The cytoplasm is filled with ribosomes, as many as 15,000 per E. coli cell. These ribosomes are 180- diameter spheres, composed of half protein and half RNA. They are built from two unequal parts, with molecular weights 1,800,000 and 900,000, and are designated "70 S" ribosomes from their sedimentation behavior in the ultracentrifuge.

Ribosomes in eucaryotes are 35% larger and are termed "80 S" ribosomes. They have a 200- diameter, and are made from two pieces with molecular weights 2,400,000 and 1,200,000. Mitochondria and chloroplasts in eucaryotic cells also have ribosomes of their own for protein synthesis, but these ribosomes are smaller, like those of bacteria. This is one of the many pieces of evidence that suggests an ancient bacterial origin for these eucaryotic organelles.

The cytoplasm also contains storage granules filled with glycogen (or starch), lipids such as poly--hydroxybutyric acid, and polymetaphosphate (endless chains of linked phosphate tetrahedra). All of these compounds are means of storing energy in bacteria.


Right: Electron micrograph of ribosomes.