Lecture 1 - Cells Structure and Transport Mechanisms (1)

7/25/2019 Lecture 1 - Cells Structure and Transport Mechanisms (1)

1/14

CELLS

All living things are made of cells, and cells are the smallest units that can be alive. Life on Earth isclassified into five kingdoms, and they each have their own characteristic kind of cell. However thebiggest division is between the cells of the prokaryote kingdom (the bacteria) and those of the other fourkingdoms (animals, plants, fungi and protoctista), which are all eukaryotic cells. rokaryotic cells aresmaller and simpler than eukaryotic cells, and do not have a nucleus.

rokaryote ! without a nucleus

Eukaryote ! with a nucleus

Eukaryotic Cells


2/14

Cytoplasm (or Cytosol)."his is the solution within the cell membrane. #t contains en$ymes for

metabolic reactions together with sugars, salts, amino acids, nucleotides and everything elseneeded for the cell to function.

Nucleus. "his is the largest organelle. %urrounded by a nuclear envelope, which is a double

membrane with nuclear pores & large holes containing proteins that control the e'it of substancessuch as A from the nucleus. "he interior is called the nucleoplasm, which is full of chromatin& a*A+protein comple' containing the genes. *uring cell division the chromatin becomescondensed into discrete observable chromosomes. "he nucleolus is a dark region of chromatin,involved in making ribosomes.

Mitochondrion (pl. Mitochondria). "his is a sausage&shaped organelle (-m long), and is

where aerobic respiration takes place in all eukaryotic cells. itochondria are surrounded by a

double membrane/ the outer membrane is simple, while the inner membrane is highly foldedinto cristae, which give it a large surface area. "he space enclosed by the inner membrane iscalled the matri', and contains small circular strands of *A. "he inner membrane is studdedwith stalked particles, which are the site of A" synthesis.

Chloroplast. 0igger and fatter than mitochondria, chloroplasts are where photosynthesis takes

place, so are only found in photosynthetic organisms (plants and algae). Like mitochondria theyare enclosed by a double membrane, but chloroplasts also have a third membrane calledthe thylakoid membrane. "he thylakoid membrane is folded into thylakoid disks, which are thenstacked into piles called grana. "he space between the inner membrane and the thylakoid is


3/14

called the stroma. "he thylakoid membrane contains chlorophyll and stalked particles, and is thesite of photosynthesis and A" synthesis. 1hloroplasts also contain starch grains, ribosomes andcircular *A.

Ribosomes."hese are the smallest and most numerous of the cell organelles, and are the sites

of protein synthesis. "hey are composed of protein and A, and are manufactured in the

nucleolus of the nucleus. ibosomes are either found free in the cytoplasm, where they makeproteins for the cell2s own use, or they are found attached to the rough endoplasmic reticulum,where they make proteins for e'port from the cell. "hey are often found in groupscalled polysomes. All eukaryotic ribosomes are of the larger, 34%3, type.

Smooth Endoplasmic Reticulum (SER).%eries of membrane channels involved in synthesising

and transporting materials, mainly lipids, needed by the cell.

Rough Endoplasmic Reticulum (RER).%imilar to the %E, but studded with numerous

ribosomes, which give it its rough appearance. "he ribosomes synthesise proteins, which areprocessed in the E (e.g. by en$ymatically modifying the polypeptide chain, or addingcarbohydrates), before being e'ported from the cell via the 5olgi 0ody.

Golgi ody (or Golgi !pparatus).Another series of flattened membrane vesicles, formed from

the endoplasmic reticulum. #ts 6ob is to transport proteins from the E to the cell membrane fore'port. arts of the E containing proteins fuse with one side of the 5olgi body membranes,while at the other side small vesicles bud off and move towards the cell membrane, where theyfuse, releasing their contents by e'ocytosis.

"acuoles."hese are membrane&bound sacs containing water or dilute solutions of salts and

other solutes. ost cells can have small vacuoles that are formed as re7uired, but plant cellsusually have one very large permanent vacuole that fills most of the cell, so that the cytoplasm(and everything else) forms a thin layer round the outside. lant cell vacuoles are filled with cellsap, and are very important in keeping the cell rigid, or turgid. %ome unicellular protoctistshave feeding vacuoles for digesting food, or contractile vacuoles for e'pelling water.

Lysosomes. "hese are small membrane&bound vesicles formed from the E containing a

cocktail of digestive en$ymes. "hey are used to break down unwanted chemicals, to'ins,organelles or even whole cells, so that the materials may be recycled. "hey can also fuse with afeeding vacuole to digest its contents.

