Chapter-2
The Cell
Syllabus: Structure of cell,function of its components with special reference to mitochondria and microsomes.
The cell is the smallest unit of living tissues. Cells of different tissues perform different functions. A cell is made up of the following structures:
1. Cell Membrane : It is the external boundary of the living cell. It is a three layered structure made up of lipids bilayer and proteins. It measures approximately 70 A° in thickness. The cell wall allows the diffusion of substances into and out of the 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) nucleolus
b) chromatin
a) Nucleolus: It is a highly coiled filamentous structure present in the nucleus. It is not surrounded by a membrane. 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 ribonucleic acid) and proteins. The chromatin threads carry genetic information. At the time of cell division, chromatin condenses into chromosomes. The number of chromosomes is constant for a particular species of organism. In man, there are 23 pairs of 46 chromosomes.
3. Cytoplasm: It is the 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 homogenate at very high speed (10000 rpm). Microsomes contain .1) Ribosomes 2) Granular matrix. Ribosomes are concerned with protein synthesis. Granular matrix contains:
i) Oxidases which generate hydroygen peroxide.
ii) Catalase which converts hydrogen peroxide into water.
5.Endoplasmic reticulum: It is the most extensive cell organelle present in cytoplasm. It consists of two membranes which are separated by a space. Endoplasmic reticulum is of two types. They are :
a) Granular or rough surfaced endoplasmic reticulum containing ribosomes. The ribosomes are the sites of protein synthesis.
b) Agranular or smooth surfaced endoplasmic recticulum. This does not contain ribosomes. This type of endoplasmic reticulum is concerned with fatty acid and steroid synthesis. Also, they store and release calcium.
6. 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 their secretion.
7.Mitochondria:
They occur in the cytoplasm at vari- able numbers e.g. few hundreds to few thousands The mitochondrion is composed of two layers of membranes. They are:
1. an outer layer which is smooth.
2. an inner layer folded into sheets of tubules called cristae.
Both these layers enclose a central cavity called matrix.
The mitochondria are made up of proteins, phospholipids and some ribonucleic acid. They also contain some important enzyme systems. The mitochondria are chiefly concerned with cellular oxidation.
8.Lysosomes: They are small spherical or oval bodies surrounded by a single membrane. They vary in number and contain a variety of hydrolytic enzymes.The lysosomes breakdown bacteria and cell debris engulfed by the cell. The damaged intracellular organelles are also broken down and digested by lysosomes.
9.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 thread like structure. It contains two centrioles.
10. Microtubules: They are straight, hollow cylinders with a diameter of about 200 A°. They are concerned with the maintenance of cell shape. They are also associated with movements of cilia, flagella and mitotic spindles.
Functions of a cell:
The following are the important functions performed by a cell.
1. Ingestion and assimilation: The cell ingests chemical substances like amino-acids 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 synthesise 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 material is used for growth and repair. ii) Catabolism in which food material is broken down to release energy for various functions of the cell.
4. Respiration: It involves transport of oxygen from lungs 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 into 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.
7. Reproduction: After growing to an optimum size, the cell divides into daughter cells. Reproduction of cells occurs by mistosis or meiosis.
MITOSIS: Mitosis is a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. This occurs in four stages namely prophase, metaphase, anaphase and telophase.
1. Prophase: Initially, the centrosome divides into two. These two new centrosomes 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 céntre of the cell. They get attached to thread like structures of the centrosomes which are now at the two poles of the cell.
3. Anaphase: The chromosomes now divide longitudinally into two equal parts. The two groups of chromosomes move away to the poles and get arranged around the centrosomes.
4. Telophase: The cell becomes narrower at the centre to facilitate division. The thread like structures disappear. The nuclear membrane appears again. At the end of telophase, the cell divides. Also, the chromosomes disappear into the nucleus. The daughter cells grow and inturn reproduce by mitosis. During mitosis, each chromosome duplicates. So, the daughter cell contains fortysix chromosomes.
MEIOSIS: It is a process of reproduction which occurs in higher animals including man. It involves the fusion of:
i) Spermatozoon from the male (which contains 23 chromosomes and
ii) Ovum from the female (which also contains 23 chromosomes).
The spermatozoon and ovum are also called as gametes. The fusion of these two (male and female) gametes results in the formation of a zygote which has the normal number of 46 chromosomes (23 pairs). This leads to mixing of the hereditary determinants or genes from the male and female.
Determination of sex: One pair of chromosomes from the father and one pair from 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 chromosome from each pair determines the sex of the child. If the child has X chromosomes from the mother and X chromosome from the father, it is a female (XX). If the child has X chromosome from the mother and Y chromosome from the father it is a male (XY).
Human Anatomy & Physiology:
(Click below link for read)
● Chapter -3, The Tissues
● Chapter -4,
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