digests the protein right down to individual amino

acids and little links of amino acids is adequate to be taken over into the blood stream.

The main purposes of amino acids are to construct and rectify muscular tissue, but the uses go further than

that. These things develop chemicals substance that grant our minds to function at its eminent potential.

Structure of Amino Acid

Amino acids are the building blocks of proteins. Each amino acid contains :

1. Amino group (-NH2 group

2. Carboxyl group (-COOH group)

3. R group (side chain) which determines the type of an amino acid

All three groups are attached to a single carbon atom called chiral carbon. There are 20 common amino

acids characterised by different R groups. These 20 amino acids can be classified according to their mass,

volume, acidity, polarity and hydrophobicity.



Essential Amino Acids

Out of the 20 amino acids in proteins, humans are able to synthesize only 11, called non-e ssential amino

acids, the other 9 called essential amino acids, must be obtained in diet. The division between essential and

nonessential amino acids is not clear cut however Tryosine for instance is sometimes considered non-

essential because humans can produce it from phenylalanine, but phenylalanine itself is essential and must

be obtained in the diet. Arginine can be synthesized by humans, but much of the arginine in proteins also

comes from the diet.

Essential amino acids are not produced by the body. We can get them by eating complete protein foods or

from a combination of incomplete vegetables. The nine essential amino acids include histidine, isoleucine,

leucine, lysine, methionine, phenylalanine, tryptophan, and valine. The thirteen non-essential amino acids

are alanine, arginine, aspartic acid, cysteine, cystine, glutamic acid, glutamine, glycine, hydroxyproline,

proline, serine, and tyrosine.

Essential amino acids Non-essential amino acids

Histidine Alanine

Isoleucine Arginine

Leucine Asparagine

Lysine Aspartic acid

Methionine Cysteine

Phenylalanine Glutamic acid

Threonine Glutamine

Tryptophan Glycine

Valine Proline

Serine

Tyrosine

Standard Amino Acids

The standard 20 amino acids differ only in the structure of the side chain or R group. They can be

subdivided into smaller groupings on the basis of similarities in the properties of their side chains.

In addition to the 20 common amino acids there are non-standard amino acids. Two non-standard amino

acids which appear in proteins and can be specified by genetic code are selenocysteine and pyrrolysine.

There are also non-standard amino acids which do not appear in proteins.

Examples include lanthionine, 2-aminoisobutyric acid and dehydroalanine. They often occur as

intermediates in the metabolic pathways for standard amino acids. Some non-standard amino acids are



formed through modification to the R-groups of standard amino acids. One example ishydroxyproline which

is made by a post translational modification of proline.

Properties of Amino Acids

The general properties of amino acids are listed below :

· Solubility: Amino acids are soluble in water, acids, alkalies, but sparingly soluble in organic

solvents.

· Color: Amino acids are colorless, white solids.

· State: Amino acids are solid crystalline compounds.

· Melting points: Amino acids have high melting points.

· Due to presence of basic and acidic groups in the same molecule, they may be regarded as salts and

hence, most of them either possess higher melting point or melt with decomposition.

· Presence of asymmetric carbon

· Except Glycine (the first member of amino acid series), all the amino acids contain at least one

asymmetric carbon atom and hence, they are optically active and can exist in d and l- forms.

· But it is very important to note that in nature they never exist in the racemic form.

· In nature they always occur in the optically active form.

· All the naturally occurring amino acids are having L- configuration.

· And this L-series of amino acids are biologically active compounds.

· They exist in the form of internal salt, zwitter ion. (as we know aminoacids have acidic and basic

functional groups in it)

Amino acids and carboxylic groups

Decarboxylation of amino acids



Deamination of amino acids

Ammonium salts from amino acids

Carboxylate salts from amino acids

Zwitterions

Since in amino acids both carboxylic group(-COOH) and amino group(-NH3) exists, in aqueous solution the

H+ ion is transformed from one end of the molecule to the other end to form zwitterions. It is considered as

both electrically charged and electrically neutral also. Since it is having both the charges (positive and

negative) the net charge of zwitterion is zero.



Amino acid in zwitter ion form reacts with both acids and bases and it shows the amp hoteric behavior. The

point at which the zwitter ion has no net charge is called Iso-electric point. At this point the amino acid is

considered as electrically neutral.

Isoelectric Point

An important pH value relative to the various forms an amino acid can have in solution, is the pH at which it

exists primarily in its zwitterion form, that is, its neutral form (no net charge). This pH value is known as the

isoelectric point for the amino acids. An isoelectric point is the pH at which an amino acid exists primarily

in its zwitter form. At the isoelectric point, almost all amino acid molecules in a solution (more than 99%)

are present in their zwitterion form.

Every amino acid has a different isoelectric point. Fifteen of the 20 amino acids, those with non polar

neutral side chain have isoelectric points in the range of 4.8-6.3. The three basic amino acids have higher

isoelectric points, and the two acidic amino acids have lower ones. The list of isoelectric points for the 20

standard amino acids are given below.

Name isoelectric point

alanine 6.01

arginine 10.76

asparagine 5.41

aspartic acid 2.77

Cysteine 5.07

glutamic acid 3.22

glutamine 5.65

glycine 5.97

histidine 7.59

isoleucine 6.02

leucine 5.98

lysine 9.74

methionine 5.74

phenylalanine 5.48

proline 6.48

serine 5.68

threonine 5.87

tryptophan 5.88

trrosine 5.66

valine 5.97



Expanded Genetic Code

Genetic code is the dictionary of nucleotide bases, which determines the sequence of amino acids in

proteins. The genetic code (or) codons have triplet base sequences in m RNA, which act as code words for

amino acids. The DNA sequence that code for a specific protein(or) polypeptide is called a gene. There are

4 different bases in m RNA - A, G, C and U. They produce 64 different triplets (43). Out of 64 codons 61

codons code for 20 amino acids. The 3 codons UAA, UAG and UGA do not code for amino acids and they

are called non-sense codons. The codons AUG and GUG are called initiating codons.

However, there being only 20 amino acids, more than one codon can code for the same amino acid.

Example: CUU and CUC both code for leucine.

The genetic code has four note worthy features

1. The genetic code is universal as the same codons code for the same amino acid in all living

organisms

2. It is degenerate, i.e., more then one codons code for an amino acid.

3. It is comma-less.

4. The third base of the codon is less specific.

PPrrootteeiinnss

Proteins are compounds found in all living cells, in animals and plants. They play a variety of important

roles and are essential to maintain the structure and function of all lifeforms. The word 'protein' is derived

from the Greek word protos, meaning "primary" or "first". Proteins are vital for the growth and repair, and

their functions are endless. Each and every property that characterizes a living organism is affected by

proteins, whether it is a bacteria or a human body.

Proteins perform many functions which are essential for life. The building blocks of proteins are the twenty

naturally occurring amino acids. The chemical and physical structure of amino acids and proteins, describe

the topology proteins and discuss an important enzyme and penicillin amidase. These amino acids are

liberated when proteins are hydrolyzed. Proteins are the polymers of -amino acids.

All proteins contain the elements carbon, hydrogen, oxygen, nitrogen and sulfur some of these may also

contain phosphorus, iodine, and traces of metals like ion, copper, zinc and manganese.



The name protein is derived from the Greek word proteins meaning of prime importance. As enzymes, they

catalyze biochemical reactions, as hormones they regulate metabolic processes and as antibodies they

protect the body against toxic substances.

Composition of Proteins

1. Proteins are made of long chains of amino acids.

2. Proteins are composed of carbon, hydrogen, oxygen and nitrogen arranged as the strands of amino

acids.

3. The digestive system breaks down protein containing food into individual amino acids.

4. The body then resembles the amino acids in different o rders to make new proteins, which can be

used for growth, repairing tissues, hormones and as enzymes.

5. As proteins are made of amino acids joined together by peptide bonds amino acids can be called the

basic molecules of life.

A strand of amino acids that makes up a protein may contain 20 different kinds of amino acids. Amino acids

are the building blocks of proteins. Each has an amine group at one end and an acid group at the other and a

distinctive side chain. The backbone is the same for all amino acids. The side chain differs from one amino

acid to the next while the nitrogen is in the amine group.

Food Protein Chart

All proteins are made of amino acids and different protein foods contain different proportions of the various

amino acids. There are, in all 23 amino acids, in food proteins. As one food cannot supply all the required

amino acids, a fair combination of various foods need to be chosen to supply us all the amino acids.



