what is cell

        THE CELL

A cell, consists of a microscopic unit of protoplasm contained in a cell membrane. It forms the structural unit of any living organism.

 A single cell performs all functions of the body in a unicellular organism. But in a multicellular organism cells get specialised to form different tissues, each tissue doing a specific function. Shape of cells vary from tissue to tissue.

1.        Cell Membrane

 (It is 7.5 n.m. thick and separates the cell cytoplasm from tissue fluid.

Generally each cell has the following parts: It is trilaminar in structure with an outer and inner phospholipid layers and a third layer of protein embeded in the lipid bilayer. The lipids and proteins have some carbohydrates attached to their peripheral surface.

 Lipid bilayer contains lecithin, cephalin, cholesterol and glycolipids. Proteins of the cell membrane are of three types:-

(a) Proteins embedded in the lipid layer and projecting out on one surface of the cell membrane.

(b) Proteins crossing the thickness of the cell membrane and projecting on its both surfaces (transmembrane proteins).

(c) Proteins present on the outer surface of the cell membrane (peripheral proteins), like glycoproteins. Transmembrane proteins act as membrane pores while surface glycoproteins act as mediators of cell interactions such as adhesion and recognition.

 It has selective permeability for substances which go in and out of the cell.

Its chemical nature gives it the ability to conduct impulses.

Cell membranes of adjacent cells are connected to each other by junctional complexes such as desmosomes, zonula occludens, zonula adherens and gap junctions.

2.    Cytoplasm-:

 It contains cytoplasmic matrix, organelles and inclusions.

             All metabolic and synthetic activities of the cell take place in the cytoplasm under the directions received from the nucleus.

(a)  Organelles-

 Centrosome (centriole)-:

 a pair of centrosomes are seen in most of the cells. Electron miscroscopically each centrosome consists of ninegroups of tubules, each group with three tubules, arranged in a circle. They form mitotic spindle fibres in dividing cells and also contrtibute to flagella and cilia. With light microscope centrioles are seen as dots embedded in dense cytoplasm.

Mitochondria-:

 They are 3 µm long and 1 µm wide granule or rod like structures.

They are more in number in cells with high metabolic activity.

Each has a two layered membranous covering, the inner layer folded inwards to form incomplete partitions called cristae.

Within the inner membrane is the matrix which contains many enzymes, some amount of DNA and RNA. DNA and RNA give them the ability to duplicate themselves during cell division.

Enzymes like flavoproteins and cytochromes are attached to the mem- branes and are involved in the respiratory processes of the cell.

Dehydrogenase enzymes are present in the matrix and are involved in Kerb's citric acid cycle and lipid and protein synthesis.

 ATP and GTP are formed in the mitochondria and provide energy for the various activities of the cell.

Endoplasmic reticulum (ER)-:

This consists of a system of membranes which form flattened tubules or vesicles.

This membranous reticulum is present in the cytoplasm. Ribonucleoprotein particles called ribosomes are seen on the ER in good number in cells engaged in active protein synthesis.

This type is known as granular endoplasmic reticulum. In some cells, like muscle cells ER is without ribosomes.

This type is known as smooth (agranular) endoplasmic reticulum. This variety of ER is seen in cells involved in the production of glycogen or steroid hormones. Both varieties of ER are seen in liver cells.

Microsomes-:

When homogenised cell preparations are centrifuged Microsomes fractions are obtained. One such fraction, called microsome fraction, consists of membraneous vesieles with or without ribosomes on them.

This is not a separate cell organelle but only fractions of rough an smooth endoplasmic reticulum.

Ribosomes-:

 These contain RNA and are seen as free bodies in the cytoplasm or on endoplasmic reticulum, They are the seat of protein syathesis.

Golgi apparatus-:

 Consists of membranes similar to smooth ER. Here membranes are seen as flattened vesicles with the ends often expended and stacked one over the other. Golgi apparatus is seen placed between the nucleus and apical part of the cell.

It is involved in the formation of secretory products rich in carbohydrates, It provides membranous covering for secretion granules and replenishes cell membrane.

 Lysosomes-:

 These are membrane bound bodies containing hydro-lytic enzymes like phosphase, cathepsin, proteases, glycosidases, lipases and carbohydrates. These enzymes are synthesised in endoplasmic retic- ulum and the covering membrane is formed by Golgi apparatus.

 Lysosomes, with the help of the enzymes within, digest foreign bod- ies like bacteria entering the cell. They also help in the break down of an injured cell.

Lysosomes that have not entered into a digestive event are called "primary lysosomes". Lysosomes in which digestion of an ingested bacterium or other foreign body is taking place is called a "secondary lysosome"

Peroxisomes-:

These contain many enzymes involved in lipid metab- olism. It also protects cell from the harmful effects of hydrogen peroxide. Smaller variety of them are called "microperoxisomes".

 Microtubules and microfilaments-:

 They are seen in cytoplasm. They are made up of proteins like tubulin or actin.

 They are rod or tube like structures and have the following functions:

 (1) act as cytoskeleton

 (2) help in intracellular transport

 (3) form the components of cilia, flagella and centrioles.

  (b) Cytoplasmie inclusions-

These have no role in the metabolic functions of the cell. They are stored food such as glycogen granules or fat droplets or pigments such as melanin or lipofuchsin.

3.         Nucleus-:

        Except human RBC all other cells have a nucleus. Shape of nucleus varies with shape of cells. Nucleus is bounded by a two layered nuclear membrane having pores on it.

Outer layer of the membrane is continuous with endoplasmic reticulum. Nuclear pores allow passage of substances between nucleoplasm and cytoplasm.

Nucleoplasm is semisolid and consists of mainly chromatin. In cells when they are not undergoing cell division, chromatin is seen as filaments. Some of it is seen as condensed chromatin (heterochromati) while some of it is seen as extended chromatin (euchromatin). Just before cell division chromatin become very much condensed to form rod sheped structures called chromosomes. Each human cell has 46 chromoshaped Two of them are sex chromosomes. and the rest are autosomes.

In male there is one 'X' and one Y' chromosome while in female both the sex chromosomes are X-chromosomes. Chromosomes are the seat of hereditary material called DNA (D-oxyribose Nucleic acid) which con- trols the expression of different characters of the individual. This it does by regulating the synthesis of proteins in the cells.

In female, cells show the presence of sex chromatin. This rod-shaped structure is present near the nuclear membrane. It is formed by one of the two X chromosomes which gets inactivated and is also known as “Barr body"

Nucleolus-:

A nucleus may contain one or more nucleoli. Nucleolus has a high RNA content. This then goes to cytoplasm through nuclear pores and takes part in protein synthesis.

Nucleoli are formed by the portions close to second- ary constrictions of certain chromosomes. Such parts of these, chromosomes are known as nucleolar organising centres. Ribosomal RNA is synthesised in nucleoli.

 Variation In The Size And Shape Of Cells-:

Diameter of a cell vary from 6.5 um to 250 um (Lymphocyte = 6.5 um, RBC = 7 um, Neve cells- 10 um to 150 um and ovum = 250 um). Similarly cells may be of different shapes; square shaped (cuboidal). flattened (squamous), cylindrical (columnar), many sided (polyhedral). oval or round (as in spinal ganglion), spindle shaped (as in smooth muscle), irregular (as in multipolar neurons) or dumb-bell shaped (as in human RBC).