Cytoskeleton."his is a network of protein fibres e'tending throughout all eukaryotic cells, used

for support, transport and motility. "he cytoskeleton is attached to the cell membrane and givesthe cell its shape, as well as holding all the organelles in position. "here are three types of proteinfibres (microfilaments, intermediate filaments and microtubules), and each has acorresponding motor protein that can move along the fibre carrying a cargo such as organelles,chromosomes or other cytoskeleton fibres. "hese motor proteins are responsible for such actionsas/ chromosome movement in mitosis, cytoplasm cleavage in cell division, cytoplasmic streaming

in plant cells, cilia and flagella movements, cell crawling and even muscle contraction in animals.

Centriole. "his is a pair of short microtubules involved in cell division.

Cilium and #lagellum. "hese are fle'ible tails present in some cells and used for motility. "hey

are an e'tension of the cytoplasm, surrounded by the cell membrane, and are full of microtubulesand motor proteins so are capable of comple' swimming movements. "here are twokinds/ flagella (pl.) (no relation of the bacterial flagellum) are longer than the cell, and there are


4/14

usually only one or two of them, while cilia (pl.) are identical in structure, but are much smallerand there are usually very many of them.

Micro$illi. "hese are small finger&like e'tensions of the cell membrane found in certain cells such

as in the epithelial cells of the intestine and kidney, where they increase the surface area forabsorption of materials. "hey are 6ust visible under the light microscope as a brush border.

Cell Membrane (or %lasma Membrane)."his is a thin, fle'ible layer round the outside of all cells

made of phospholipids and proteins. #t separates the contents of the cell from the outsideenvironment, and controls the entry and e'it of materials. "he membrane is e'amined in detaillater.

Cell &all."his is a thick layer outside the cell membrane used to give a cell strength and rigidity.

1ell walls consist of a network of fibres, which give strength but are freely permeable to solutes(unlike membranes). lant cell walls are made mainly of cellulose, but can also containhemicellulose, pectin, lignin and other polysaccharides. "here are often channels through plantcell walls called plasmodesmata, which link the cytoplasms of ad6acent cells. 8ungal cell walls aremade of chitin. Animal cells do not have a cell wall.


5/14

%rokaryotic Cells

Cytoplasm. 1ontains all the en$ymes needed for all metabolic reactions, since there are no

organelles

Ribosomes. "he smaller (94 %) type.

Nuclear 'one. "he region of the cytoplasm that contains *A. #t is not surrounded by a nuclear

membrane.

N!. Always circular, and not associated with any proteins to form chromatin.

%lasmid. %mall circles of *A, used to e'change *A between bacterial cells, and very useful

for genetic engineering.

Cell membrane. made of phospholipids and proteins, like eukaryotic membranes.

Mesosome. A tightly&folded region of the cell membrane containing all the membrane&bound

proteins re7uired for respiration and photosynthesis.

Cell &all. ade of murein, which is a glycoprotein (i.e. a protein+carbohydrate comple'). "here

are two kinds of cell wall, which can be distinguished by a 5ram stain/ 5ram positive bacteriahave a thick cell wall and stain purple, while 5ram negative bacteria have a thin cell wall with anouter lipid layer and stain pink.


6/14

Capsule(or Slime Layer). A thick polysaccharide layer outside of the cell wall. :sed for sticking

cells together, as a food reserve, as protection against desiccation and chemicals, and asprotection against phagocytosis.

#lagellum. A rigid rotating helical&shaped tail used for propulsion. "he motor is embedded in the

cell membrane and is driven by a H;gradient across the membrane. 1lockwise rotation drives the

cell forwards, while anticlockwise rotation causes a chaotic spin. "his is an e'ample of a rotatingmotor in nature.

Summary o the ierences et*een %rokaryotic and Eukaryotic Cells

%R+,!R-+/CCELLS E0,!R-+/CCELLS

small cells (< 5 m) larger cells (> 10 m)

always unicellular often multicellular

no nucleus or any membrane-bound

organelles

always have nucleus and other membrane-

bound organelles

DN is circular! without "roteins DN is linear and associated with "roteins to

form chromatin

ribosomes are small (#0$) ribosomes are large (%0$)

no cytos&eleton always has a cytos&eleton

cell division is by binary fission cell division is by mitosis or meiosis

re"roduction is always ase'ual re"roduction is ase'ual or se'ual

Endosymbiosis

rokaryotic cells are far older and more diverse than eukaryotic cells. rokaryotic cells have probablybeen around for .= billion years longer than eukaryotic cells. #t is thought thateukaryotic cell organelles like mitochondria and chloroplasts are derived from prokaryotic cells thatbecame incorporated inside larger prokaryotic cells. "his idea is called endosymbiosis, and is supportedby these observations/

organelles contain circular *A, like bacteria cells.

organelles contain 94% ribosomes, like bacteria cells.

organelles have double membranes, as though a single&membrane cell had been engulfed and

surrounded by a larger cell.