Proteins after digestion are finally broken down into amino acids. Thereafter, they are synthesized for body

use. Excess of dietary protein is converted into fat. The food rich in proteins are beef, chicken, pork, turkey,

fish, beans, milk, cheese, ice cream, peanut butter, eggs, nuts, cottage cheese and yogurt.

Human body use proteins to build new cells, maintain tissues, and synthesize new proteins that make it

possible for performing basic bodily functions. Proteins are present in the outer and linear membranes of

every living cell. Proteins play an important part in the creation of every new cell and every new individual.

The hair, nails and the outer layers of skin are made of keratin, a scleroprotein, or a protein resistant to

digestive enzymes.



Essential and Nonessential Proteins

To make all the proteins that body needs, it requires 22 different amino acids. Out of these ten are

considered essential, which means these cannot synthesize them in body and must obtain them from food.

Several more are nonessential. If we don't get them in food, we can manufacture them from fats,

carbohydrates and other amino acids.

Glutamine, ornithine and taurine are somewhat in between essential and nonessential for human

beings. They are essential under certain conditions such as with injury or disease. The essential and

nonessential amino acids are listed below.

Essential amino acids Nonessential amino acids

Arginine Alanine

Histidine Asparagine

Isoleucine Aspartic acid

Leucine Citrulline

Lysine Cysteine

Methionine Glutamic acid

Phenylalanine Glycine

Threoniwne Hydroxyglutamic acid

Tryptophan Norleucine

Valine Proline

Serine

Tyrosine

Structure of Proteins

The 20 amino acids commonly found in proteins are joined together by peptide bonds. The complexity of

protein structure is best analyzed by considering the molecule in terms of four organizational levels, namely

primary, secondary, tertiary and quaternary.

Primary structure

Primary structure is the order in which the amino acid are covalently linked together. The primary structure

is the one-dimensional first step in specifying the three-dimensional structure of a protein. Understanding

the primary structure of proteins is important because many genetic diseases result in proteins with

abnormal amino acid sequences, which cause improper folding and loss or impairment of normal function.



Secondary structure

Secondary structure is the arrangement in space of the atoms in the peptide backbone. The α-helix and β-

pleated sheet arrangements are two different types of secondary structure. In many proteins the folding of

parts of the chain can occur independently of the folding of other parts. Such independently folded portions

of proteins are referred to as domains or super-secondary structure.

Tertiary structure

Tertiary structure includes the three-dimensional arrangement of all the atoms in the protein, including those

in the side chains and in any prosthetic groups. A specific three-dimensional shape of a protein resulting

from interactions between R groups of the amino acid residues in the protein.



Quaternary structure

Many proteins consist of a single polypeptide chain, and are defined as monomeric proteins. However,

others may consist of two or three polypeptide chain that may be structurally identical or totally unrelated.

The arrangement of these polypeptide subunits is called the quaternary structure of the protein. All the

subunits are held together by the non-covalent interaction like hydrogen bond, ionic bond and hydrophobic

bond.

Properties of Proteins

A protein is a biological macro molecule composed of one or more chain of amino acids linked by peptide

bonds. In general, we speak of protein when the string contains more than 50 amino acids. For smaller

sizes, we speak of peptide and polypeptide, but more often they are simply "small protein".

The Dutch chemist Gerhard Mulder (1802-1880) discovered proteins. The word protein comes from the

Greek "protos" which means first, essential. This probably refers to the fact that proteins are essential to

life and they often constitute the majority share (60%) of the dry weight of cells. Another theory that would

make reference protein as the adjective protean, with the Greek God Proteus who could change shape at

will. The proteins indeed adopt many forms and provide multiple functions. But, this was not discovered

until much later, during the twentieth century.

Solubility in Water

1. The relationship of proteins with water is complex. The secondary structure of proteins depends

largely on the interaction of peptide bonds with water through hydrogen bonds.

2. Hydrogen bonds are also formed between protein (alpha and beta structures) and water. The protein-

rich static ball are more soluble than the helical structures.

3. At the tertiary structure, water causes the orientation of the chains and hydrophilic radicals to the

outside of the molecule, while the hydrophobic chains and radicals tend to react with each other

within the molecule (cf. hydrophobic effect).



4. The solubility of proteins in an aqueous solution containing salts depends on two opposing effects

on the one hand related to electrostatic interactions ("salting in") and other hydrophobic interactions

(salting out).

Denaturation

A protein is denatured when its specific three-dimensional conformation is changed by breaking some bonds

without breaking its primary structure. It may be, for example, the disruption of helix areas. The

denaturation may be reversible or irreversible. It causes a total or partial loss of biological activity. This is

an important property of protein.

There are a number of Denaturing agents as follows.

1. Physical agents: Heat, radiation, pH

2. Chemical agents: Urea solution which forms new hydrogen bonds in the protein, organic solvents,

detergents.

CC)) NNuuccll eeii cc AAccii ddss

Biomolecules like carbohydrates, lipids, proteins, vitamins, hormones are basically organic compounds

which are mainly composed of carbon and hydrogen with some other atoms like oxygen, sulphur,

nitrogen, phosphorus etc. Some Biomolecules like adenosine triphosphate (ATP) and adenosine

diphosphate (ADP) act as energy coin and mainly involve in the energy transfer process during metabolic

activities. Carbohydrates are composed of monomer units which are bonded with each other through

glycosidic linkage.

Proteins are also formed by condensation polymerisation of amino acid molecules . The amino acid

molecules are bonded with each other through peptide linkage which is an amide bond and formed by the

condensation reaction between amino group and the carboxyl group of similar or different types of amino

acids.

Nucleic acid found in both nucleus and cytoplasm.

The term nuclic acids was first introduced by Altman in 1889.

Nucleic acids are little different from other Biomolecules in their structure and functions. They are mainly

formed by three structural units; phosphate group, sugar molecule and nitrogenous bases. These units are

bonded with each other in a certain manner to give the specific geometry to the molecule .



Functional

group

Chemical

formula

Chemical

property Structure Biomolecules

Hydroxyl -OH Polar

Carbohydrate, proteins, Nucleic

acid

Sulfhdryl -SH Polar

Vitamins, hormones

Phosphate -HPO4- Polar

Nucleic acid

Carboxyl -COOH Acidic

Proteins

Amino -NH2 Basic

Proteins

Nucleic Acids Definition

1. Every generation shows some resemblance to their ancestors which is because of nucleus of living

cell.

2. Nucleus has some bio molecules which can transmit the characteristics from one generation to

next one, called as heredity.

3. There are some particles which are responsible for heredity is called as chromosomes.

4. Chromosomes are made up of proteins and some bio molecules known as nucleic acids.

5. Nucleic acids are bio polymers made up of due to polymerization of monomer unit called as

nucleotides.

Nucleotide:

There are mainly three components in each nucleotide, a sugar molecule, a heterocyclic nitrogenous

base and a phosphate group linked together by covalent bond.

Nuclieotide also take part in energy transfer system of cells. And form about 2% of the cell content.

Nucleotide = Nucleoside + Phosphate

Nucleoside = Nitrogen base + Sugar

Hence NNuucclleeoottiiddee == NNiittrrooggeenn bbaassee ++ SSuuggaarr ++ PPhhoosspphhaattee

And no. of nucleotide combines to form nucleic acid



Mainly two types of nucleic acids are present in nucleus which involve in heredity process; DNA

(deoxyribose nucleic acid) and RNA (ribose nucleic acid).

Both nucleic acids have their special features. DNA is a hereditary molecule which used to store and

transmits genetic information, thus it is also termed as “code of life”. During the cell division in a living cell

each DNA forms exact copy of that which carry information from parent cell to daughter cell.

RNA can be three types; mRNA, tRNA and rRNA. mRNA involves in protein synthesis in the cell

nucleus with the use of specific part of DNA by using transcription process. tRNA involves in transfer

of amino acids to the exact place in the cytoplasm where the proteins are synthesized.

Building Blocks of Nucleic Acids

Nucleic acids are bio polymers composed of monomer units called as nucleotides, thus they are the building

blocks of all nucleic acids. Each nucleotide has three components which are bonded together in a certain

manner to form complete unit. These components are as follow.

Nucleiotide : is a component of nitrogen base pentose sugar and phosphate

1. Nitrogen-containing "base"

There are two type of nitrogenous base present in nucleotides, pyrimidine (one ring) or purine (two rings )

which are quite differ from each other in there structure.

(a) Purine

Are 9 membered double ring nitrogen bases which possess nitrogen at 1, 3, 7,9, position.

