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 The cells of all living organisms consist of protoplasm, which is bound by a cell membrane. Prokaryotic cells and cells of algae, fungi and higher plants contain a cell wall outside the cell membrane. Some protozoans cells show the presence of a cell coat outside the cell membrane. |
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 The cell wall is rigid and as such maintains shape and size of the cell, apart from providing mechanical support. It is porous and hence, freely permeable. |
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 In prokaryotic cells, cell wall is composed of amino sugars; in fungi, it is composed of chitin and in higher plants it is composed of cellulose. The cell coat is generally composed of glycoproteins. |
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 Protoplasm is represented by cytoplasm and nucleus. Cytoplasm is jelly-like homogenous ground substance called cytosol. It encloses living inclusions called cell organelles and nonliving inclusions called ergastic substances. |
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 Ergastic substances are represented by various kinds of chemical substances represented by storage products (reserve food) or secretory products (gums, alkaloids, hormones, etc) or excretory products (calcium carbonate and other salts). These substances are generally stored in the vacuoles. |
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 Cell organelles are mainly of two types membrane-bound and non-membranous. Membrane bound organelles include endoplasmic reticulum, golgi complex, mitochondria, chloroplasts, lysosomes. Non-membranous organelles include ribosomes, centrosomes, microbodies, microtubules and microfilaments. |
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 Endoplasmic reticulum (ER) is a branched membranous network spread all over the cell, connecting at places, the cell membrane and nuclear membrane also. It is of two types smooth ER, without ribosomes and rough ER, with ribosomes. ER functions mainly as an intracellular transport channel. |
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 Mitochondria are oval and rod shaped organelles formed by a double membrane. The inner membrane is thrown into projections called cristae, which contain numerous tiny particles. These particles enclose the enzyme complexes responsible for oxidation. |
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 Plastids are spherical or oval cell organelles found only in plant cells. Plastids are of two types: leukoplasts without any colouring pigments and chromoplasts containing colouring pigments. The most familiar chromoplasts are the chloroplasts containing chlorophyll. |
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 Lysosomes are organelles bound by a single membrane, found only in animal cells. Lysosomes enclose lytic enzymes, which can dissolve old and worn-out cell organelles, and if necessary the entire cell itself. |
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 Golgi complex is an organelle consisting of membranous cisternae, tubules and vesicles. It is concerned with modifying, sorting and packaging of cell products into secretory vesicles. |
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 Ribosomes are non-membranous cell organelles usually found on the surface of membranes of ER, as well as in the nucleolus inside the nucleus. Each ribosome particle has 2 sub units, which join together at the time of protein synthesis. |
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 Centrioles are short-paired cylindrical structures found near the nucleus. They show a (9+o) arrangement of microtubules. They are involved in the formation of spindle fibres during cell division and the basal bodies of cilia and flagella. |
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 Cilia and flagella are found in the surface of motile cells. They show a (9+2) arrangement of microtubules. These structures help in locomotion. |
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 The cytoskeleton is composed of structures called microtubules, microfilaments and microbodies. Microtubules are composed of tubulin protein while microfilaments are composed of actin and myosin. Microbodies enclose a set of enzymes. |
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 Vacuoles are membrane-bound spaces, which enclose sap (water and mineral salts) or ergastic substances. In animal cells vacuoles are small and helps in osmoregulation. |
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 Nucleus is the most conspicuous organelle of the cell. A cell may be uninucleate or binucleate or multinucleate. |
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 Nucleus is bound by a porous nuclear membrane that encloses a nucleoplasm. It contains a chromatin network and a nucleolus. |
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 Chromatin network is formed by the long and coiled DNA molecule and histone proteins. The chromatin network condenses into chromosomes at the time of cell division. |
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 Nucleolus is composed of ribosomes and RNA. |
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 Nucleus is the controlling centre of the cell. It is also involved in the transmission of hereditary characters. |
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