CYTOPLASM, PROKARYOTIC

The cytoplasm of a prokaryotic cell is everything that is present inside the bacterium. In contrast to a eukaryotic cell, there is not a functional segregation inside bacteria. The cytoplasm houses all the chemicals and components that are used to sustain the life of a bacterium, with the exception of those components that reside in the membrane(s), and in the periplasm of Gram-negative bacteria.

The cytoplasm is bounded by the cytoplasmic membrane. Gram-negative bacteria contain another outer membrane. In between the two membranes lies the periplasm.

When viewed in the light microscope, the cytoplasm of bacteria is transparent. Only with the higher magnification available using the transmission electron microscope does the granular nature of the cytoplasm become apparent. The exact structure of the cytoplasm may well be different than this view, since the cytoplasm is comprised mainly of water. The dehydration necessary for conventional electron microscopy likely affect the structure of the cytoplasm.

The cytoplasm of prokaryotes and eukaryotes is similar in texture. Rather than being a free-flowing liquid the cytoplasm is more of a gel. The consistency has been likened to that of dessert gel, except that the bacterial gel is capable of flow. The ability of flow is vital, since the molecules that reside in the cytoplasm must be capable of movement within the bacterium as well as into and out of the cytoplasm.

The genetic material of the bacteria is dispersed throughout the cytoplasm. Sometimes, the deoxyribonucleic acid genome can aggregate during preparation for microscopy. Then, the genome is apparent as a more diffuse area within the granular cytoplasm. This artificial structure has been called the nucleoid. Smaller, circular arrangements of genetic material called plasmids can also be present. The dispersion of the bacterial genome throughout the cytoplasm is one of the fundamental distinguishing features between prokaryotic and eukaryotic cells.

Also present throughout the cytoplasm is the ribonucleic acid, various enzymes, amino acids, carbohydrates, lipids, ions, and other compounds that function in the bacterium. The constituents of the membrane(s) are manufactured in the cytoplasm and then are transported to their final destination.

Some bacteria contain specialized regions known as cytoplasmic inclusions that perform specialized functions. These inclusions can be stored products that are used for the nutrition of the bacteria. Examples of such inclusions are glycogen, poly-B-hydroxybutyrate, and sulfur granules. As well, certain bacteria contain gas-filled vesicles that act to buoy the bacterium up to a certain depth in the water, or membranous structures that contain chlorophyll. The latter function to harvest light for energy in photosynthetic bacteria.

The cytoplasm of prokaryotic cells also houses the ribosomes required for the manufacture of protein. There can be many ribosomes in the cytoplasm. For example, a rapidly growing bacterium can contain upwards of 15,000 ribosomes.

The processes of transcription, translation, protein import and export, and at least some degradation of compounds occurs in the cytoplasm. In Gram-negative bacteria, some of these functions also occur in the periplasmic fluid. The mechanisms that underlie the proper sequential orchestration of these functions are still yet to be fully determined.