There are two purine base commonly found in nucleic acid, Adenine (A) and guanine (G).

(b) Pyrimidine

are 6 membered single ring nitrogen bases that contains nitrogen at 1 & 3 position. There are three

pyrimidine base, Thymine (T) and cytosine(C) present in DNA and Uracil (U).



2. Five carbon sugar

Two pentose sugars are present in nucleic acids, ribose or deoxyribose sugar. DNA contains deoxyribose

sugar while RNA contains ribose sugar. Both sugars are differing only in the presence of one oxygen atom

at C2 position.

· In any nucleotide, the combination of these two components, a base and sugar is known as a

nucleoside.

· The bonding between nucleosides and phosphoric acid molecules results the formation of

nucleotides.

· In nucleosides, the 1-position of pyrimidine and 9-position of purine bonded with C1 of the sugar

molecule through a Î²-linkage also known as N-glycosidic linkage.

On the basis of five different bases, there are five nucleosides are possible in DNA and RNA.

Abbreviation Base Nucleoside Nucleic Acid Structure of nucleoside

A Adenine Deoxyadenosine DNA



Adenosine RNA

G Guanine

Deoxyguanosine DNA

Guanosine RNA

C Cytosine

Deoxycytidine DNA

Cytidine RNA



T Thymine Deoxythymidine (thymidine) DNA

U Uracil Uridine RNA

3. A phosphate group

A phosphate group acts as a linking chain between two sugar molecules to make a complete strand of

nucleic acid. The oxygen part of phosphate group linked with carbon atom of sugar to form nucleotide. A

Nucleotide can also exist in activated forms with two or three phosphates, known as nucleotide diphosphates

or triphosphates.

On the basis of sugar, they also termed as deoxynucleotide and ribonucleotide.



· Thus nucleotides are nucleoside monophosphate and can be written as Base-Sugar-Phosphate.

· On the basis of sugar, nucleotide can be abbreviated by using three capital letters with prefix d- in

case of deoxy series.

For example, nucleotides and nucleosides of DNA are as follow.

Base Nucleoside 5'-Nucleotide Abbreviation

Adenine 2'-Deoxyadenosine 2â€™-Deoxyadenosine-5'-monophosphate dAMP

Cytosine 2'-Deoxycytidine 2'-Deoxycytidine-5'-monophosphate dCMP

Guanine 2'-Deoxyguanosine 2'-Deoxyguanosine-5'-monophosphate dGMP

Thymine 2'-Deoxythymidine 2'-Deoxythymidine-5'-monophosphate dTMP

During the formation of polynucleotide like DNA or RNA, monomer units (nucleotides) are bonded

with phospho diester bond in which a bond formed between the 3' -OH group and the 5' phosphate group

through condensation reaction.

Structure of Nucleic Acids

The structure of nucleic acid is two types.

1. Primary structure



2. Secondary structure

1. Primary structure

Primary structure represents the backbone of polynucleotide which forms with bonding between three

components of nucleotides. In the polymerization of nucleotides, the C5 carbon atom of sugar get

condensed with hydroxyl group (-OH) of phosphate group which present at C3â€™ of the other nucleotide.

Hence the backbone of the nucleic acids made up of sugar and phosphate residue arranged in an alternate

manner. Further each sugar bonded with nitrogenous base to form complete primary structure of nucleic

acid.

2. Secondary structure

· Secondary structure of nucleic acid is based on Chargaff rule, given by E. Chargaff ; which states that in

all cases the amount of adenine is equal to thymine (A=T) and cytosine is equal to guanine(C=G).

· Hence the base composition of DNA varied from one species to another but the total amount of purines is

equal to the total amount of pyrimidine.

· Secondary structure shows the complete helix form of nucleic acid.

· DNA exist as double helical form while RNA as single strand. DNA consist of two strands runs in

opposite direction (the free phosphate residues at 3’ or 5’ positions of the two strands lie on the opposite

sides of the Î±-helix) giving a double helix structure.

· The nitrogenous base gets pair with each other through hydrogen bonding.

· A purine base of one strand paired with a pyrimidine base of another strand.

· Due to size and geometry of bases, guanine and cytosine get paired through three hydrogen bonds.

· Similarly adenine and thymine are bonded through two hydrogen bonds.

· The two helix of DNA are complementary to each other.



· These two strands of DNA are not identical and the base sequence in one strand automatically fixes due

to the base pairing.

· The distance between base pairs is about 0.34 nm and the distance between two successive turns of the

helix is 3.4 nm with the diameter 2.0 nm.

· Because of the winding of strands of DNA around each other, some gaps created between each set of

phosphate backbones which called as groove.

· There are two types of grooves; major and minor.

· A wide and deep groove is called as major groove while a shallow and narrow is termed as minor groove .



Examples of Nucleic Acids

Nucleic acids are mainly two types.

1. Deoxyribose nucleic acid (DNA) and

2. Ribose nucleic acid (RNA).

Both nucleic acids are polynucleotide and differ from each other in the presence of different nitrogenous

base and sugar. Since each nucleotide has three components; a five member sugar molecule, a nitrogenous

base and a phosphate group. In both nucleic acids (DNA and RNA), the phosphate group is same.

DNA consists of deoxyribose sugar, however a ribose sugar is present in RNA backbone. Purine bases are

similar in both nucleic acids while pyrimidine base are different. DNA contains thymine (T) and cytosine(C)

but RNA is made up of cytosine(C) and Uracil (U). Unlike DNA, RNA is a single strand nucleic acid

involves in protein synthesis.



Function of Nucleic Acids

There are two important functions of nucleic acids in any living cells; Replication and protein synthesis .

1. Replication

The process of formation of two identical copies of DNA strand by a single DNA strand is known

as replication or mitosis or cell division. These identical DNA strands transfer to daughter cell and

responsible for hereditary.

Replication process completed in certain steps.

· Unwinding of DNA strands.

· Each strand acts as template and involve in synthesis of two new strands.

· New copy of DNA strand passes to daughter cell.

Hence replication is a semi conservative process as only one half of the parental DNA s conserved and one

half gets synthesizes. This process is an enzyme catalyzed process involves many enzymes like,

1. Topoisomerase: Initiates unwinding of the double helix of DNA.

2. Helicase: Assists the unwinding by cleavage of hydrogen bonds between base pairs.

3. Single-strand binding-protein: Stabilizes the separated strands by preventing them to recombine

again.

4. DNA Polymerase: Check for errors and make corrections during replication process.

5. Ligase: United with small unattached DNA segments on a strand.



2. Synthesis of proteins

RNA involves in protein synthesis and the message for the synthesis of proteins is coded in DNA. There are

three types of RNAs involves in protein synthesis.

· Transfer RNA (tRNA): It is a smallest but stable and long lived molecule contains around 65-110

nucleotides. It used to carry activated amino acids to the site of protein synthesis.

· Ribosomal RNA: it involves in the formation of the ribosome which is a main site for protein synthesis.

· Messenger RNA (mRNA): It is a short- lived RNA which acts as a carrier of genetic information on the

primary structure of proteins from DNA with special features which allow it to attach to riboso mes and

function in protein synthesis.

dd)) EEnnzzyymmeess

Enzymes are macromolecules, usually proteins, produced in living systems, which act as catalysts in

physiological reactions. The striking characteristics of enzymes are their catalytic power and specificity.

Enzymes have immense catalytic power. They accelerate reactions by factors of at least a million. Most

reactions in living systems do not occur at perceptible rates in the absence of enzymes. A simple reaction

like the hydration of CO2 is catalyzed by the enzyme carbonic anhydrase and is called as enzyme catalyzed

reaction.The transfer of CO2 from tissues into the blood and then to the alveolar air would be very slow in

the absence of this enzyme. The enzyme can hydrate 105 molecules of CO2 per second, which is 107 times

faster than the uncatalyzed one.

This example illustrates the fact that enzymes have immense catalytic power. Enzymes increase at the rate

of 106 to 1020 times. The catalytic power of proteins comes from their capacity to bind substrates (reactant)

molecules in precise orientations and to stabilize transition states in the making and breaking of bonds.

Enzymes are highly specific. An enzyme usually catalyses a single chemical reaction or a set of closely

related reactions.

For e.g., urease catalyses the hydrolysis of urea only.

Other examples of enzyme catalyzed reactions are as follows:



The remarkable specificity of enzymes comes from the ability of the enzymes to bring the substrates into a

favorable orientation in the enzyme-substrate (ES) complexes such that the substrates are bound to specific

regions of the enzymes.

