Essential structures comprised in a plant cell

Essential Structures in a Plant Cell

The essential structures comprised in a plant cell may be summarized as follows:- 

1. The Cell Wall, basically composed of cellulose but often chemically altered by the incorporation of other materials. This encloses a space, the Cell Lumen, in which the following occur. 
2. The Cytoplasm, which includes all the protoplasm outside the nucleus, with certain differentiated structures :- 

The Plasma Membrane, an external surface layer in contact with the wall. 

The Vacuolar Membrane or Tonoplast, bounding the vacuoles. 

Embedded in the cytoplasm are the following structures :- 

• (a) The Plastids, which are protoplasmic bodies, denser than the cytoplasm and not separated from it by any definite membrane. They comprise :- 
• Chloroplasts, the bearers of the green pigment, Chlorophyll. 
• Chromoplasts, with colours other than green (the term is sometimes used to include all coloured plastids). eucoplasts, which are colourless and found chiefly in underground organs and in the meristem cells. 
• (b) The Vacuoles, sac-like enclosures in the cytoplasm, filled with a liquid" cell-sap." 
• (c) The Mitochondria or Chondriosomes, granules, rods or threads apparently composed of phospholipins and proteins, which are found scattered through the cytoplasm of all cells. 
• (d) The Centrosomes, granules associated with nuclear division, which are characteristic of animals, but are found only in a few of the lower plants. 
• (e) The Ergastic Substances, or materials secreted by the cytoplasm either as food materials or as by-products. Here are included such things as oil-drops, protein grains or crystalloids, starch grains (in the plastids) and crystals of Calcium oxalate. 

3. The Nucleus, almost always single in the cell and composed of the following parts ;- 

• (a) The Nuclear Membrane, separating it from the cytoplasm. (b) The Chromatin, which is organised as :- 
• The Nuclear Reticulum, formed of fine threads, which is characteristic of the non-dividing or " metabolic" nucleus. 
• The Chromosomes, which are relatively thick rods, formed from the nuclear reticulum during nuclear diyision and con­stant both in number and form. Chromatin is a compound of nucleic acid with basic protein. 
• (c) The Nuclear Sap, or Karyolymph, which is colourless and fills all the central parts of the nucleus. It may also contain ergastic reserves, e.g., protein crystals. 
• (d) The Nucleolus, one or more in each nucleus, which is a spherical granule of material, attached to the reticulum and associated with certain chromosomes. It consists of a mixture of protein and lipin, and normally stains differently from the chromatin. 

A cell in a higher organism cannot be looked upon as an independent unit. There is a considerable degree of physiological unity pervading all tissues and the cell must be largely controlled by the functioning of the tissues as a whole. Although itself a synthesis of many smaller components, as we haye seen aboye, it plays a part in the synthesis of a still higher unit, the organism, to which it is subordinate. Modern studies no longer treat the 

cell as a static object but as a functioning mechanism, and the growth of experimental cytology, including the micro-dissection of cells, has opened roads of the highest promise towards a fuller understanding of life processes. 

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