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Chapter – 8 Cell — Structure and Functions Cell: The smallest structural and functional unit of anorganism, which istypically
microscopic andconsists of cytoplasm and a nucleus enclosed in amembrane. All organisms aremade of smaller parts called organs. Organs are madeof still smaller parts. The smallest living part of an organism is a ‘cell’. Cells were first observed in cork by Robert Hooke in 1665. Cells exhibit variety ofshapes and sizes. Number of cells also varies fromorganism to organism. Some cells are big enough to be seen with the unaided eye. Hen’s egg is an example. Some organisms are single-celled, while others contain large number of cells. The single cell of unicellular organisms performs all the basic functions performed by a variety ofcells in multicellular organisms. The cell hasthree main parts, (i) the cellmembrane, (ii) cytoplasm which contains smaller components called organelles, and(iii) the nucleus. Cell membrane: The basiccomponent of a cell. Thecytoplasm and nucleus are enclosed within cell membrane. There is an outer thick layer in cells of plants called cell wall. Cytoplasm: The jelly-like substance present between thecell membrane andthe
nucleus. Various organelles present in thecytoplasm are: (i) Mitochondria (ii) Plastids (present onlyin plant cell) (iii) Endoplasmic reticulum (ER) (iv) Ribosome (v) Lysosomes (vi) Vacuole (vii) Golgi body (viii) Centrosome (present onlyin animal cell). Nucleus: Nucleus is separated from cytoplasm by a nuclear membrane. It is generally spherical in the centre of the cell. Nuclear membrane: Nucleus is separated from the cytoplasm by a membrane called the nuclear membrane. Nucleolus: Nucleus contains a still smaller round body known as nucleolus. Chromosomes: Nucleus contains thread-like structures called chromosomes. These carry genes andhelp in inheritance or transfer ofcharacter from theparents to theoff springs Cells without wellorganised nucleus, i.e.lacking nuclear membrane, are called prokaryotic cells. Plant cells differ from animal cellsin having an additional layeraround the cellmembrane termed cellwall. Coloured bodies called plastids are found in the plantcells only. Greenplastids containing chlorophyll are called chloroplasts. Plant cell has a big central vacuole unlike a number of small vacuoles in animal cells. Size of cells: The size may be as small as a millionths of a metreor may be as largeas a fewcentimetres. Size ofthe cells hasno relation withthe size ofthe body ofthe animal or plant. It is related to its function.
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