Cell is the fundamental unit of life. It is tiny microscopic mass of protoplasm enclosing a denser spherical or oval body the nucleus. Cell is bounded by cell membrane, protoplasm, nucleus and cell membrane are all living parts of cell. Recently cell is defined, as structural and fundamental unit of living organism or the cell is the basic unit of life.
Animal cell consists of plasma membrane, nucleus, cytoplasm and its organelles.
Plasma membrane: All cells are covered by membrane that serves as their outer boundary separating the cytoplasm from external environment. This is called plasma membrane. It allows the cell to take up and retain certain substances while excluding others. It consists of double layer of phospholipids bearing proteins.
Nucleus: Nucleus lies in the cytoplasm of the cell. Nucleus is most important part of the cell and controls all the activities of the cell. Nucleus is spherical or oval in shape surrounded by nuclear membrane which bears nuclear pores. Certain substances pass freely through these pores between the nucleus and surrounding cellular substances.
Nucleus is filled with protein rich substance called nucleoplasm or karyolymph. In nucleoplasm are numerous fine strands in the form of network called chromatin network or nuclear reticulum composed of nucleic acid DNA and protein. During cell division chromatin changes its shape to form chromosomes which contain hereditary units called genes that carry the heredity information from generation to generation. Within nucleus is spherical body called nucleolus which may be more than one in one nucleus. It disappears during cell division and reappears afterwards.
Nucleolus is believed to play important role in the synthesis of ribonucleic acid and ribosome in eukaryotic cells.
Cytoplasm: Protoplasm outside nucleus is called cytoplasm. Cytoplasm is composed of several types of organelles and a blind matrix the cytosal in which organelles reside. Cytoplasm contains following organelles:
(1) Endoplasmic reticulum:
It extends from plasma membrane to the nuclear membrane. It is an elaborate tube like system of lipoprotein. There are two types of it (a) Agramulated or smooth endoplasmic reticulum and (b) Rough or granulated endoplasmic reticulum. Smooth endoplasmic reticulum is not associated with ribosomes. Endoplasmic reticulum serves as supporting platform for ribosome. It forms structural framework of the cell with increased surface for various metabolic reaction.
(2) Mitochondria or chondriosomes:
They are present in the cytoplasm and appear as minute granules, vesicles, rodlets, threads or strings. They are the centre of aerobic respiration. There are two this membranes of mitochondrion formed of lipids and proteins. Interior of mitochondrion contains blind like organic matrix with a number of chemical compounds. Mitochondria have semi autonomous existence in the cell. They have their own DNA. Mitochondria are called ‘power house of the cell’.
(3) Goolgi apparatus (Dictyosome):
It is canalicular system with sacs and has parallel arranged flattered membrane bound vesicles which lack ribosomes. Golgi was scientist who first found out these bodies in the cytoplasm. Each golgi body is dise shaped and has central flattered plate like compartments called histernae peripheral network of inter connecting tubules and peripherally occurring vesicles and Golgian vauoles. Golgi bodies are especially prominent in glandular cells. Golgi bodies manufacture certain macromolecules by itself.
(4) Lysosomes:
These are spherical bodies, few micrometer in diameter, surrounded by single membrane originated by Golgi apparatus and containing digestive enzymes. They occur only in the cytoplasm of animal cells and function the digestion of material taken into the cell by phagocytosis. Normally they function as destroyers of foreign particles and worn out cellular components. Then the membrane of lysosomes in ruptured the cell undergoes chemical break down or lysis.
(5) Ribosomes:
They are so named because they contain high concentration of Ribonucleic acid (RNA). These small structures are the sites of protein synthesis in all cells. Ribosomes are found freely dispersed in cytoplasm in prokaryotic cells. Ribosomes are composed of 50 or more different kinds of proteins. There are millions of these per cell and are all similar. Ribosomes are regarded as protein factories. Under direction of nucleus they produce protein needed by the cell. Ribosomes are among the most vital cellular components. Recent investigations reveal that ribosomes are manufactured in the nucleolus from where they are transferred to the cytoplasm through nucleopores.
(6) Vacuoles:
They are non protoplasmic liquid filled cavities in the cytoplasm and are surrounded by membrane called tonoplast which permeable; it allows certain substances to enter in the vacuole. In animal cell they are temporarily formed at the time of need.
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