Structure and Function of Bacterial Cells (page 8)
(This chapter has 10 pages)
© Kenneth Todar, PhD
The cell membrane is the most dynamic structure in the cell. Its
main function is as a permeability barrier that regulates the passage
of substances into and out of the cell. The plasma membrane is the
definitive structure of a cell since it sequesters the molecules of
life in the cytoplasm, separating it from the outside environment. The
bacterial membrane freely allows passage of water and a few small
uncharged molecules (less than molecular weight of 100 daltons), but it
does not allow passage of larger molecules or any charged substances
except when monitored by proteins in the membrane called transport
Transport of Solutes
The proteins that mediate the passage of
solutes through membranes
referred to variously as transport systems, carrier proteins,
and permeases. Transport systems operate by one of three transport
processes as described below in Figure 22. In a uniport
a solute passes through the membrane unidirectionally. In symport processes
(also called cotransport) two solutes must be transported in
same direction at the same time; in antiport processes ( also
diffusion), one solute is transported in one direction
as a second solute is transported in the opposite direction.
in bacterial cells. Solutes enter or exit from bacterial cells by means
of one of three processes: uniport, symport (also called cotransport)
antiport (also called exchange diffusion). Transport systems (Figure 23
below) operate by one or another of these processes.
Types of Transport
Bacteria have a variety of types of transport
systems which can be
alternatively in various environmental situations. The elaborate
of transport processes and transport systems in procaryotes probably
their need to concentrate substances inside the cytoplasm against the
(gradient) of the environment. Concentration of solutes in the
requires the operation of an active transport system, of which
are two types in bacteria: ion driven transport systems (IDT)
dependent transport systems (BPDT). The definitive feature of an
transport system is the accumulation of the solute in the cytoplasm at
concentrations far in excess of the environment. According to the laws
of physical chemistry, this type of process requires energy.
23. Operation of
transport systems. Bacterial transport systems are operated by
proteins (sometimes called carriers, porters or permeases) in the
membrane. Facilitated diffusion is a carrier-mediated system that does
not require energy and does not concentrate solutes against a gradient.
Active transport systems such as Ion-driven transport and Binding
transport, use energy and concentrate molecules against a concentration
gradient. Group translocation systems, such as the phosphotransferase
in Escherichia coli, use energy during transport and modify the
solute during its passage across the membrane.
There are four types of carrier-mediated
transport systems in
The carrier is a protein (or group of proteins) that functions
the passage of a small molecule from one side of a membrane to the
side. A transport system may be a single transmembranous protein that
a channel that admits passage of a specific solute, or it may be a
system of proteins that binds and sequentially passes a small molecule
through the membrane. Transport systems have the property of specificity
for the solute transported. Some transport systems transport a
solute with the same specificity and kinetics as an enzyme. Some
systems will transport (structurally) related molecules, although at
efficiency compared to their primary substrate. Most transport systems
transport specific sugars, amino acids, anions or cations that are of
value to the bacterium.
Facilitated diffusion systems (FD)
are the least common type
transport system in bacteria. Actually, the glycerol uniporter in E.
coli is the only well known facilitated diffusion system. FD
the passage of a specific solute through a carrier that forms a channel
in the membrane. The solute can move in either direction through the
to the point of of equilibrium on both sides of the membrane. Although
the system is carrier-mediated and specific, no energy is expended in
transport process. For this reason the glycerol molecule cannot be
against the concentration gradient.
Ion driven transport systems (IDT) and Binding-protein
transport systems (BPDT) are active transport systems that
used for transport of most solutes by bacterial cells. IDT is used for
accumulation of many ions and amino acids; BPDT is frequently used for
sugars and amino acids. IDT is a symport or antiport process that uses
a hydrogen ion (H+) i.e., proton motive force (pmf), or some
i.e., chemiosmotic potential, to drive the transport process. IDT
such as the lactose permease of E. coli utilize the consumption
of a hydrogen ion during the transport of lactose. Thus the energy
during active transport of lactose is in the form of pmf. The lactose
is a single transmembranous polypeptide that spans the membrane seven
forming a channel that specifically admits lactose.
Binding-protein dependent transport systems
(BPDT), such as
histadine transport system in E. coli, are composed of four
Two proteins form a membrane channel that allows passage of the
A third protein resides in the periplasmic space where it is able to
the amino acid and pass it to a forth protein which admits the amino
into the membrane channel. Driving the solute through the channel
the expenditure of energy, which is provided by the hydrolysis of ATP.
Group translocation systems (GT), more
commonly known as the
system (PTS) in E. coli, are used primarily for the
of sugars. Like binding protein-dependent transport systems, they are
of several distinct components. However, GT systems specific for one
may share some of their components with other group transport systems.
