Environmental Engineering Reference
In-Depth Information
of the wall weight. In
Escherichia coli
, the Gram-negative walls are about 1 µm
thick and contain only one or two layers of peptidoglycan. The outer mem-
brane lies outside the thin peptidoglycan layer and is essentially a lipopro-
tein bilayer. The outer membrane and peptidoglycan are so firmly linked by
this lipoprotein that they can be isolated as one unit.
Plasma Membrane (Cytoplasmic Membrane)
Surrounded externally by the cell wall and composed of a lipoprotein com-
plex, the
plasma membrane
is the critical barrier separating the inside from
the outside of the cell. About 7 to 8 nm thick and comprising 10 to 20% of
the dry weight of a bacterium, the plasma membrane controls the passage of
all material into and out of the cell. The inner and outer faces of the plasma
membrane are embedded with water-loving (hydrophilic) lips, whereas the
interior is hydrophobic. Control of material into the cell is accomplished by
screening, as well as by electric charge. The plasma membrane is the site of
the surface charge of the bacteria. In addition to serving as an osmotic barrier
that passively regulates the passage of material into and out of the cell, the
plasma membrane participates in the active transport of various substances
into the bacterial cell. Inside the membrane, many highly reactive chemical
groups guide the incoming material to the proper points for further reac-
tion. This active transport system provides bacteria with certain advantages,
including the ability to maintain a fairly constant intercellular ionic state in
the presence of varying external ionic concentrations. In addition to partici-
pating in the uptake of nutrients, the cell membrane transport system par-
ticipates in waste excretion and protein secretions.
Cytoplasm
Within a cell and bounded by the cell membrane is a complicated mixture
of substances and structures called the
cytoplasm
. The cytoplasm is a water-
based fluid containing ribosomes, ions, enzymes, nutrients, storage gran-
ules (under certain circumstances), waste products, and various molecules
involved in synthesis, energy metabolism, and cell maintenance.
Mesosome
A common intracellular structure found in the bacterial cytoplasm is the
mesosome.
Mesosomes
are invaginations of the plasma membrane in the
shape of tubules, vesicles, or lamellae. Their exact function is unknown.
Currently, many bacteriologists believe that mesosomes are artifacts gener-
ated during the fixation of bacteria for electron microscopy (Singleton and
Sainsbury, 1994).