Each enzyme has a specific active site

The active site of an enzyme binds the substrate and contains the residues that directly participate in the

making and breaking of bonds. Some features of the active site are :

1. The active site takes up a relatively small part of the total volume of an enzyme.

2. The active site is a three dimensional entity formed by groups that come from different parts of the

linear amino acid sequence.

3. Enzymes bind the substrate by multiple weak interactions. These weak interactions are electrostatic

bonds, hydrogen bonds, van der Waals' forces and hydrophobic interactions. van der Waals' forces

become significant in binding only when many substrate species can come simultaneously close to

many enzymes species. Hence, the enzyme and substrate should have complementary shapes. The

directional nature of hydrogen bonds between substrate and enzyme brings about a high degree of

specificity.

4. Active sites are clefts and crevices. Substrate molecules bind to the clefts or crevices.

5. The specificity of binding of a substrate depends on the precisely defined arrangement of atoms in

the active site. To fit into the active site the substrate must have a matching shape, just like how a

key fits into a lock.

Given above is the Lock and Key model of interaction between a substrate and enzyme. The active site of

the enzyme alone is complementary in shape to that of the enzyme This model was proposed in the late 19th century. Since then, modern X-ray crystallographic and spectroscopic methods have revealed that in many cases, enzymes are markedly modified by the binding of the substrate. The active sites of these enzymes have shapes that are complementary to that of the substrate only after the substrate is bound. Such enzyme-substrate interaction are described by induced fit model.



Given above is the Induced-fit model of interaction between substrates and enzymes. The enzyme changes shape upon binding to the substrate. The active site has a shape complementary to that of the substrate only after the substrate is bound In an enzyme catalyzed reaction, the rate of enzyme catalyzed reaction varies with the concentration of the substrate, as shown in the figure.

Given above is the representation of the rate of enzyme catalyzed reaction graph At a given concentration of the enzyme, the rate of reaction is almost linearly proportional to [S] when [S] is low. At high [S], r is nearly independent of [S]. To account for such behavior, Leonor Michaelis and Maud Menten proposed the following steps in enzyme catalyzed reactions.

The formation of ES complex is fast and reversible, while the formation of the product (step 2) is the slow, rate determining step. The rate of enzyme catalyzed reaction changes from first order to zero order as the

concentration of the substrate is increased. A chemical reaction cannot be of practical use unless it proceeds at suitable speed. If a reaction is too slow, raising the temperature can speed it up. It has been found that the rate of a chemical reaction may be considerably influenced by the presence of a small amount of a specific substance. Such a



substance is called a catalyst and the process is called catalysis. Berzelius first introduced the term catalysis in 1836.A catalyst may be defined as follows. A catalyst is a substance which alters the rate of a chemical reaction without being used up in the reaction and can be recovered chemically unchanged at the end of the reaction.

Characteristics of Enzymes

· Enzymes are proteins involved in all the biochemical conversions in the plant and animal cells. Enzymes act as biochemical catalysts and accelerate the chemical conversion but remain unmodified themselves. The enzymes work by decreasing the activation energy required for the reaction.

· The reactant in a biochemical conversion is called a substrate. The site in the enzyme molecule where the substrate binds and the reaction takes place is called the active site. It is usually a small

pocket like cleft found deep inside the enzyme molecule. · Enzymes are selective in terms of the substrate and act upon closely related molecules. On one hand,

certain enzymes act on only one substrate thus displaying absolute selectivity while others recognize only the bond that they act on and are not affected by the type of the molecule containing the bond.

· Being protein in nature, enzymes require specific conditions o f pH and temperature for optimal activity. At high temperatures and non-optimal pH the enzymes lose their tertiary structure (are

denatured) which is critical for its activity. ·

Co-factors, Co-enzymes and Prosthetic Group

· Enzymes often require a metal ion or a chemical compound, usually organic in nature, that binds to it and assists in the catalytic process.

· These helper molecules can bind to the enzyme tightly and are referred to as prosthetic groups while

loosely bound ones are classified as co-enzymes. The term co-factor is used at times to describe the inorganic substances.

· Organic co-factors are often derivatives of vitamins. Some examples of co-enzymes and their parent vitamins are thiamine pyrophosphate (thiamine or Vitamin B1), flavin adenine dinucleotide (riboflavin), nicotinamide adenine dinucleotide (niacin, Vitamin B3), and lipoamide (lipoic acid).

· Non-vitamin co-enzymes are ATP, S-adenosyl methionine, heme, glutathione, and co-enzyme Q. · Inorganic co-factors are metals like iron, copper, magnesium, manganese, cobalt, nickel, selenium,

and molybdenum as well as iron-sulphur clusters.

Types of Enzyme Catalysis

Though a huge number of reactions occur in living organisms, these reactions can be classified into half a dozen types. These reactions are as follows:

· Oxidation and reduction reactions - Enzymes that catalyze these reactions are

called oxidoreductases. Example: The enzyme alcohol dehydrogenase converts primary alcohols to aldehydes. H3CCH2OH + NAD → H3CCHO + NADH + H+ In the above reaction, ethanol is converted to acetaldehyde, and the, NAD is converted to NADH. Here NAD is a co factor. Or we can also say that, ethanol is oxidized, and NAD is reduced. As we



already know, in redox reactions, one of the substrates is oxid ized and one is reduced, so here ethanol is oxidized and NAD is reduced.

· Group Transfer Reactions - The enzymes that catalyze this type of reactions are called as, transferees. They move functional groups from one molecule to another. For an example, alanine amino transferase shifts the alpha-amino group in between alanine and aspartame:

· Some transferases move the phosphate groups between ATP and other compounds, sugar residues to

form disaccharides.

· Hydrolysis - The enzymes catalyzing these reactions are termed hydro lases. They break single bonds by adding the elements of water. For example, phosphatase breaks the bond between oxygen and phosphorus of phosphate esters:

Some other hydrolases function as digestive enzymes, for example, they break the peptide bonds in proteins.

· Formation or removal of a double bond along with group transfer - The enzymes which catalyze these reactions, are known as Lyase. The common functional groups transferred by this lyase include amino groups, water, and ammonia. For example, decarboxylases remove CO2 from alpha- or beta-keto acids. Dehydratases remove water, as in fumarase.

Deaminases remove ammonia, as in the removal of amino groups from amino acids



· Isomerization of Functional Groups - In most of the biochemical reactions, the position of the

functional group is changed within a molecule, where as the molecule contains the same number and same atoms that it did in the beginning. In other words we can also say that the substrate and product of the reaction areisomers. The enzymes catalyzing these rearrangement reactions are known asisomerases. For example, triose phosphate isomerase.

· Single bond formation by eliminating the elements of water - The enzymehydrolases break the bonds

by adding the elements of water and ligates carry out the opposite reaction. It removes the elements of water from two functional groups to form a single bond.

Enzyme Catalysis Reaction

· The catalysts which catalyze the biochemical reactions are known as Enzymes.· Catalysis of a reaction in biological systems by enzymes is an important example of homogeneous

catalysis.

· Enzymes are complex nitrogenous substances having molecular weights of the order of 10,000 or even more.

· Enzymes are derived from living organisms.

· Enzymes catalyze most of the biochemical processes such as digestion and biosynthesis etc. Therefore, enzymes are termed as bio-chemical catalysts and the phenomenon is known as biochemical catalysis.

· Many enzymes have been isolated in pure crystalline state from living cells. The first enzyme was synthesized in the laboratory in 1969.

Some important examples of enzyme-catalyzed reactions are given below in the table:



S.No Enzymes Source Enzymatic reaction 1 Invertase Yeast Sucrose → Glucose + Fructose

2 Zymase Yeast Glucose → Ethyl alcohol + CO2 3 Maltese Yeast Maltose → Glucose 4 Diastage Malt Starch → Maltose

5 Amylase Saliva Starch → glucose 6 Ptylin Saliva Starch → sugar

7 Lipase Castorseed Sugar → Glycerol 8 Urease Soyebean Urea → Ammonia + CO2 9 Pepsin Stomach Protein → Amino acids

10 Trypsin Intestine Protein → Amino acids 11 Lactic bacilli Curd Fermentation of milk

Characteristics of Enzyme Catalyzed Reactions:

· Enzymes are highly specific and each enzyme catalyzes a particular reaction. For example: An enzyme called unease catalyzes the hydrolysis of urea and no other reactions. NH2CONH2 + H2O → 2NH3 + CO2 Enzyme zymase converts glucose into ethyl alcohol.