E. coli, glucose may be transported by a group translocation
process that involves the phosphotransferase system. The actual carrier
in the membrane is a protein channel fairly specific for glucose.
specifically enters the channel from the outside, but in order to exit
into the cytoplasm, it must first be phosphorylated by the
system. The PTS derives energy from the metabolic intermediate
pyruvate (PEP). PEP is hydrolyzed to pyruvate and glucose is
to form glucose-phosphate during the process. Thus, by the expenditure
of a single molecule of high energy phosphate, glucose is transported
changed to glucose-phosphate.
of bacterial transport systems
PD = passive
FD = facilitated diffusion
IDT = ion-driven transport
BPDT = binding protein
GT = group translocation
|conc. against gradient
|solute modified during transport
Generation of Energy
Unlike eucaryotes, bacteria don't have intracellular organelles for
energy producing processes such as respiration or photosynthesis.
Instead, the cytoplasmic membrane carries out these functions. The
membrane is the location of electron transport systems (ETS) used to
produce energy during photosynthesis and respiration, and it is the
location of an enzyme called ATP synthetase (ATPase) which is
used to synthesize ATP.
When the electron transport system operates, it establishes a pH
gradient across of the membrane due to an accumulation of protons (H+)
outside and hydroxyl ion (OH-) inside. Thus the outside is
the inside is alkaline. Operation of the ETS also establishes a charge
on the membrane called proton motive force (pmf). The outer face
of the membrane becomes charged positive while inner face is charged
negative, so the membrane has a positive side and a negative side, like
a battery. The pmf can be used to do various types of work including
the rotation of the flagellum, or active transport as described above.
The pmf can also be used to make ATP by the membrane ATPase enzyme
which consumes protons when it synthesizes ATP from ADP and phosphate.
The connection between electron transport, establishment of pmf, and
ATP synthesis during respiration is known as oxidative phosphorylation;
during photosynthesis, it is called photophorylation.
Figure 24 below illustrates the membrane of E. coli. The topographical features
of the membrane from top to bottom are 1. lactose transport system; 2.
the flagellar motor coupled to the hook and filament; 3. Na+ transport (export)
system; 4. Ca++ transport (export) system; 5.
electron transport system; 6. ATPase enzyme; 7. proline transport
system. The operation ot the electron transport system during
respiration produces the H+ charge on the membrane (pmf).
The pmf ( H+) is used by the transport
systems to move molecules from one side of the membrane to the other;
by the flagellar motor ring to rotate the flagellar
filament; and by the ATPase enzyme to synthesize ATP.
Figure 24. Schematic view of
the plasma membrane of Escherichia coli. The S and M rings
constitute the flagellar motor are shown. The motor ring is imbedded in
the phospholipid bilayer. It is powered by pmf to rotate the flagellar
filament. The electron transport system is shown oxidizing NAD by
of a pair of electrons, passing them through its sequence of carriers
to O2. ATPase is the transmembranous protein enzyme that
from the outside to synthesize ATP on the inside of the membrane.
other transmembranous proteins are transport systems which are
by either symport or antiport processes.
The plasma membrane of
into the cytoplasm or form stacks or vesicles attached to the inner
surface. These structures are sometimes referred to as mesosomes.
Such internal membrane systems may be analogous to the cristae of
or the thylakoids of chloroplasts which increase the surface area of
to which enzymes are bound for specific enzymatic functions. The
apparatus (light harvesting pigments and ATPase) of photosynthetic
is contained in these types of membranous structures. Mesosomes may
represent specialized membrane regions involved in DNA replication and
segregation, cell wall synthesis, or increased enzymatic activity.
foldings and vesicles sometimes appear in electron micrographs of
cells as artifacts of preparative techniques. These membranous
of course, are not mesosomes, but their existence does not prove that
are not present in procaryotes, and there are several examples of
membrane topology and appearance that are suggestive of mesosomes.
There are a few antibiotics (e.g. polymyxin),
bile salts), and proteins (e.g. complement) that can damage bacterial
Between the inner (plasma) and outer membranes of Gram-negative
bacteria and spirochetes is a space called the periplasm or periplasmic space
(See Figures 9 and 18). Actually, the peptidoglycan sheet resides
periplasm. The periplasm is a very active compartment of the cell,
containing enzymes for assembly of cell wall and membrane components,
various degradative or detoxifying enzymes, secretion systems, sensing
proteins for chemotaxis and signal transduction, and binding proteins
solutes taken up by BPDT transport systems. Components of the
periplasm are needed in this region of the cell and are bounded or
"trapped" by the two membranes of the cell. In the case of spirochetes,
their flagella (called endoflagella or periplasmic flagella) rotate
within the periplasm and impart the flexing and screw-like rotation
characteristic of spirochete motility.
Table 9. Representative
proteins in E. coli.
For amino acids (e.g.
For sugars (e.g. glucose,
For vitamins (e.g. thiamine,
For ions (e.g. phosphate,
For murein assembly (e.g.
For fimbrial subunit secretion
and assembly (e.g. chaperonins)