Cellstructure

and functions

In biology, cell theory is a scientific theory that describes the properties of cells, the basic unit of structure in every living thing. The initial development of the theory, during the mid-17th century, was made possible by advances in microscopy; the study of cells is called cell biology. Cell theory is one of the foundations of biology.The three parts to the cell theory are as described below:All living organisms are composed of one or more cells.The cell is the basic unit of structure, function, and organization in all organisms.All cells come from preexisting, living cells.

What is cell?

KEYWORDS

CELL The smallest structural unit of an organism thats capable of independent functioning, consisting of one or more nuclei, cytoplasm, and various

organelles, all surrounded by a semipermeable cell membrane.

CELL MEMBRANE The semipermeable membrane that encloses the cytoplasm of a cell. Also calledcytomembrane, plasmalemma, plasma membrane.

CELL WALL The rigid outermost cell layer found in plants and certain algae, bacteria, and fungi but characteristically absent from animal cells.

CHLOROPLAST A chlorophyll-containing plastid found in algal and green plant cells.

CHROMOSOME A threadlike linear strand of DNA and associated proteins in the nucleus of eukaryotic cells that carries the genes and functions in the transmission of hereditary information.

CYTOPLASM The protoplasm outside the nucleus of a cell.

EUKARYOTES A single-celled or multicellular organism whose cells contain a distinct membrane- bound nucleus.

GENE A hereditary unit consisting of a sequence of DNA that occupies a specific location on a chromosome and determines a particular characteristic in an organism. Genes

undergo mutation when their DNA sequence changes.

MULTICELLULAR Having or consisting of many cells.

NUCLELOUS A small, typically round granular body composed of protein and RNA in the nucleus of a cell. It is usually associated with a specific chromosomal site and involved in

ribosomal RNA synthesis and the formation of ribosomes.

ORGANELLES A differentiated structure within a cell, such as a mitochondrion, vacuole, or chloroplast, that performs a specific function.

ORGAN A differentiated part of an organism, such as an eye, wing, or leaf, that performs a specific function.

SUMMARY

A microscope is an instrument used to see objects too small for the naked eye. An English scientist, Robert Hooke, discovered the existence of cells in 1665.Heis known for his book Micrographia, and for first using the word “cell” to describe the basic unit of life.Scanning electron microscopes are used to examine the external parts of various organisms; It is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample, producing signals that contain information about the sample’s surface topography, composition, and other properties such as electrical conductivity.

The transmission electron microscope (TEM) is used to view the internal structure of a cell and its organelles. TEM is a microscopy technique in which a beam of electrons is transmitted through an ultra-thin specimen, interacting with the specimen as it passes through it. An image is formed from the interaction of the electrons transmitted through the specimen; the image is magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or to be detected by a sensor, such as a CCD camera.

Organisms that are made up of a single cell and perform all their vital activities, like reproduction, locomotion and digestion, are called unicellular organisms. Unicellular organisms can be found everywhere. The oldest forms of life, unicellular organisms existed 3.8 billion years ago, if not longer.

Organisms that are made up of more than one cell are called multi-cellular organisms. Most life that can be seen with the naked eye are multi-cellular, as are all animals and plants.

Amoeba uses small finger-like projections, called pseudopodia, for locomotion and to capture prey. Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells with these structures are called ameboids. Paramecium is a single cell, and is built in such a way that can perform all vital activities, like reproduction, locomotion, digestion, and so on.

The White Blood Corpuscle (WBC) is the only animal cell that changes its shape. WBC’s are responsible for protecting our bodies against invading bacteria and harmful micro-organisms. To find invading bacteria, they travel along with blood in blood vessels. Whenever they find invading bacteria, they squeeze through the blood vessels and intercellular spaces, catch the bacteria and kill it. To perform this task, WBC’s change their shape using pseudopodia similar to those in an amoeba.

The branched structure of a neuron helps it transfer messages to all parts of the body. A neuron, also known as a nerve cell, is an electrically excitable cell that processes and transmits information by electrical and chemical signaling.

The black layer or scab (a hard coating on the skin formed during the wound healing reconstruction phase) that you see on a wound is a result of the aggregation of dead Red Blood Cells (RBC’s).The human respiratory system consists of the nostrils, nasal passage, internal nares, pharynx, larynx, trachea, bronchi, bronchioles and alveoli. Alveoli are the functional units of the lungs.All organ systems work together to form a complex organism. Organ systems include the digestive system, respiratory system, circulatory system, excretory system and nervous system.

Cytoplasm is a thick liquid in a cell that holds the organelles, except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm. Within the cells of eukaryotes organisms, the contents of the nucleus are separated from the cytoplasm.

The cell membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It consists of the phospholipid bilayer with embedded proteins. Cell membranes are involved in a variety of cellular processes, such as cell adhesion, ion conductivity and cell signalling, and serve as the attachment surface for extracellular material and intercellular cytoskeleton.

The nucleus controls the activities of a cell. The nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organised as multiple long linear DNA molecules to form chromosomes. The genes within these chromosomes are the cell's nuclear genome. The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression. The nucleus is, therefore, the control centre of the cell.

Methylene blue is the stain placed on plant and animal cells to differentiate the nucleus under a microscope. Solutions of this substance are blue when in an oxidising environment, but will turn colourless if exposed to a reducing agent.

Chromosomes are called the vehicles of heredity because they carry genes, and help in the inheritance or transfer of characteristics from the parents to the offspring. A chromosome is an organised structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Bacteria and blue-green algae that contain nucleoids are called prokaryotes.