C6H12O62C2HOH → + 2CO2

· For all enzyme reactions there exists an optimum temperature at which its efficiency is maximum.

Above this temperature, an enzyme loses its catalytic activity and below this temperature the reaction rates are slow because of the temperature effects. The optimum temperature range for enzymatic activity is 298-310K. Therefore, human body temperature (310K) is suited for enzymatic

activity. · Enzymes become inactive in the presence of electrolytes and on exposure to ultraviolet radiations. · Enzymes are highly efficient. Extremely small quantities of an enzyme catalyst, as small as millionth

of a mole, can increase the speed of the reaction up to million times as compared to un analyzed reactions. It is because the enzyme molecules are generated during their catalytic act ivity. This regeneration may be million times a minute. An example is renin enzyme, which is used in making

cheese. It coagulates over a million times its own weight of milk protein. · Enzyme reactions are highly sensitive to catalytic poisoning. Some typica l poisons are HCN, H2S,

CS2, etc. The inhibitors or catalytic poisons often reduce or completely destroy the catalytic activity of the enzymes by interacting with active functional groups on the enzyme surface.

· The activity of an enzyme is maximum at a particular pH. This has been explained by assuming that there are three forms of enzyme in equilibrium,

E + S ES → E + P

of which only EH can combine with the substrate to yield an intermediate, EHS, that can react to form products. The other intermediates, EH2S and ES do not form products. Since the concentration of ES is maximum at a particular pH, the activity of the enzyme is also maximum at a particular pH, which

is between pH value 5-7.



· There are certain enzymes which require non-proteinous substances for their activity. These can be metal ions (Na, K, Cu, Fe, Mn, Zn, Mg) or small organic molecules called co enzymes. A large number of co enzymes are derived from vitamins. Amylase in the presence of Na+ ions (NaCl) is catalytically very active.

9. Reproduction & cell division (cell cycle).

CCeellll CCyyccllee

All cells arise from pre-existing cells by division. Every living cell today is said to be descended from a

single ancestral cells that lived 3-4 billion years ago. In the vast time period, evolution of cells and

organisms was seen, thus continuing the success of life on Earth. The genetic information has been

preserved through cell division.



Reproduction of cells is fundamental to the development and function of all life. In single-celled organisms,

cell division creates are a entire new organism. In multicellular organisms cell division transform a single

founder cell into different communities of cells that constitute the tissues and organs. In adult organisms cell

division replaces cells those die from natural causes or those that are lost to environmental changes.

What is Cell Cycle

The cell cycle is the sequence of events that takes place in cells. It leads to cell division and replication

(duplication). In prokaryotes, the cell cycle is through the process of binary fission. In eukaryotes, the cell

cycle can be divided into two phases - interphase and mitotic phase. Interphase is the stage during which the

cell prepares, grows and accumulates nutrients needed for mitosis and also duplicates the DNA. In the

mitotic phase, the cell splits itself into two distinct cells known as 'daughter cells' and the final phase is

cytokinesis, where the newly formed cells are completely divided. The cycle of cell division is a vital

process by which a fertilized egg that is single-celled develops into a mature organism. It is by this process

by which skin, hair, blood cells and some internal organs are renewed.

Phases of Cell Cycle

There are four distinct phases in the cell cycle - G1 phase, S phase (synthesis), G2 phase and M phase. The

G1, S phase and G2 phase together are known as interphase. The M phase or the mitotic phase is of two

processes, one where the chromosomes of the cell is divided between two sister cells and the other is

cytokinesis where the cell's cytoplasm divides into half forming two distinct cells.



The cells that have stopped dividing temporarily or reversibly are said to be in the state of quiescence called

G0 phase. Progression from phase to another depends on the proper completion of the previous one. After

the process of cell division, the daughter cells begin the interphase of the new cycle. The stages of

interphase are not morphologically distinguishable, yet each phase has a distinct biochemical process that

prepares the cell for initiation of cell division.

G0 Phase

Sometimes the cells in the quiescent and senescent cells are referred to as post mitotic. The cells which

are indivisible in multicellular eukaryotes generally enter the quiescent G0 state from G1. They may remain

in the quiescent state for long periods of time. This state can be for indefinite like in neurons and is very

common in cells that are fully differentiated. Death of the cells in response to damage of DNA or

degradation would make the progeny of the cells nonviable. Some cells like the cells of liver and kidney

enter the G0 phase semi-permanently.

Interphase

Earlier to to the cell division process, the cell needs to accumulate nutrients. During the interphase all the

preparations are done. In interphase of a newly formed cell, a series of changes takes place in the cell and

the nucleus, before it is capable of division. This phase is also known as intermitosis. Earlier this stage was

known as resting stage because no remarkable activiyt realated to cell division takes place here. Interphase

proceeds in a series of three stages, G1,S, and G2. Division of cell operates in a cycle, hence the interphase

of the cycle is preceded by the previous cycle of M phase and cytokinesis. Interphase is also called the

preparatory phase. In the interphase stage the division of nucleus and cytosol does not occur. The cell

prepares for division. This is a stage between the end of mitosis and start of the next phase. Many events

occur in this stage and most significant event that occurs is the replication of genetic material.

G1 Phase

This is the first phase in the interphase. From the end of the previous M phase till the beginning of the DNA

synthesis in the next cycle is called the G1 phase, here G indicates gap. This phase is also called growth

phase. In this phase the biosynthetic activities of the cell, which shows a considerable slow down during the

M phase of resumes it activities at a high rate. In this phase there is a marked production of proteins by the

use of 20 amino acids. Also enzymes that are required in S phase needed during DNA replication. The

duration of the G1 phase is highly variable, also among different cells of the same species. The G1 phase is

under the control of the p53 gene.



S phase

The start of the S phase is when the DNA replication commences. When the phase completes all the

chromosomes have been replicated. Each chromosome has two sister chromatids. During this phase, the

amount of DNA in the cell is doubled but the ploidy of the cell remains unchanged. In this phase the

synthesis is completed as soon as possible as the exposed base pairs are sensitive to external factors like

drugs or mutagens.

G2 phase

It is again the gap phase which happens during the gap between the DNA synthesis and mitosis. During this

phase the cell will continue to grow. The G2 checkpoint mechanism controls to ensure that the cell is ready

to enter the M (mitosis) phase and divides.

Mitosis or M phase

The M phase consists of karyokinesis - nuclear division. The M phase is of several distinct phases, known

as Prophase,

Metaphase,

Anaphase,

Telophase,

Cytokinesis.

The process of mitosis takes place only in eukaryotes, the chromosomes in the nucleus of the cell into two

identical nucleus. This stage is followed by cytokinesis. In cytokinesis the cell, nuclei, cytoplasm, organelles

and cell membrane is divided into two equal shares. Mitosis and cytokinesis together make the mitotic (M)

phase of the cell cycle. The mother cell divides into two daughter cells that are genetically identical to each

other. Mitosis is seen only in eukaryotic cells, but it occurs in different ways in different species. The

process of mitosis is a sequence of events divided into three stages - prophase, metaphase, anaphase and

telophase. During this process of mitosis the chromosome pairs condense and they attach to fibres that pull

sister chromatids to opposite sides of the cell. The cell with the process of cytokinesis produces two

identical daughter cells.

Cell Cycle Control System

Regulation of the cell cycle is a crucial process to the survival of the cell. Cell regulation includes the

detection and repair of genetic damage and also prevention of uncontrolled cell division. The molecular

events that control the cell cycle occurs in a sequential fashion and is impossible to reverse the cycle.



The major event in the cell cycle is the replication of DNA, which occurs in the S phase, and separation of

the duplicated chromosomes and the constituents of the cell which occurs in the M phase. Regulation and

the initiation and completion of S and M phases ensures the genetic information and other cellular

components are duplicated and divided equally between the daughter cells with each cycle.

Cell cycle checkpoints are the regulatory pathways that control the order and the timing of the transitions of

the cell cycle. The checkpoints also ensures that critical events such as replication of DNA and segregation

of chromosomes are completed before the cell progresses further through the cycle. The cell-cycle

checkpoints respond to the cellular damage by slowing the cycle to provide time for repair and it also

induces transcription of genes that facilitate the repair. The loss of the checkpoints results in instability of

chromosomes and it can result in the transformation of normal cells into cancer cells.

