An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 31

When stained properly, the area occupied by a prokaryotic cell's DNA can be easily seen.
Prokaryotes may also have other visible structures when viewed under the microscope, such
as ribosomes, storage granules, spores, and volutins. Ribosomes are the site of protein synthesis.
A typical bacterial cell contains approximately 10,000 ribosomes per cell, although this
number can vary greatly with growth rate. The size of a typical ribosome is 10 e 20 nm and
consists of approximately 63% RNA and 37% protein. Storage granules (which are not
present in every bacterium) can be used as a source of key metabolites and often contain
polysaccharides, lipids, and sulfur granules. The sizes of storage granules vary between
0.5 and 1 m m. Some bacteria make intracellular spores (often called endospores in bacteria).
Bacterial spores are produced as a resistance to adverse conditions such as high temperature,
radiation, and toxic chemicals. The usual concentration is 1 spore per cell, with a spore size of
about 1 m m. Spores can germinate under favorable growth conditions to yield actively
growing bacteria.
Volutin is another granular intracellular structure, made of inorganic polymetaphos-
phates, that is present in some species. Some photosynthetic bacteria, such as Rhodospirillum,
have chromatophores that are large inclusion bodies (50 e 100 nm) utilized in photosynthesis
for the absorption of light.
Extracellular products can adhere to or become incorporated within the surface of the cell.
Certain bacteria have a coating or outside cell wall called capsule, which is usually a polysac-
charide or sometimes a polypeptide. Extracellular polymers are important to biofilm forma-
tion and response to environmental challenges (e.g. viruses). Table 2.3 summarizes the
architecture of most bacteria.
2.1.5.2. Archaebacteria
Under the microscope, archaebacteria appear to be nearly identical to eubacteria.
However, these cells differ greatly at the molecular level. In many ways, the archaebacteria
are as similar to the eukaryotes as they are to the eubacteria. Some examples of differences
between archaebacteria and eubacteria are as follows:
(1) Archaebacteria have no peptidoglycan.
(2) Nucleotide sequences in ribosomal RNA are similar within the archaebacteria but distinctly
different from those of eubacteria.
(3) The lipid composition of the cytoplasmic membrane is very different for the two groups.
Archaebacteria usually live in extreme environments and possess unusual metabolism.
Methanogens, which are methane-producing bacteria, belong to this group, as well as ther-
moacidophiles, which can grow at high temperatures and low pH values. Halobacteria,
which can live only in very strong salt solutions, are also members of this group. These
organisms are important sources for catalytically active proteins (enzymes) with novel
properties.
2.1.6. Eukaryotes
Fungi (yeasts and molds), algae, protozoa, and animal and plant cells constitute the
eukaryotes. Eukaryotes are five to ten times larger than prokaryotes in diameter (e.g. yeast
about 5 m m, animal cells about 10 m m, and plants about 20 m m). Eukaryotes have a true