Q.:- What is Cell?

Ans.:-

INTRODUCTION:-

The cell is the smallest unit of living tissues. Cell of different tissues perform different functions. A cell is made up of the following structure:

(1.) Cell wall: it is the external boundary of the living cell. It is a three layered structure made up of lipids and proteins. It measures approximately  70 A% in thicknesses the cell wall allows the diffusion of substances in to and out of cell. 

(2.) Nucleus: IT is the largest structure present almost in the centre of a cell. It is more or less spherical in shape. It is bounded by nuclear membrane. The nucleus contains: 

(a) Nucleus  (b) Chromatin 

a) Nucleolus: It is a highly coiled filamentous structure present in the nucleus. It not surrounded by a memberane. But it contains numerous granules. Nucleolus is the site of ribosomal RNA (ribonucleic acid ) synthesis. 

b) Chromatin: These are fibrous threads present in the nucleus. They are composed of DNA ( deoxy ribonucleric acid ) and proteins. The Chromatin threads carry genetic information. At the time of cell division, Chromatin condensed in to Chromosomes. The number of Chromosomes is constant for a particular spcies of organisms. In man there are 23 pairs of  46 Chromosomes. 

(3.) Cytoplasm:-

It is region lying between the cell membrane and nucleus. The cytoplasm contains cell organelles like endoplasmic reticulum, golgi apparatus, mitochondria, lysosomes and centrosome.

 (4.) Microsomes: They are extremely small bodies present in the cytoplasm. They can be separated by centrifuging a tissue homogenste at Very high speed ( 10000 rpm ) microsomes contains 1 ) Ribosomes 2 ) Granuler Matrix Ribosomes are concerned with proteins synthesis. Granular Matrix contains: 

• Oxidases which generate hydrogen peroxide.
• Catalase which converts hydrogen peroxide in to water.

Organelles of cytoplasm: Following are the important organelles present in the cytoplasm:

i) Endoplasmic reticulum: It is the most expensive cell organelle present in cytoplasm. It consists of two membernes which are separated by a space. Endoplasmic reticulum is of two types. They are:

a) Granular: or rough surfaced endoplasmic reticulum bcontaining ribosomes. The ribosomes are the sites of proteins synthesis,

b) Agranular: or smooth surfaced endoplasmic reticulum. This does not contain ribosomes. This type of endoplasmic reticulum is concerned with fatty acid steroid synthesis. Also, they store and release calcium. 

ii) Golgi apparatus: It is a cup shaped structure and contains vesicles. It is situated between the nucleus and the apex of the cell. It is concerned with concentration of proteins prior to secretion. 

iii) Mitochondria: They occur in the cytoplasm at variable numbers e.g. few hundreds to few thousand membranes. They are:

1. An outer layer which is smooth.

2. An inner layer folded in to sheets of tubules called cristae. 

Both these layer enclose a central cavity called Matrix.

The mitochondria are made up of proteins, phospholipid and some ribonucleic acid. They also contain some important enzyme system. The mitochondria are chiefly concerned with cellular oxidation. 

iv) Lysosomes: They are small spherical or oval bodies surrounded by a single memberane . They very in numbers and contain a variety of hydrolytic enzymes . The lysosomes breakdown bacteria and cell debris engulfed by the cell. The damaged intracellular organelle are also broken down and digested by lysosomes.

v) Centrosome: it is a small rod shaped body found near the nucleus. It plays an important role during cell division. The centrosome is surrounded by a radiating threads like structure. It contains two centrioles. 

vi) Microtubules: They are straight, hollow cylinders with a diameter of about 200 A% they are concerned with the maintenance of cell shape. They also associated with movements of cilia flagella and mitotic spindles. 

 Functions of a cell:-

They following are the important functions performed by a cell.

1.) Ingestion and assimilation: The cell ingests chemical substances like amino- acid from intercellular or interstial fluid. These substances are used to build up complicated substances like proteins.

2.) Growth and repair: The ingested and assimilated materials are used to synthesis new protoplasm. This leads to increase in size and growth of the cell. Also worn out parts of the cell are replaced by this process. 

3.) Metabolism: This involves two processes: 

i) Anabolism in which the ingested and assimilated food materials is used for growth and repair. 

ii) Catabolism in which food materials is broken down to release anegry for various functions of the cell.

4.) Respiratory: It involves transport of oxygen from lung through blood to the tissues and removal of waste products like carbondioxide. This is essential for the survival and functions of a cell.

5.) Excretion: The cell eliminates waste products resulting from Catabolism in to the interstitial fluid. These products are carried by blood for elimination though lungs and kidneys. 

6.) Irritability and contractility: The cell is active by means of these two functions. The cell responds To any stimulus ( like physical, chemical, thermal, electrical and mechanical,) by contracting. Or the impulse is conducted as that occur in a nerve cells.

7.) Reproduction: After growing To an optimum size, the cell divides in to daughter cells. Reproduction of cell occurs by mistosis or meiosis.

MITOSIS:-

This occurs in four stage namely prophase, metaphase, anaphase and telophase. 

1.) Prophase: Intially, the centrosome divides in to two. These two new centrosome move away from each other to the two ends of the nucleus forming two poles.

2.) Metaphase: Now, the nuclear membrane disappears The Chromosomes arrange themselves around the centre of the cell. They get attacheds to thread like structure of the centrosomes which are now at the two poles of the cell.

3.) Anaphase: The Chromosomes now divide longitudinally in two equal parts. The two groups of Chromosomes move away to the poles and get arranged around the centrosomes.

4.) Telophase: The cell become narrower at the centre to facilitate division. The there'd like structure disappears. The nuclear membrane appersb again.

At the end of telophase, the cell divides. Also the Chromosomes disappears in to the nucleus. The daughter cells grow and intrur reproduce by mistosis.during mitosis each Chromosomes duplicates . The daughter cell contains forty-six Chromosomes. 

MEIOSIS:-

It is a process of reproduction which occur in higher animals including man. It involves the fusion of: 

i) spermatozon from the male ( which contains 23 Chromosomes and ii) Ovum from the female ( which also contain 23 Chromosomes). The spermatozon and Ovum are female) gametes results in the formation of a zygote which has the normal of 46 Chromosomes ( 23 pairs ) this lead to mixing of the hereditary determine nantsv or genes from the male and female. 

Determination of sex:-

One pair of Chromosomes from the father and one pair from with the mother are sex Chromosomes. These sex Chromosomes determine the sex of the child. In the female, the sex Chromosomes are the same and are called XX . In the male , they are different and are called XY. One Chromosomes from each pair determine the sex of the child.

If the child has X Chromosomes from the mother and X Chromosomes from the father, it is a female ( XX) Chromosomes from the father it is a male ( XY).

Written by:- DRX Aman

Read more:-

Anatomy- Introduction 

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