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Structure and Function of Bacterial Cells (page 9)

(This chapter has 10 pages)

© Kenneth Todar, PhD

The Cytoplasm

The cytoplasm of bacterial cells consists consists of an aqueous solution of three groups of molecules: macromolecules such as proteins (enzymes), mRNA and tRNA;  small molecules that are energy sources, precursors of macromolecules, metabolites or vitamins;  and various inorganic ions and cofactors (see Tables 9, 10,11). The primary structural components found in the cytoplasm are the nucleoid and ribosomes, and possibly some type of inclusion. The cytoplasm of procaryotes is more gel-like than that of eucaryotes and the processes of cytoplasmic streaming, which are evident in eucaryotes, do not occur.

Table 9. Molecular composition of E. coli under conditions of balanced growth. Percentage of  dry weight refers to all structural and  cytoplasmic components.
Molecule Percentage of dry weight

Total RNA 






Small molecules: precursors, metabolites, vitamins, etc.

Inorganic ions 

Total dry weight 











Table 10. Small molecules present in the cytoplasm of a growing bacterial cell.
Molecule Approximate number of kinds
Amino acids, their precursors and derivatives 

Nucleotides, their precursors and derivatives 

Fatty acids and their precursors 

Sugars, carbohydrates and their precursors or derivatives

quinones, porphyrins, vitamins, coenzymes and prosthetic groups and their precursors 






Table 11. Inorganic ions present in the cytoplasm of a growing bacterial cell. 
Ion  Function
K+ Maintenance of ionic strength; cofactor for certain enzymes
NH4+ Principal form of inorganic N for assimilation
Ca++ Cofactor for certain enzymes
Fe++ Present in cytochromes and other metalloenzymes
Mg++ Cofactor for many enzymes; stabilization of outer membrane of Gram-negative bacteria
Mn++ Present in certain metalloenzymes
Co++ Trace element constituent of vitamin B12 and its coenzyme derivatives and found in certain metalloenzymes
Cu++ Trace element present in certain metalloenzymes
Mo++ Trace element present in certain metalloenzymes
Ni++ Trace element present in certain metalloenzymes
Zn++ Trace element present in certain metalloenzymes
SO4-- Principal form of inorganic S for assimilation
PO4---  Principal form of P for assimilation and a participant in many metabolic reactions

The bacterial chromosome (nucleoid) is typically one large circular molecule of DNA, more or less free in the cytoplasm, although coiled and supercoiled and anchored by proteins. Procaryotes sometimes possess smaller extrachromosomal pieces of DNA called plasmids. The total DNA content of a procaryote is referred to as the cell genome. The cell chromosome is the genetic control center of the cell which determines all the properties and functions of the bacterium. During cell growth and division, the procaryotic chromosome is replicated in a semiconservative fashion to make an exact copy of the molecule for distribution to progeny cells.
However, the eucaryotic processes of meiosis and mitosis are absent in procaryotes. Replication and segregation of procaryotic DNA is coordinated by the membrane and various proteins in the cytoplasm.

Figure 25. When a bacterium such as E. coli is "gently lysed" the chromosomal DNA  leaks out of the cell as a continuous molecule that is many times longer than the length of the cell.

The distinct granular appearance of procaryotic cytoplasm is due to the presence and distribution of ribosomes. Ribosomes are composed of proteins and RNA. The ribosomes of procaryotes are smaller than cytoplasmic ribosomes of eucaryotes. Procaryotic ribosomes are 70S in size, being composed of 30S and 50S subunits. The 80S ribosomes of eucaryotes are made up of 40S and 60S subunits. Ribosomes are involved in the process of translation (protein synthesis), but some details of their activities differ in eucaryotes, bacteria and archaea. The 70S ribosomes that occur in eucaryotic mitochondria and chloroplasts contain ssrRNA closely related to bacterial ribosomal RNA. his is taken as a major line of evidence that these organelles are descended from procaryotes.

Figure 26. The bacterial chromosome or nucleoid is the nonstaining region in the interior of the cell cytoplasm. The granular structures distributed throughout the cytoplasm are cell ribosomes.


Often contained in the cytoplasm of procaryotic cells is one or another of some type of inclusion granule. Inclusions are distinct granules that may occupy a substantial part of the cytoplasm. Inclusion granules are usually reserve materials of some sort. For example, carbon and energy reserves may be stored as glycogen (a polymer of glucose) or as polybetahydroxybutyric acid (a type of fat) granules. Polyphosphate inclusions are reserves of PO4 and possibly energy; elemental sulfur (sulfur globules) are stored by some phototrophic and some lithotrophic procaryotes as reserves of energy or electrons. Some inclusion bodies are actually membranous vesicles or intrusions into the cytoplasm which contain photosynthetic pigments or enzymes.

Table 12. Some inclusions in bacterial cells.
Cytoplasmic inclusions Where found Composition Function
glycogen many bacteria e.g. E. coli polyglucose reserve carbon and energy source
polybetahydroxybutyric acid (PHB) many bacteria e.g. Pseudomonas polymerized hydroxy butyrate reserve carbon and energy source
polyphosphate (volutin granules) many bacteria e.g. Corynebacterium linear or cyclical polymers of PO4 reserve phosphate; possibly a reserve of high energy phosphate
sulfur globules phototrophic purple and green sulfur bacteria and lithotrophic colorless sulfur bacteria elemental sulfur reserve of electrons (reducing source) in phototrophs; reserve energy source in lithotrophs
gas vesicles aquatic bacteria especially cyanobacteria protein hulls or shells inflated with gases buoyancy (floatation) in the vertical water column
parasporal crystals endospore-forming bacilli (genus Bacillus) protein unknown but toxic to certain insects
magnetosomes certain aquatic bacteria magnetite (iron oxide) Fe3O4  orienting and migrating along geo- magnetic field lines
carboxysomes many autotrophic bacteria enzymes for autotrophic CO2 fixation site of CO2 fixation
phycobilisomes cyanobacteria phycobiliproteins light-harvesting pigments
chlorosomes Green bacteria lipid and protein and bacteriochlorophyll light-harvesting pigments and antennae

Figure 27. A variety of bacterial inclusions.  a. PHB granules; b. a parasporal BT crystal in the sporangium of Bacillus thuringiensis; c. carboxysomes in Anabaena viriabilis, showing their polyhedral shape; d. sulfur globules in the cytoplasm of Beggiatoa

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Kenneth Todar has taught microbiology to undergraduate students at The University of Texas, University of Alaska and University of Wisconsin since 1969.

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