7/14

he Cell Membrane

"he cell membrane (or plasma membrane) surrounds all living cells. #t controls how substances can movein and out of the cell and is responsible for many other properties of the cell as well. "he membranes thatsurround the nucleus and other organelles are almost identical to the cell membrane. embranes arecomposed of phospholipids, proteins and carbohydrates arranged in a fluid mosaic structure, as shown inthis diagram.

"he phospholipids form a thin, fle'ible sheet, while the proteins 3float3 in the phospholipid sheet likeicebergs, and the carbohydrates e'tend out from the proteins.


8/14

he phospholipids are arranged in a bilayer, with their polar, hydrophilic phosphate heads facingoutwards, and their non&polar, hydrophobic fatty acid tails facing each other in the middle of the bilayer."his hydrophobic layer acts as a barrier to all but the smallest molecules, effectively isolating the twosides of the membrane. *ifferent kinds of membranes can contain phospholipids with different fatty acids,affecting the strength and fle'ibility of the membrane, and animal cell membranes also contain cholesterollinking the fatty acids together and so stabilising and strengthening the membrane.

he proteinsusually span from one side of the phospholipid bilayer to the other (intrinsic proteins), butcan also sit on one of the surfaces (e'trinsic proteins). "hey can slide around the membrane very 7uicklyand collide with each other, but can never flip from one side to the other. "he proteins have hydrophilicamino acids in contact with the water on the outside of membranes, and hydrophobic amino acids incontact with the fatty chains inside the membrane. roteins comprise about =4? of the mass ofmembranes, and are responsible for most of the membrane2s properties.

roteins that span the membrane are usually involved in transporting substances across the

membrane (more details below).

roteins on the inside surface of cell membranes are often attached to the cytoskeleton and are

involved in maintaining the cell2s shape, or in cell motility. "hey may also be en$ymes, catalysing

reactions in the cytoplasm.

roteins on the outside surface of cell membranes can act as receptors by having a specific

binding site where hormones or other chemicals can bind. "his binding then triggers other eventsin the cell. "hey may also be involved in cell signalling and cell recognition, or they may been$ymes, such as maltase in the small intestine (more in digestion).

he carbohydrates are found on the outer surface of all eukaryotic cell membranes, and are usuallyattached to the membrane proteins. roteins with carbohydrates attached are called glycoproteins. "hecarbohydrates are short polysaccharides composed of a variety of different monosaccharides, and forma cell coat or glycocaly' outside the cell membrane. "he glycocaly' is involved in protection and cellrecognition, and antigens such as the A0@ antigens on blood cells are usually cell&surface glycoproteins.

emember that a membrane is not 6ust a lipid bilayer, but comprises the lipid, protein and carbohydrateparts.

ransport !cross he Membrane

1ell membranes are a barrier to most substances, and this property allows materials to be concentrated

inside cells, e'cluded from cells, or simply separated from the outside environment. "hisis compartmentali$ation is essential for life, as it enables reactions to take place that would otherwise beimpossible. Eukaryotic cells can also compartmentali$e materials inside organelles. @bviously materialsneed to be able to enter and leave cells, and there are five main methods by which substances can moveacross a cell membrane/

. %imple *iffusion

>. @smosis


9/14

@,@>and 1@>. 8or these molecules the membrane is no barrier at all. %ince lipid diffusion is (obviously) apassive diffusion process, no energy is involved and substances can only move down their concentrationgradient. Lipid diffusion cannot be controlled by the cell, in the sense of being switched on or off.

2. +smosis

@smosis is the diffusion of water across a membrane. #t is in fact 6ust normal lipid diffusion, but sincewater is so important and so abundant in cells (its concentration is about =4 ), the diffusion of water hasits own name & osmosis. "he contents of cells are essentially solutions of numerous different solutes, andthe more concentrated the solution, the more solute molecules there are in a given volume, so the fewer

water molecules there are. Dater molecules can diffuse freely across a membrane, but always down theirconcentration gradient, so water therefore diffuses from a dilute to a concentrated solution.

&ater %otential. @smosis can be 7uantified using water potential, so we can calculate which way water

will move, and how fast. Dater potential (, the 5reek letter psi, pronounced 3sy3) is a measure of the


10/14

water molecule potential for movement in a solution. #t is measured in units of pressure (a, or usuallyka), and the rule is that water always moves by osmosis from less negative to more negative water

potential (in other words it2s a bit like gravity potential or electrical potential). 44? pure water has ! 4,

which is the highest possible water potential, so all solutions have 4 (i.e. a negative number), and you

cannot get F 4.

