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
Right: Electron micrograph of ribosomes.