Proteins like the cyclin dpendent kinase, kinases and cyclins control the switches for the cell cycle causing

the cell to move from G1 to S or G2 to M.Regulatory molecules are of two classes cyclins and cyclin-

dependent kinases. The genes encoding cyclins and CDKs are conserved among all eukaryotes. Proteins like

the p27 and p53 prevent the cells from passing check points. They are also knwon as protein suppressors.

P27 protein binds to cyclin and CDK blocking the entry into the S phase. P53 protein blocks the cell cycle at

the M checkpoint if the DNA is damaged. P53 mutation is the most frequent mutation found in cancer cells.

P53 fucntions by blocking the cycle giving the cell time to repair its DNA. If there is severe damage in the

DNA the protein causes the cell to apoptosis.



MMiittoossiiss

Cell division is a process with sequence of steps that enables organisms to grow and reproduce. Genetic

material is replicated in parent cells and is distributed equally to the two daughter cells. Cells undergo a

period of growth called interpahse before entering mitosis. During the interphase, the genetic material

replicates and the organelles prepare for division. In the process of mitosis, the parent's cell genome is

transferred into the two daughter cells. The daughter cells are similar to each other and to their parent

cell.The cell's genome is composed of chromosomes that are complexes of tightly coiled DNA that contain

the genetic material which is vital for the proper functioning of the cell.

The process of mitosis begins when the chromosomes condense. In most eukaryotic cells, the nuclear

membrane segregates the DNA from the cytoplasm into membrane vesicles. The ribosomes also dissolve,

the chromosomes align themselves. Microtubules pull apart the sister chromatids of each chromosome. The

daughter chromosomes are pulled towards opposite ends. Nuclear membrane forms around the separate

daughter chromosomes. In animal cells, the area of cell membrane pinches inwards, to form the two

daughter cells, the imaginary line is called the cleavage furrow which separates the developing nuclei. In

plant cells, the new dividing cell wall is constructed in between the daughter cells. The parent cell will thus

split in half and give rise to two daughter cells.

Mitosis Definition

The process by which a cell which has previously replicated chromosomes in the nucleus of the cell is

separated into two identical sets of chromosomes is known as mitosis. Mitosis is the division of the mother

cell into two daughter cells, these daughter cells are genetically identical to each other and to the parent cell.

It is a form of nuclear division. Mitosis is generally followed by cytokinesis, this process divides the nuclei,

cytoplasm, cellular organelles and cell membrane into two cells of roughly equal shares of these cellular

constituents. The M phase of the cell cycle is of mitosis and cytokinesis together.



Mitosis is divided into the following stages

· Prophase

· Metaphase

· Anaphase

· Telophase

·

Phases of Mitosis

Mitosis is a fast and highly complex process. The events of mitosis is divided into the following stages

prophase, prometaphase, metaphase, anaphase and telophase.

Below figure shows the stages of mitosis:

Interphase

· Mitosis alternates with interphase. In interphase the cell prepares itself for division.

· The interphase is divided into three phases G1, S and G2.

· During these phases the cells grow by producing proteins and cytoplasmic organelles.

· In the S phase the chromosomes replicate. In G1 phase the cell grows, in S phase the chromosomes

duplicate and in G phase the cell grows more and prepares for mitosis and finally divides in the

Mitotic cycle.

· The cell cycle is regulated by proteins.

· The phases of the interphase follow strict order and have checkpoints.

· There is another phase in the interphase G0 where the cell has the option to enter this stage.



· Interphase takes about 90% of the cell's life span.

Prophase

· In the nucleus the genetic material is loosely bundled in coil called chromatin.

· At the onset of prophase the chromatin fibres become tightly coiled and condense into discrete

chromosomes.

· Inside the nucleus, the nucleolus also disappears from view.

· The centrioles begin to move to opposite ends of the cell and the spindle fibres extend from the

centromere.

· Some fibres cross the cell to form the mitotic spindle fibres.

Prometaphase

· Prometaphase is sometimes considered as the end of prophase and early metaphase.

· During the early stage of prometaphase the nuclear membrane disintegrates and the microtubules

enter the nuclear space.

· This is known as "open mitosis" and it occurs in most multicellular organisms.

· Organisms like fungi, some portists like algar or trichomonads undergo "closed mitosis" where the

spindle formation happens inside the nucleus.

· The nuclear membrane stays intact and the microtubules are not able to penetrate the intact nuclear

membrane.

· During the late prometaphase, at the centromere of each chromosome forms two kinetochores.

· Kinetochore is a complex protein structure, it is the point where the microtubules attach themselves

to the chromosome.

Metaphase

· The term metaphase is derived from Greek word 'meta' which means 'after'.

· In the prometaphase after the microtubules are attached to the prometaphase the chromosomes start

pulling the chromosomes towards the ends of the cell.

· The centromeres of the chromosomes assemble along the metaphase plate also known as the

equatorial plane.

· It is an imaginary line that is in between the centrosome poles and is called the spindle equator.

· This helps to ensure that when the chromosomes are separated the new nucleus will receive one copy

of each chromosome.



Anaphase

· After the metaphase stage the chromosomes proceed to the anaphase stage.

· The term anaphase is derived from the Greek word "ava" which means "up", or "against", or "back",

or "re".

· First the proteins that bind the sister chromatids are cleaved making the sister chromatids as separate

daughter chromosomes and are pulled apart towards the respective centrosomes to which they are

attached.

· The microtubules at the poles pull the set of chromosome that are attached to it the opposite ends of

the cell. At the end of anaphase the microtubules all degrade.

Telophase

· Telophase is derived from the Greek word "telos" meaning "end".

· It is a reversal of prophase and prometaphase events. In the telophase stage the polar microtubules

continue to lengthen elongating the cell.

· The daughter chromosomes attach at opposite site ends of the cell.

· New membranes are formed around the daughter nuclei.

· The chromosomes spread and are no longer visible under the light microscope.

· The spindle fibers also disperse, cytokinesis may also begin during this stage.

Cytokinesis

· Cytokinesis is a separate process that begins at the same time as the telophase.

· Cytokinesis is not a phase of mitosis, it is a separate process necessary for completing cell division.



· In animal cells a pinch like cleavage furrow containing a contractile ring develops at the position of

the metaphase plate separating the nuclei.

· In the animal and plant cells the division of cell is driven by vesicles derived from the Golgi

apparatus.

· In plant cells, the rigid wall requires a cell plate be synthesized between the two daughter cells.

Significance of Mitosis

· Mitosis is an equational division.

· It is the division through which daughter cells that are identical are produced.

· The daughter cells have the same amount and type of genetic constitution as that of the parent cell.

· Mitosis division is responsible for growth and development of a single-celled zygote into a

multicellular organism.

· The chromosome number remains the same in the cells produced by this division.

· The daughter cells have the same characters as those of the parent cell.

· Mitosis division helps in maintaining the proper size.

· Mitosis also helps in restoring wear and tear in body tissues, replacing damaged or lost part, healing

wounds and regeneration of detached parts.

· This method of multiplication is seen in unicellualr organisms.

· Mitotic division of cell is unchecked and it may result in uncontrolled growth of cells leading to

cancer or tumor.

Plant Cell Mitosis

Mitosis in plant cell is different from the animal cells in the following characteristics

· There is a preprophase stage which occurs only in plant cell prior to the prophase stage.

· Centiroles are absent.

· Aster is not formed.

· Cytokinesis in plant cell occurs by the cell plate formation.



· This kind of division is seen in the meristems of the plant body.

Preprophase stage - This occurs only in the plant cell. The prophase stage is preceded by the pre-prophase

stge. In vacuolated plant cells, the nucleus migrates to the centre of the cell before the onset of mitosis.

Preprophase is characterized by the formation of phragmosome, phragmosome is a sheet of cytoplasm that

bisects the cell along the future plane of cell division. There is also formation of a ring of microtubules and

actin filaments known as the preoprophase band under the plasma membrane around the equatorial plane of

the mitotic spindle. This preprophase band is the position where the cell wall eventually divide. The cell of

flowering plants lack centrioles, microtubules form a spindle on the nuclear surface and are then organized

into spindle by the chromosomes.

The cells of plant have cell wall and hence the cytokinesis cannot proceed with a cleavage furrow. During

the telophase stage in plant cell, a cell plate is formed across the cell in the location of the old metaphase

plate.

MMeeiioossiiss

Every living eukaryote organism is or has been a single cell. New cells are made by division of existing

cells, which involves the division of both nucleus and cytoplasm. In eukarytots, two types of division takes

place mitosis and meiosis. Meiosis is unique form of cellular differentiation and it is initiated usually only

once in the life cycle of a eukaryote. Meiosis is a unique and distinctive event in the life of an organism. It is



a event that normally involves an accurate and quantitative reduction in chromosome number and also

precise partitioning of genetic material.

