Somatic nucleated cells are totipotent or able to produce a complete organism. This is especially so in case of plant cells. It was first suggested by Haberlandt (1902) and proved by Steward et al in 1957, through tissue or cell culture experiments. For its survival and proper functioning, a cell always requires:
(i) Flow of Energy for overcoming entropy (tendency for disorderliness) and performing various life activities.
(ii) Flow of Information which is of two types, intrinsic or genetic and extrinsic through nerves and hormones. Intrinsic or genetic information is contained in DNA. Specific triggers activate DNA segments to transcribe m RNAs which then help synthesise specific polypeptides/proteins/enzymes, etc. Stimulated nerves secrete chemicals to activate certain cells. Hormones are recognised by either membrane receptors or intracellular receptors. The complexes allow particular activities.
Surface Volume Ratio:
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Metabolically active cells are small since small cells have higher nucleo- cytoplasmic ratio for better control and higher surface- volume ratio for quicker exchange of materials: Larger cells can be efficient only if they are elongated, branched or possess membrane extensions like microvilli. Large sized animals do not possess large sized cells but more numerous small cells.
Size & Shape of Cells:
The diameter of most of the cells ranges from 0.5 n to 20 n (1µ = 0.001 mm). The largest known cell is the egg of an Ostrich which can be as long as 170 mm. The smallest cell is perhaps yet to be discovered. The shape also is varied and depends upon the environmental conditions; surroundings function and need of the organism. Simple bacteria can be rod shaped, spiral, oval, round etc.
The shapes of desmids and diatoms are varied. Amoeba keeps on changing its shape. The body of unicellular marine alga, Acetabularia is differentiated into a rhizoid like base, a long and slender stalk and an umbrella like cap. The RBCs of human beings are round, whereas, the muscle cells are long and with pointed ends. Animals cells are capable of changing their shapes; but the shape of plant cell is fixed due to the presence of cellulose cell-wall.
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Nerve cells are the longest cells of the human body, reaching 90cm. Certain plant fibres are similarly very long, e.g. Jute (30-90 cm); Ramie (55 cm), Hemp dm).
Prokaryotic and Eukaryotic Cells:
Basically cells are of two types, procaryotic and eucaryotic.
A procaryotic cell has one envelope system with no membrane lined internal organelles, except thylakoids, if present. A nucleus is absent, instead, a nucleoid made of naked and folded DNA lies freely in cytoplasm. Internal compartments are absent. Respiratory enzymes are located on the cell membrane. Cytoplasm does not show streaming. Sap vacuoles are absent. Gas vacuoles may occur-Cell wall, if present, contains peptidoglycan or muco- peptide. Flagella, if present are single stranded and made of flagellin.
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An eucaryotic cell has internal compartments and double envelope system, that is, besides cell membrane, the cell organelles are also covered by membranes. Mitosis and meiosis occurs in the life cycle. Cytoplasm show streaming, sap vacuoles occur. Flagella if present are double-stranded and made of tubulin.