Localization of Ribosomes in the Cell

Ribosomes are abundant in cells involved in intense protein synthesis. In the bacterial cell they are dispersed throughout the protoplasm and account for about 30%, sometimes even more, of its dry weight. In electron micrographs all the intracellular space, except nucleoid (DNA) regions, looks stuffed with ribosomes (Fig. 4.1). About 104 ribosomes, on average, are present in one bacterial cell.

The relative content (concentration) of ribosomes in eukaryotic cells is lower; here, the number of

Figure 4.1. Electron micrograph of ribosomes on an ultra-thin section of a bacterium Vibrio alginolyticus. The cells are fixed with osmium tetraoxide. Ribosomes look as the abundant granular material filling in the cytoplasm. (Courtesy of L.Ye. Bakeyeva, Moscow State University).

Figure 4.2. Electron micrograph of ribosomes on an ultra-thin section of a rat liver cell. Fixation with glutaraldehyde. Ribosomes on the membranes of rough endoplasmic reticulum, as well as some clusters of free ribosomes, are seen. (Courtesy of Yu.S. Chentsov, Moscow State University).

ribosomes varies considerably depending on the protein-synthesizing activity of the corresponding tissue or individual cell. Most of the ribosomes are found in the cytoplasm. In the cells with an active protein secretion and a developed network of endoplasmic reticulum, a marked proportion of cytoplasmic ribosomes are attached to the endoplasmic reticulum membrane, specifically to the surface facing the cytoplasmic matrix (Fig. 4.2). The ribosomes are distributed non-uniformly on the reticulum: they may be abundant in one part and virtually nonexistent in others. These ribosomes synthesize proteins which are directly transported into the membrane lumen for subsequent secretion. Protein synthesis for "house-keeping" purposes inside the cell takes place primarily upon the free cytoplasmic ribosomes that are not associated with the membrane but are scattered in the cytoplasmic matrix. That is why the cytoplasm of embryonic, non-differentiated, rapidly growing or proliferating cells contains mainly free ribosomes.

The formation of all ribosomes present in the cytoplasmic matrix, both membrane-bound and free ones, takes place in the nucleolus of the eukaryotic cell, and ribosomes can naturally also be detected in this compartment of the cell nucleus; it is thought, however, that nucleolar ribosomes are not active in protein synthesis.

In addition, the eukaryotic cell contains different populations of ribosomes in such intracellular organelles as mitochondria and, in the case of plant cells, chloroplasts. Ribosomes of these organelles differ from cytoplasmic ribosomes in that they are slightly smaller and have a different chemical composition and different functional characteristics. These ribosomes are formed directly in the organelles.

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