Meiosis fulfills two interrelated functions which are connected with the sexual reproduction process. It

produces a haploid phase in the life cycle of an organism, also known as reduction and also provides the

production of genetically distinct progeny, also referred as recombination. Deviations from this behavior are

usually lethal or sublethal to the organism, as the a proper functional chromosome is an essential

requirement for the basic function of cell during development. As meiosis is a ordered process, genes and

proteins control the process and conserve the event throughout eukaryotes.

What is Meiosis

Meiosis is a unique type of cell division, it is necessary to sexual reproduction in eukaryotic organisms. The

cells that are produced by the process of meiosis are referred to as gametes or spores. Meiosis shuffles the

genes between the chromosomes in a pair, which are received from each parent. It produces chromosomes

with new genetic combinations in every gamete the process generates. Meiosis division produces genetically

unique four cells, the chromosome number is half as that is in the parent cell.

Meiosis Chart



Stages of Meiosis

Meiosis is a one way process, unlike mitosis is a cell cycle. The preparatory phase to meiosis is identical in

pattern and name to the interphase of the mitotic cell cycle.

Interphase

Meiosis interphase is divided into three phases

· G1 phase or Growth 1 phase is a very active period. In this period the cell synthesizes vast range of

proteins which includes the enzymes and structural proteins necessary for growth of the cell. In the

G1 phase the chromosome are made of single molecule of DNA, at this point, in humans, the number

of chromosomes per cell is 46 which is 2N and identical to the somatic cells.

· S phase or the Synthesis phase - There is replication of genetic material in this phase. Chromosomes

duplicate, each of the 46 chromosomes become a complex of two sister, identical chromatids.

· G2 phase or Growth phase is not present in meiosis.

The interphase stage is followed by meiosis I and meiosis II.

Meisois is divided into meiosis I and meiosis II stages. It is further divided into Karyokinesis I

and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively.

Meiosis I

· The pairs of homologous chromosomes, made up of two sister chromatids are split into two cells.

· The resulting daughter cells contains one entire haploid set of chromosomes.

· The first meiotic division reduces the ploidy of original cell by a factor of two.

· It produces two haploid cells (N chromosomes, 23 in humans).



· Hence meiosis I is referred to as a reductional division.

· A diploid human cell contains 46 chromosomes and is said to be 2N because it contains 23 pairs of

homologous chromosomes.

· Meiosis II is an equational division similar to mitosis, where the sister chromatids split and creating

4 haploid cells, two from each daughter cells from meiosis I.

Prophase I

· Prophase I is the longest phase of meiosis I.

· During this phase, there is exhange of DNA between homologous chromosomes, this process is

known as homologous recombination. This process often results in chromosomal crossover.

· The DNA created are of new combinations, and during crossover they are a s ignificant source of

genetic variation. This may result in beneficial new combinations of alleles.

· The chromosomes that are paired and replicated are called bivalents or tetrads.

· They have two chromosomes and four chromatids, each chromosome comes from each parent.

· Pairing of homologous chromosomes is called synapsis. At the stage of synapsis formation, the non-

sister chromatids may cross-over at points called chiasmata.

Leptotene

· Leptotene is the first stage of prophase I and is also known as leptonema, which is derived from a

Greek word which means "thin threads".

· In this stage, individual chromosomes consists of two sister chromatids.

· The chromosomes condense into visible strands within the nucleus.

· The two sister chromatids are tightly bound, that they are not distinguishable from one other.

· During this phase the lateral elements of the synaptonemal complex assemble.

· This stage is of very short duration and progressive condensation and coiling of chromosome takes

place.

Zygotene

· Zygotene is also known as zygonema, it is derived from Greek word which means 'paired threads'.

· The chromosomes in this line up with each other into homologous chromosome pairs.

· This stage is known as bouquet stage, due to the way the telomeres cluster at on end of the nucleus.

· Synapsis of homologous chromosomes takes place in this stage, it is facilitated by the assembly of

central element of the synaptonemal complex.

· Pairing of chromosomes happens in a zipper like fashion and starts at the centromere (procent ric) or

at the chromosome ends (proterminal) or at any other portion (intermediate).



· Two chromosomes in a pair are equal in length and in position of the centromere, making the pairing

highly specific and exact.

· These paired chromosomes are called bivalent or tetrad chrmosomes.

Pachytene

· The pachytene stage is also known as pachynema and is derived from Greek which means "thick

threads".This is the stage where chromosomal crossing over occurs.

· Nonsister chromatids of homologous chromosomes exchange segments over homologous regions.

· Sex chromosomes are not identical and they exchange information over a small region of homology.

· Chiasmata is formed where exchange happens.

· There is exchange of information between the non-sister chromatids and this results in a

recombination of information.

· Every chromosome has a complete set of information and there are no gaps formed as the result of

the process.

Diplotene

· The diplotene stage is also known as diplonema, which is derived from Greek word meaning "two

threads".

· During this stage there is degradation of synaptonemal complex and the homologous chromosomes

separate a little from one another.

· The chromosomes in this stage uncoil a little, this allows transcription of DNA.

· The bivalent homologous chromosomes remain tightly bound at the region of the chiasmata,where

crossing over occurred.

· The chiasmata regions remains on the chromosomes until they are separated in the anaphase.

In the oogenesis of humans the developing oocytes in the fetal stage stop at this stage of diplotene before

birth. This state is referred to as the dictyotene stage and it remains in this suspended stage until puberty.

Diakinesis

· During the stage of diakinesis the chromosomes condense further.

· The word diakinesis is derived form Greek word which means "moving through".

· This stage is the first part of meiosis where the four arms of the tetrads are visible.

· The sites where crossing over has occurred entangle together, overlapping effectively and making

the chiasmata visible clearly.

· This stage resembles the prometaphase of mitosis, where the nucleoli disappear and the nuclear

membrane disintergrates into vesicle and also there is formation of the meiotic spindle.



Metaphase I

· The homologous pairs of chromatids move together along the metapahse plate.

· The kinetochore microtubules from the centrioles attach to their kinetochores respectively.

· The homologous chromosomes align along the equatorial plane, this alignment happens due to the

continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the

kinetochores of the homologous chromosomes.

Anaphase I

· During this phase the kinetochore microtubules shorten, this severs the recombination nodules and

pulls the homologous chromosomes apart.

· As each chromosome has only one functional unit of a pair of kinetochores, the whole chromosomes

are pulled towards the opposite poles which results in the formation of two haploid sets.

· Each chromosomes contains a pair of sister chromatids.

· Disjunction occurs during this time, this is one of the process that leads to genetic diversity as the

chromosomes end up in either of the daughter cells.

· The nonkinetochore microtubules lengthen and pushes the centrioles farther apart. The cell is

elongated and it prepares for division at the center.

Telophase I

· The first phase of meiotic division ends when the chromosomes arrive at the poles.

· The daughter cells now have half the number of chromosomes, the chromosomes consists of a pair

of chromatids.



· The microtubules of the spindle network disappear and nuclear membrane surrounds each haploid

set.

· The chromosomes uncoil and return back to the chromatin stage.

· The process of cytokinesis occurs where, the cell membrane in the animals cells is pinched off, and

in plant cells there is formation of the cell wall in between the daughter cells.

· This completes the creation of two daughter cells. The sister chromatids remain attached during the

telophase I stage.

Meiosis II

It is the second part of the meiotic process and is also known as equational division. Meiosis II is similar to

mitosis. The genetical results are fundamentally different from that of mitosis. The end result of mitosis II is

the production of four haploid cells from two haploid cells produced in meiosis I, each cell consisting of 23

chromosomes in humans and the chromosomes consists of two sister chromatids.

In meiosis II there are four steps

· Prophase II

· Metaphase II

· Anaphase II

· Telophase II.

Prophase II

· During this stage there is disappearance of the nucleoli and the nuclear envelope, also there is

shortening and thickening of the chromatids.

· The centrioles move to the polar region and the spindle fibers are arranged for the second meiotic

division.



Metaphase II

· During this stage the centromeres that contain two kinetochores attach to the spindle fibers at each

pole from the centrioles.

· The equatorial plate formed here is rotated by 90 degrees, compared to meiosis I and is

perpendicular to the previous metaphase plate.