Organisms that have a well-organised nucleus are called eukaryotes. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear envelope, within which the genetic material is carried. The presence of a nucleus gives eukaryotes their name, which comes from the Greek eu, meaning "good", andkaryon, meaning "nut" or "kernel". Most eukaryotic cells also contain other membrane-bound organelles.

The nucleoid is an irregularly-shaped region within the cell of prokaryotes, which has nuclear material without a nuclear membrane.

The nucleolus is a small, spherical body in the nucleus. It is a non-membrane bound structure composed of proteins and nucleic acids. Ribosomal RNA is transcribed and assembled within the nucleolus.Chloroplast is a type of plastid involved in photosynthesis in plants. These are specialised organelles found in all higher plant cells. These organelles contain the plant cell's chlorophyll, providing the green colour. They have a double outer membrane. Within the stroma are other membrane structures, the thylakoids and grana where photosynthesis takes place.

A vacuole is a membrane-bound organelle present in all plant, fungal cells, and some animal and bacterial cells. Vacuoles are essentially enclosed compartments filled with water containing inorganic and organic molecules, including enzymes, in solution. In certain cases, though, vacuoles may contain solids that have been engulfed. Vacuoles are formed by the fusion of multiple membrane vesicles, and are effectively just larger forms of these. The organelle has no basic shape or size, and its structure varies according to the needs of the cell.

CORK CELLSIn 1665, a Englishman called Robert. Hooke used a microscope which made by himself to  observe a cork, he then discovered that it had many small holes, just like honeycomb, so he named these small holes be cells, which the original meaning as small rooms. Robert Hooke hadn't describe the things inside cells. He just simply said that cells contain air and liquid. Now, we know that the cells that Hooke had discovered was only the cell membrane of dead cell. Afterwards, many scholars also discovered cells in different living things. Plants and animals also have cells, but these scholars hadn't noticed the things inside the cells. In 1831,Robert Brown discovered nucleus in some plants' epidermis cells. In 1835, someone discovered cytoplasm inside cells. That's the way people learn about cell's basic structure and the form of cells.

ROBERT HOOKEBorn 28 July [O.S. 18

July] 1635Freshwater, Isle

of Wight, EnglandDied 3 March 1703

(aged 67)London, England

Nationality EnglishFields Physics and

chemistryInstitutions Oxford UniversityAlma mater Christ Church,

OxfordAcademic advisors Robert Boyle

Known for Hooke's lawMicroscopy

applied the word 'cell'

Influences Richard Busby

An artist's impression of Robert Hooke.

EXERCISESQuestion.1: Indicate True(T) or False(F)a.    Unicellular organisms have one-celled body.b.    Muscle cells are branched structures.c.    The basic living structure of an organism is an organ.d.    Amoeba has irregular shape.Answer: a)T

b)T (spindle shaped structures) c)F d)T

Question.2: Make a sketch of the human nerve dell. What function do nerve cells perform?Answer: Functions of human nerve cell:Nerve cells receive message from different parts of body.They further transfer these messages to brain and accordingly brain send commands for functioning of different organs of body. 

Question.3: Write short notes on the following:(a) Cytoplasm (b) Nucleus of a cellAnswer:     (a)   Cytoplasm: Cytoplasm is a jelly like substance which is present between the cell membrane and the nucleus. Various other organelles of cells are present in the cytoplasm. Cytoplasm is made up of chemical substances like carbohydrates, proteins and water. These chemical substances are present in cells of all types and sizes. Cytoplasm contains many important tiny substances called Organelles.     (b)   Nucleus of a cell: Nucleus is the master of the cell. It commands all the functioning of the cell. It is generally located in the center of the cell and is spherical in shape. A membrane called nuclear membrane separates it from cytoplasm. It contains the genetic material DNA and RNA in it. This porous membrane allows the transfer of material in the nucleus and cytoplasm. Nucleus contains a dense body called Nucleolus which actually contains Chromosomes, the genetic material.

Question.4: Which part of the cell contains organelles?Answer: Cytoplasm.

Question.5: Make sketches of animal and plant cells. State three differences between them.

Plant Cells Animal Cells

They are usually larger than animal cells. They are generally small in size.

Cell wall is present. Cell wall is absent.

Vacuoles are larger in size. Vacuoles are small in size.

Cell

wall

Chloroplast

Cell MembraneVacuole

Nucleus

Cytoplasm

Cell Membrane Vacuole

Nucleus

Cytoplasm

Question.6: State a difference between eukaryotes and prokaryotes.Answer: Prokaryotes do not have a well designed nuclear membrane while, eukaryotes have a well designed nuclear membrane.

Question.7: Where are the chromosomes found in cell? State their functions.Answer: Chromosomes are found in the nucleus of a cell. Their function is to carry characteristic features of parent cells to the daughter cell means, from parent to offspring.

Question.8: ‘Cells are the basic structural units of living organism’. Explain.Answer: In Biology, the basic unit of which all living things are composed is known as ‘cell’. The ‘cell’ is the smallest structural unit of living matter that is capable of functioning independently. A single cell can be a complete organism in itself, as in bacteria and protozoans. A unicellular organism also captures and digests food, respires, excretes, grows, and reproduces. Similar functions in multi-cellular organisms are carried out by groups of specialized cells which are organized into tissues and organs such as, the higher plants and animals. Hence, ‘cell’ is known as the basic structural and functional unit of life.       

Question.9: Explain why chloroplasts are found only in plant cells.Answer: Chloroplasts are found only in plant cells because they are required for photosynthesis.

ThankYou.