Sub-cellular architecture

Apart from the division of labour brought about by cell differentiation, and the grouping of similar cells into organs, a further division exists at the sub-cellular level. In this way, the many conflicting chemical reactions that take place simultaneously inside cells, achieve the required isolation in specialized compartments formed of selectively permeable membranes. The importance of these membranes is indicated by the fact that they comprise about 80% of the dry weight of an animal cell (O Brien, 1967). Our present understanding of sub-cellular structure stems from the adaptation of the electron microscope to this task by Albert Claude in 1940. 

nucleolus M, mitochondrion ER, endoplasmic reticulum to which ribosomes are attached. 

Such differences in comparative cytology can provide a basis for the exercise of selectivity in drugs and other agents. Nowhere is this more evident than in bacteria whose small size, compared to eukaryotic cells, leaves no space for a nucleus or even one mitochondrion. In place of a nucleus, the DNA is gathered into a single chromosome, laced through the plasma membrane in place of 

Even in mammals, selectivity between the various tissues is possible because of the great specialization in shape and structure of the cells, and this is true also of the cells ultrastructure. Nuclei from the various human tissues differ so much in appearance that forensic pathologists have long based an entire system of tissue identification on these differences. Mammalian mitochondria, too, differ from organ to organ (Section 5.4.3). Such cytological differences (i.e. between two tissues in the one organism) could be used in therapy. 

The remainder of this chapter outlines the comparative morphology and function of organelles. For further reading on the division of labour inside cells, see Bourne (1970) for a descriptive atlas of the structure of cells and tissues, see Porter and Bonneville (1973). 

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