Centrosomes-In animal cells some minute cytoplasmic bodies are universally present near the nucleus called centrosomes.
Centrosomes are also found to occur in cells of some lower plants e.g. Dictyota, Fucus, Yeast cells etc(Sharp, 1943 ;White, 1973), but not yet found in seed plants. In the centre of the centrosome a deeply stained granule is present called centride or centriole. This centride is embedded within a mass of hyaline substance known as centrosphere. During cell division, around the cytoplasm, some radiating rays are formed-these rays are known as astral rays.
Functions- (i) to take part in spindle formation during Cell division and (ii) to help in cilia formation.
Ribosomes-Ribosomes are small, sub-spherical particulate components of the cytoplasm ; they are often found connected to the outside of the endoplasmic reticulum. Ribosomes also occur free in the cytoplasm and in the nucleus, chloroplasts and mitochondria (Cutter, 1978). These organelles have a diameter of about 10=15 nm and are Composed of 15% RNA and 50% protein (mainly histone)--for this composition ribosomes are often called ribo-nucleo protein (RNP) particles.
Functions-Protein synthesis and fatty acid metabolism are generally carried out by ribosomes. Clusters of ribosomes known as polyribosomes or polysomes may be the actual structures associated with protein synthesis.
Spherosomes-spherosomes are small granular organelles which occur free in the cytoplasm and are highly mobile in living cells. Spherosomes have a diameter of about 0.25-1 micron and consist of lipids and protein (Esau, 1965). According to Gunning and Steer (1975), the majority of spherosomes consists of lipid droplets which are not bounded by a membrane. According to some investigator the spherosomes are surrounded by a membrane, whereas according to others a surface skin consisting of an outer layer of oriented lipid molecules is formed in response to aqueous cytoplasm surrounding them (Fahn, 1982).
Spherosomes were called formerly microsomes (Perner 1958), these organelles are probably associated with lipid synthesis and they originate as oil containing vesicles detached from the ER.
Microbodies-Microbodies are small bodies, 0.5-1.5 um in diameter ; microbodies occur in the cytoplasm of a variety of tissues, they are bounded by a single membrane and their matrix appears fibrillar or granular. There are two types of microbodies, such as peroxisomes and glyoxysomes-they differ mainly in the enzymes present in them. Peroxisomes occur in leaves of higher plants, often closely associated with chloroplast envelope ; they contain most of the enzymes for the glycolate pathway from the photosynthetic carbon cycle. Glyoxysomes occur in the storage tissues of fatty seeds, they contain enzymes necessary for the breakdown of fatty acids to acetyl-CoA and the synthesis of succinate from acetyl-CoA.
Microtubules and Microfilaments-Microtubuies are straight, elongated, hollow structures having the diameter 23-27 nm ; these are composed of globular protein sub units called tubulin. Microtubules are present in the peripheral cytoplasm close to cell wall still growing in area and thickness, in mitotic and meiotic spindles, in the phragmoplast arising between the daughter nuclei at the telophase stage etc. It is supposed that the microtubules might be responsible for orientation of developing microfibrils.
Microfilaments occur in some plant cells, especially components of vascular tissues. They are involved in the control of cytoplasmic streaming.
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