Cytoplasm - Its structure and features

CYTOPLASM
The living cytoplasm, where it can be observed in uninjured cells, appears as a colourless, viscous fluid, which frequently shows streaming movements or a circulation round the cell, called cyclosis. This striking move­ment of the living substance has been the subject of a great deal of investiga­tion but no final explanation has emerged. The energy is certainly provided by respiration, and the most probable view of the mechanism is that it is due to rhythmic contraction and relaxation of the protoplasmic structure.
Cytoplasm is not homogeneous in appearance. There is a hyaline fluid medium, which shows no microscopic structure, called the hyaloplasm, in which are suspended immense numbers of minute granules or droplets consisting of ergastic substances, including oil-drops, protein, volutin and lipin granules, and the larger more definitely visible granules known as mitochondria. None of these is an essential component of the cytoplasm itself, which is the hyaloplasm, and they can be centrifuged out of suspension in the hyaloplasm without injuring it. It is in the hyaloplasm, therefore, that the true living structure of protoplasm must be looked for.
The cytoplasm is usually differentiated into zones. Firstly, there is a very thin external layer of hyaloplasm with relatively few granules, which has sometimes been called the ectoplasm, but is more properly described as the hyaline layer, as the term ectoplasm has been diferently employed in Amoeba. In plant cells this hyaline layer is fluid and extremely thin. It is most easily revealed in the form of the very fine extensions which often connect a plasmolyzed protoplast to the pit areas of the cell wall. Thin as it is, it possesses a coherent surface film, probably a monomolecular layer of non-aqueous substance, which is referred to as the protoplasmic membrane, or sometimes the plasmalemma. A similar but tougher membrane forms the boundaries of the vacuoles and is called the tonoplast.
The portion of the cytoplasm which is enclosed by these two layers shows the granular appearance previously mentioned, and is known as the endoplasm. This part of the cytoplasm is differentiated into three com­ponent systems. The outer portion (the ectoplasm of Amoeba) next to the hyaline layer, is elastic and almost solid (geloid) in texture, while the inner portion is definitely fluid and much less cohesive than the outer layer.
When protoplasm is immersed in water the external layer remains coherent, but if this is ruptured the internal portion mixes readily with the water. The external membrane appears to consist of a mixture of lipin and protein molecules, and is probably of molecular thickness, but it is the controlling agent in determining the entry of substances into the cell or, as it is called, the protoplasmic permeability. For example, the dye, Eosine, will not enter a living cell, but if ·it is injected through the proto­plasmic membrane it diffuses readily in the liquid endoplasm.
A third component, called kinoplasm, has been distinguished, con­sisting of a system of very delicate fibrils, only visible with difficulty, but distinguishable by the streams of granules which they carry along. The kinoplasm appears to be connected to both the inner and outer protoplasmic membranes.
It would be a mistake to view these protoplasmic systems as fixed and permanent structures. The most essential characteristic of protoplasm is its changeability, and not only do reversible alterations occur in the different components but probably a continuous exchange of material occurs between them.
The living quality of protoplasm lie's in the interaction of its components, not in the properties of any individual part of it. It is an active system, neither a substance nor simply a mixture of substances, but a chemical machine in which all the components have a part to play which may be more or less essential to the whole.
The vacuoles are part of the cytoplasmic system, though) in a mature plant cell only one large vacuole may be present, occupying tht whole central part of the cell and confining the cytoplasm to a thin peripheral layer, the protoplasmic utricle. In the youngest cells, however, either no vacuoles occur or they are very small, and one of the main features of the development of cells towards maturity is the appearance of vacuoles in the cytoplasm, a process called vacuolization. It is still an open question if vacuoles arise de novo in cells, but the weight of evidence seems to be on the side of the vievv which regards them as always present, even in meristematic cells. Large vacuoles are probably derived chiefly from the growth and fusion of small pre-existing ones. Passing through a phase which suggests a network of canals, possibly due to cytoplasmic streaming movements, these separate vacuoles eventually fuse into larger units or even finally into one. The cell sap, which the vacuoles contain, is an assemblage of practically all the materials which figure in metabolism, from inorganic salts to enzymes and even proteins. Some of these, notably the sugars and organic acids, are highly osmotic, and are the cause of the osmosis by which water is drawn in through the cytoplasm. The entry of water creates a positive pressure in the vacuole, which is called the turgor pressure, by means of which the cell as a whole is kept extended. It is interesting to note that the concentra­tions of some inorganic salts in the vacuole may be different from that outside, showing that the function of the protoplasm in absorption is by no means merely passive.
The term mitochondria is applied to granular elements found in the cytoplasm, some of which are relatively large or filamentous, and readily recognizable even in living cells. Granules of all sizes occur, however, and it is not certain where the limit should be drawn, or whether all particles down to visible limits should be included under this name. There is in cytoplasm a certain class of granules, or short rods with certain chemical properties in common, which may be regarded as true mitochondria. They disintegrate in highly acid or alkaline solutions, and consist of a mixture or compound of phospho-lipins and nucleo-proteins. The nucleic acid in­volved here is, however, not the same as that in the nucleus itself, as described later, but is ribo-nucleic acid, a cytoplasmic material, which may also occur free in the form of granules, named originally volutin grains.
The universal occurrence of mitochondria suggests that they have some metabolic function, but although theories have been plentiful, facts are scarce and no definite statement can yet be made on the subject.
The plastids are persistent cell organs, which appear to arise from minute granules present in the youngest cells and ultimately derived from the oosphere. These pro-plastids have often been confused with mitochondria, and whether they are distinct structural elements is still uncertain. The mature chloroplast has been the subject of intensive research on account of its importance in photosynthesis. The variability of its form in the Algae is as striking as is its constancy of character in all the higher plants. This constant form is roughly that of a disc, like a watch, with convex faces, which seems to have proved so efficient that it has remained unchanged through vast periods of evolution. Fully grown chloroplasts multiply in the cells by a process of division. The plastid elongates and a median constriction or waist appears, which narrows until the two halves are completely separated. The plastid is protoplasmic in nature, though of gel consistency, and there is evidence that it has a semipermeable surface membrane. The protoplasmic body or stroma is colourless, but in chloroplasts it includes numerous small bodies, called grana, composed of protein and lipoid, which contain the chlorophyll. Where these are large and not too closely packed, as in many Mosses, they may be seen as green spots, giving the plastid a mottled appearance.
In the Algae the chloroplasts may contain a protein body called the pyrenoid, around which the starch grains are formed during photo­synthesis.
Leucoplasts are similar in development and structure but lack chlorophyll. Some at least, however, form chlorophyll when they are exposed to light, as in potato tubers. Whether the plastids are in fact permanent cell structures cannot yet be decided. In some plants, notably ~1osses, they can be traced through the reproductiye cells from one generation to the next, but in most cases the idea of permanence rests on supposition, as the pro-plastids cannot be adequately distinguished from other cell granules.

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