The dense cytosol is bounded by the cell membrane. It appears granular because it is densely packed with ribosomes, which are much more abundant than in the cytoplasm of eukaryotic cells. This is a reflection of the higher growth rate of bacteria. Each ribosome is a ribonucleoprotein particle consisting of three species of rRNA (5 S, 16 S, and 23 S) and about 56 proteins. The overall subunit structure (one 50 S plus one 30 S particle) of the 70 S bacterial ribosome resembles that of eukaryotic ribosomes (which are 80 S, com-posed of one 60 S and one 40 S particle), but is smaller and differs sufficiently in function that a very large number of antimicrobics have the prokaryotic ribosome as their target.
The number of ribosomes varies directly with the growth rate of the cell . At all but the slowest growth rates about 70% of the ribosomes at any one time exist as polysomes and are engaged in translating mRNA. Except for the functions associated with the cell membrane, all of the metabolic reactions of the cell take place in the cytosol. Accordingly, it is found to be the major location of a great fraction of the 2000 to 3000 different enzymes of the cell. The cytosol of some bacterial species also contains nutri-tional storage granules called reserve granules. The most prevalent kinds consist of glycogen or polymetaphosphate. Their presence and abundance depend on the nutritional state of the cell.