Anaphase II

· The metaphse II is followed by the anaphase II, in the anaphase II stage the centromeres are cleaved,

this allows the microtubules attached to the kinetochores to pull the sister chromatids apart.

· The sister chromosomes move towards the opposing poles.

Telophase II

· The meiosis II process ends at this stage, this stage is similar to the telophase I.

· In this phase there is uncoiling and lengthening of the chromosomes and disappearance of the

spindle. There is also reformation of nuclear envelope.

· Cleavage or cell wall forms which eventually produces a total of four daughter cells, each cell

having its own haploid set of chromosomes.

Mitosis vs Meiosis

Meiosis Mitosis

End result Normally there are four cells, each cell has half the number of chromosomes as the parent cell.

Two cells, each cells has the same number of chromosomes as that of the parent.

Function Sexual reproduction , production of gametes (sex cells).

Cellular reproduction, growth, repair wear and tear of cells, sexual reproduction.

Where it occurs? Animals, fungi, plants, and some protists. Occurs in all eukaryotic organisms.

Stages

Steps in the process Prophase I, Metaphse I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II.

Prophase, Metaphase, Anaphase, Telophase.

Genetical composition Not similar to parents Usually similar to parents.

Crossing over process Occurs in Prophase I Sometimes

Pairing of homologous chromosomes

Yes No

Cytokinesis Occurs in Telophase I and Telophase II Occurs in Telophase

Splitting of chromosomes

Does not occur in Anaphase I; occurs in Anaphase II

Occurs in Anaphase



Asexual Reproduction

Reproduction is a characteristic of a living organism. In multicellular organisms, reproduction is the

production of progeny which possess features which are similar to those of parents. This is known as sexual

reproduction and involves the fusion of male and female gametes to form a progeny.

Asexual reproduction is also a means of reproduction in some organisms. Fungi spread easily by the

production of asexual spores. In organisms like yeast and hydra budding is the way of reproduction. In flat

worms like Planarians reproduction is by fragmentation. Unicellular organisms like bacteria and unicellular

algae reproduce by increasing the number of cells by cell division.

Most of the prokaryotes reproduce asexually without the formation of gametes, while processes like

conjugation, transformation and transduction are regarded as sexual reproduction. In multicellular

organisms, asexual reproduction is very rare. Asexual reproduction may have short term benefits with rapid

growth in population a stable environment while sexual reproduction allows more rapid genetic diversity in

generations and also allows adaptation to changing environments.

Asexual Reproduction Definition

Asexual reproduction is a form of reproduction by which a single parent produces a progeny and the genes

inherited are from that parent only. The offspring here are usually produced by mitosis. In this type of

reproduction, the process of meiosis, reduction of chromosome number and fertilization does not takes



place. The offspring of asexual reproduction will have exact copies of the genes of the parent. Asexual

reproduction is also referred to as agamogenesis. Agamogenesis is the type of reproduction which does not

involve the fusion of gametes. This type of reproduction is the primary form of reproduction for single-

celled organisms like archarea, bacteria and protists. Plant and fungi also show asexual reproduction.

Characteristics of Asexual Reproduction

· Asexual reproduction involves only one parent.

· All the offspring are identical to the parent.

· Asexual reproduction does not involve the process of meiosis and fertilization, hence the

process does not require a mate for reproduction.

· This type of reproduction is seen in lower forms of organisms.

· Asexual reproduction does not require time and energy to be spent on seeking a receptive mate.

· It is efficient method as large number of offspring are produced quickly, this enable animals to take

advantage of favorable environmental conditions.

· Asexual reproduction is sometimes known as cloning.

· There are many types of asexual reproduction like vegetative propagation, fission, budding and

fragmentation.

· All the forms of sexual reproduction are the variations of process of mitosis.

Types of Asexual Reproduction

The types of asexual reproduction are of several types like fission, budding, fragmentation and vegetative

propagation.

Fission

· Fission is a type of asexual reproduction where there is splitting of an individual to form two

individuals this is known as binary fission or more may also produce more than two individuals

known as multiple fission, all the progeny are approximately of the same size.

· This is a common form of reproduction in single-celled organisms and bacteria.

· Eukaryotic organisms like some protists and unicellular fungi also divide by fission. These

organisms divide through binary fission and most of them are capable of sexual reproduction.

· Multiple fission occurs in protists like the sporozoans and algae.

· The parent organism nucleus divides many times by the process of mitosis producing several

nuclei. The cytoplasm is then separated creating multiple daughter cells.



Budding

· In this mode of asexual reproduction, offspring buds off from the body of the parent cell.

· The genetic material of the parent cell is divided equally but there is unequal division of

the cytoplasm.

· Hydra reproduces by budding.

· The buds formed on the parent body of the hydra matures and eventually breaks away from the

parent organism.

Vegetative Propagation

· It is a type of asexual reproduction and is seen in plants where progeny are formed without the

production of seeds or spores by the process of meiosis or syngamy.

· In this type of reproduction plants grow from vegetative parts of the plant like the roots, stems and

leaves.



Spore formation

· During the biological life of many multicellular organisms spores are formed and this process is

known as sporogenesis.

· These spores formed during sporic meiosis in some plant and algae grow into multicellular

individuals without the event of fertilization.

· These haploid spores give rise to gametes through the process of mitosis.

· This process of spore formation and gamete formation occurs in separate generations of life cycle

and is known as alteration of generation.

Fragmentation

· Fragmentation is a type of asexual reproduction where a new organism grows from a fragment of the

parent organism.

· The body of the parent breaks into distinct pieces and these fragments grow into a full and mature

individual.

· This type of asexual reproduction is seen in many organisms like annelid worms, sea stars, and some

fungi and plants.



Gemmules (Internal Buds)

· This form of asexual reproduction is seen in sponges where a parent organism releases a specialized

mass of cells that develops into offspring.

Regeneration

· In the regeneration type of asexual reproduction, if a piece of parent organism breaks into distinct

pieces, each piece develops and grows into a completely new individual. Example: Planarians.



Agamogenesis

· Agamogenesis is a type of reproduction in which the reproduction does not involve a male gamete.

Types of agamogenesis are parthenogenesis and apomixis.

· Parthenogenesis is a form of reproduction in which a egg that is unfertilized develops into a new

individual.

· This type of agamogenesis occurs naturally in many plants and invertebrates like water feas, aphids,

stick insects, etc. and in vertebrate like some reptiles, amphibians and rarely in birds.

· Apomixis is the formation of new sporophyte without the process of fertilization. It is usually seen

in ferns and flowering plants and is a rare process in seed plants.

·

Advantages of Asexual Reproduction

· This type of reproduction enable organisms to reproduce without a mate.

· It does not require the time and energy that takes to search a mate.

· It results in the reproduction of large number of offspring rapidly.

· Like in plants, it enables to spread and colonize an area in short period of time.

· Animals that are confined to one particular place and unable to look for a mate reproduce asexually.

· Stable environments with very little change are favorable for organisms to reproduce asexually.

Disadvantages of Sexual Reproduction

· In this type of reproduction the offspring produced are genetically identical to each other and to the

parent.

· This causes no or very little genetic variation within a population.

· Any mutation in the parent cell, can cause harmful effects on the survival ability of the offspring.

· If there is harmful mutation in the organisms, environment changes could be deadly to all the

individuals.

Asexual Reproduction in Animals

Asexual reproduction is less common in animals. It is often seen in simpler animals like hydra.

Animals reproduce asexually by many different methods. The methods are budding,

parthenogenesis, gemmule formation and polyembryony.

· Budding is the process where the parent cell buds off forming daughter organisms.

· In parthenogenesis eggs are produced by the females develop into adult organisms without

undergoing the process of fertilization.



· Gemmules are internal buds, gemmules are cell masses that are released from their body and these

cells develop into independent offspring.

· Polyembryony is a condition in which a single egg leads to the development of two or more

embryos.

Asexual Reproduction in Plants

Plants show two main types of asexual reproduction, in which new plants are produced that are

genetically identical to the parent individual. Plants reproduce asexually by vegetative reproduction

and apomixis.

· In vegetative reproduction a vegetative part of the original plant like the roots, stems and leaves and

the new plants grows from these parts of the plant.

· Apomixis in plants are similar to parthenogenesis in animals. The seeds produced by apomixis are

involved in the formation and dispersal of sedds that are not originated from the fertilization of the

embryos.

Unit 3 by kailash sir cell structure & function kp

Documents

Transcript of Unit 3 by kailash sir cell structure & function kp