Cells and +smosis. "he concentration (or @) of the solution that surrounds a cell will affect the state ofthe cell, due to osmosis. "here are three possible concentrations of solution to consider/

#sotonic solution a solution of e7ual @ (or concentration) to a cell

Hypertonic solution a solution of higher @ (or concentration) than a cell

Hypotonic solution a solution of lower @ (or concentration) than a cell

"he effects of these solutions on cells are shown in this diagram/


11/14

"he diagram below shows what happens when > fresh raw eggs with their shells removed with acid areplaced into sucrose solution (hypertonic) and distilled water (hypotonic). Dater enters the egg in water(endosmosis) causing it to swell and water leaves the egg in sucrose causing it to shrink (e'osmosis).

"hese are problems that living cells face all the time. 8or e'ample/

%imple animal cells (proto$oans) in fresh water habitats are surrounded by a hypotonic solution

and constantly need to e'pel water using contractile vacuoles to prevent swelling and lysis.

1ells in marine environments are surrounded by a hypertonic solution, and must actively pump

ions into their cells to reduce their water potential and so reduce water loss by osmosis.

Goung non&woody plants rely on cell turgor for their support, and without enough water they wilt.

lants take up water through their root hair cells by osmosis, and must actively pump ions intotheir cells to keep them hypertonic compared to the soil. "his is particularly difficult for plantsrooted in salt water.


12/14

3. #acilitated iusion.

8acilitated diffusion is the transport of substances across a membrane by a trans&membrane proteinmolecule. "he transport proteins tend to be specific for one molecule (a bit like en$ymes), so substancescan only cross a membrane if it contains the appropriate protein. As the name suggests, this is a passivediffusion process, so no energy is involved and substances can only move down their concentrationgradient. "here are two kinds of transport protein/

1hannel roteins form a water&filled pore or channel in the membrane. "his allows charged

substances (usually ions) to diffuse across membranes. ost channels can be gated (opened orclosed), allowing the cell to control the entry and e'it of ions.

1arrier roteins have a binding site for a specific solute and constantly flip between two states so

that the site is alternately open to opposite sides of the membrane. "he substance will bind on theside where it at a high concentration and be released where it is at a low concentration.

"he rate of diffusion of a substance across a membrane increases as its concentration gradientincreases, but whereas lipid diffusion shows a linear relationship, facilitated diffusion has a curved

relationship with a ma'imum rate. "his is due to the rate being limited by the number of transportproteins.

4. !cti$e ransport (or %umping).

Active transport is the pumping of substances across a membrane by a trans&membrane proteinpump molecule. "he protein binds a molecule of the substance to be transported on one side of themembrane, changes shape, and releases it on the other side. "he proteins are highly specific, so there isa different protein pump for each molecule to be transported. "he protein pumps are also A"aseen$ymes, since they catalyse the splitting of A" into A* ; phosphate (i), and use the energy released


13/14

to change shape and pump the molecule. umping is therefore an active process, and is the onlytransport mechanism that can transport substances up their concentration gradient.

he Na5,5%ump. "his transport protein is present in the cell membranes of all animal cells and is themost abundant and important of all membrane pumps. De look at it in more detail in module B (A>course)

6. "esicles

"he processes described so far only apply to small molecules. Large molecules (such as proteins,polysaccharides and nucleotides) and even whole cells are moved in and out of cells by using membranevesicles.

Endocytosisis the transport of materials into a cell. aterials are enclosed by a fold of the cellmembrane, which then pinches shut to form a closed vesicle. %trictly speaking the material has not yetcrossed the membrane, so it is usually digested and the small product molecules are absorbed by the

methods above. Dhen the materials and the vesicles are small (such as a protein molecule) the processis known as pinocytosis (cell drinking), and if the materials are large (such as a white blood cell ingestinga bacterial cell) the process is known as phagocytosis (cell eating).

E7ocytosisis the transport of materials out of a cell. #t is the e'act reverse of endocytosis. aterials tobe e'ported must first be enclosed in a membrane vesicle, usually from the E and 5olgi 0ody.Hormones and digestive en$ymes are secreted by e'ocytosis from the secretory cells of the intestine andendocrine glands.


14/14

%ometimes materials can pass straight through cells without ever making contact with the cytoplasm bybeing taken in by endocytosis at one end of a cell and passing out by e'ocytosis at the other end.

Summary o Membrane ransport

ME8+0SES

ENERG-0SES%R+E/NS S%EC/#/C C+NR+LL!LE

$im"le Diffusion N N N N

smosis N N Y N

acilitated Diffusion N Y Y Y

ctive *rans"ort Y Y Y Y

+esicles Y N Y Y

Lecture 1 - Cells Structure and Transport Mechanisms (1)

Documents

Transcript of Lecture 1 - Cells Structure and Transport Mechanisms (1)