Rudiments of Prokaryotic Cell Structure
A typical
prokaryote, E. coli, is a rod capped with hemispheres (Fig. 1.1). It is 1–3 µ (10-4 cm = 1 µ = 104 Å) long and
0.75 µ in
diameter. Such a
The cell
envelope consists of three parts, an inner and outer mem-brane and an
intervening peptidoglycan layer (Fig. 1.2). The outer surface of the outer
membrane is largely lipopolysaccharides. These are attached to lipids in the
outer half of the outer membrane. The polysac-charides protect the outer
membrane from detergent-like molecules found in our digestive tract.outer
membrane The outer membrane also consists of matrix proteins that form pores
small enough to exclude the detergent-like bile
salts, but large enough to permit passage of small molecules and phospholipids.
Figure
1.3 Structure of the cell wall
showing the alternating N-acetylglu-cosamine N-acetylmuramic acid units. Each
N-acetylmuramic acid possesses a peptide, but only a few are crosslinked in E. coli.
The major
shape-determining factor of cells is the peptidoglycan layer or cell wall (Fig.
1.3). It lies beneath the outer membrane and is a single molecule containing
many polysaccharide chains crosslinked by short peptides (Fig. 1.4). The outer
membrane is attached to the pepti-doglycan layer by about 106
lipoprotein molecules. The protein end of each of these is covalently attached
to the diaminopimelic acid in the peptidoglycan. The lipid end is buried in the
outer membrane.
The
innermost of the three cell envelope layers is the inner or cytoplasmic
membrane. It consists of many proteins embedded in a phospholipid bilayer. The
space between the inner membrane and the outer membrane that contains the
peptidoglycan layer is known as the periplasmic space. The cell wall and
membranes contain about 20% of the cellular protein. After cell disruption by
sonicating or grinding, most of this protein is still contained in fragments of
wall and membrane and can be easily pelleted by low-speed centrifugation.
The
cytoplasm within the inner membrane is a protein solution at about 200 mg/ml,
about 20 times more concentrated than the usual cell-free extracts used in the
laboratory. Some proteins in the cytoplasm may constitute as little as 0.0001%
by weight of the total cellular protein whereas others may be found at levels
as high as 5%. In terms of concentrations, this is from 10-8 M to 2 × 10-4 M, and in a
bacterial cell this is from 10 to 200,000 molecules per cell. The
concentrations of many of the proteins vary with growth conditions, and a
current re-search area is the study of the cellular mechanisms responsible for
the variations.
Figure 1.4 Structure
of the peptide crosslinking N-acetylmuramic acid units. DAP is diaminopimelic
acid
The majority of the more than 2,000 different types of proteins found within a bacterial cell are located in the cytoplasm. One question yet to be
answered about these proteins is how they manage to exist in the cell without
adhering to each other and forming aggregates since polypep-tides can easily
bind to each other. Frequently when a bacterium is engineered for the
over-synthesis of a foreign protein, amorphous precipitates called inclusion
bodies form in the cytoplasm. Sometimes these result from delayed folding of
the new protein, and occasionally they are the result of chance coprecipitation
of a bacterial protein and the newly introduced protein. Similarly, one might
also expect an occasional mutation to inactivate simultaneously two apparently
unre-lated proteins by the coprecipitation of the mutated protein and some
other protein into an inactive aggregate, and occasionally this does occur.
The
cell’s DNA and about 10,000 ribosomes also reside in the cyto-plasm. The ribosomes
consist of about one-third protein and two-thirds RNA and are roughly spherical
with a diameter of about 200 Ã…. The DNA in the cytoplasm is not surrounded by a
nuclear membrane as it is in the cells of higher organisms, but nonetheless it
is usually confined to a portion of the cellular interior. In electron
micrographs of cells, the highly compacted DNA can be seen as a stringy mass
occupying about one tenth of the interior volume, and the ribosomes appear as
granules uniformly scattered through the cytoplasm.
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