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Structure and Function of Bacterial Cells (page 8)

(This chapter has 10 pages)

© Kenneth Todar, PhD

Permeability Barrier

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 systems.

Transport of Solutes 

The proteins that mediate the passage of solutes through membranes are referred to variously as transport systems, carrier proteins, porters, and permeases. Transport systems operate by one of three transport processes as described below in Figure 22. In a uniport process, a solute passes through the membrane unidirectionally. In symport processes (also called cotransport) two solutes must be transported in the same direction at the same time; in antiport processes ( also called exchange diffusion), one solute is transported in one direction simultaneously as a second solute is transported in the opposite direction.

Figure 22. Transport processes in bacterial cells. Solutes enter or exit from bacterial cells by means of one of three processes: uniport, symport (also called cotransport) and antiport (also called exchange diffusion). Transport systems (Figure 23 below) operate by one or another of these processes.

Types of Transport Systems

Bacteria have a variety of types of transport systems which can be used alternatively in various environmental situations. The elaborate development of transport processes and transport systems in procaryotes probably reflects their need to concentrate substances inside the cytoplasm against the concentration (gradient) of the environment. Concentration of solutes in the cytoplasm requires the operation of an active transport system, of which there are two types in bacteria: ion driven transport systems (IDT) and binding-protein dependent transport systems (BPDT). The definitive feature of an active 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.
 


Figure 23. Operation of bacterial transport systems. Bacterial transport systems are operated by transport proteins (sometimes called carriers, porters or permeases) in the plasma 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 protein-dependent transport, use energy and concentrate molecules against a concentration gradient. Group translocation systems, such as the phosphotransferase (pts) system 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 procaryotes. The carrier is a protein (or group of proteins) that functions in the passage of a small molecule from one side of a membrane to the other side. A transport system may be a single transmembranous protein that forms a channel that admits passage of a specific solute, or it may be a coordinated 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 single solute with the same specificity and kinetics as an enzyme. Some transport systems will transport (structurally) related molecules, although at reduced efficiency compared to their primary substrate. Most transport systems transport specific sugars, amino acids, anions or cations that are of nutritional value to the bacterium.

Facilitated diffusion systems (FD) are the least common type of transport system in bacteria. Actually, the glycerol uniporter in E. coli is the only well known facilitated diffusion system. FD involves 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 membrane 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 the transport process. For this reason the glycerol molecule cannot be accumulated against the concentration gradient.

Ion driven transport systems (IDT) and Binding-protein dependent transport systems (BPDT) are active transport systems that are 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 other cation, i.e., chemiosmotic potential, to drive the transport process. IDT systems such as the lactose permease of E. coli utilize the consumption of a hydrogen ion during the transport of lactose. Thus the energy expended during active transport of lactose is in the form of pmf. The lactose permease is a single transmembranous polypeptide that spans the membrane seven times forming a channel that specifically admits lactose.

Binding-protein dependent transport systems (BPDT), such as the histadine transport system in E. coli, are composed of four proteins. Two proteins form a membrane channel that allows passage of the histadine. A third protein resides in the periplasmic space where it is able to bind the amino acid and pass it to a forth protein which admits the amino acid into the membrane channel. Driving the solute through the channel involves the expenditure of energy, which is provided by the hydrolysis of ATP.

Group translocation systems (GT), more commonly known as the phosphotransferase system (PTS) in E. coli, are used primarily for the transport of sugars. Like binding protein-dependent transport systems, they are composed of several distinct components. However, GT systems specific for one sugar may share some of their components with other group transport systems. In 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. Glucose specifically enters the channel from the outside, but in order to exit into the cytoplasm, it must first be phosphorylated by the phosphotransferase system. The PTS derives energy from the metabolic intermediate phosphoenol pyruvate (PEP). PEP is hydrolyzed to pyruvate and glucose is phosphorylated to form glucose-phosphate during the process. Thus, by the expenditure of a single molecule of high energy phosphate, glucose is transported and changed to glucose-phosphate.

Table 8. Distinguishing characteristics of bacterial transport systems
PD = passive diffusion
FD = facilitated diffusion
IDT = ion-driven transport
BPDT = binding protein dependent transport
GT = group translocation
Property  PD FD IDT BPDT GT
carrier mediated  - + + + +
conc. against gradient  - - + + NA
specificity  - + + + +
energy expended  - - pmf  ATP PEP
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 acidic and 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 which 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 removal of a pair of electrons, passing them through its sequence of carriers eventually to O2. ATPase is the transmembranous protein enzyme that utilizes protons from the outside to synthesize ATP on the inside of the membrane. Several other transmembranous proteins are transport systems which are operating by either symport or antiport processes.

The plasma membrane of procaryotes may invaginate into the cytoplasm or form stacks or vesicles attached to the inner membrane surface. These structures are sometimes referred to as mesosomes. Such internal membrane systems may be analogous to the cristae of mitochondria or the thylakoids of chloroplasts which increase the surface area of membranes to which enzymes are bound for specific enzymatic functions. The photosynthetic apparatus (light harvesting pigments and ATPase) of photosynthetic procaryotes is contained in these types of membranous structures. Mesosomes may also represent specialized membrane regions involved in DNA replication and segregation, cell wall synthesis, or increased enzymatic activity. Membrane foldings and vesicles sometimes appear in electron micrographs of procaryotic cells as artifacts of preparative techniques. These membranous structures, of course, are not mesosomes, but their existence does not prove that mesosomes are not present in procaryotes, and there are several examples of procaryotic membrane topology and appearance that are suggestive of mesosomes.

There are a few antibiotics (e.g. polymyxin), hydrophobic agents (e.g. bile salts), and proteins (e.g. complement) that can damage bacterial membranes.

The Periplasm

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 within the 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 for 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 periplasmic proteins in E. coli.
    Binding proteins
    For amino acids (e.g. histadine, arginine)
    For sugars (e.g. glucose, maltose)
    For vitamins (e.g. thiamine, vitamin B12)
    For ions (e.g. phosphate, sulfate)

    Biosynthetic enzymes
    For murein assembly (e.g. transglycosylases, carboxypeptidases, transpeptidases)
    For fimbrial subunit secretion and assembly (e.g. chaperonins)

    Degradative enzymes
    phosphatases
    proteases

    Detoxifying enzymes
    Beta-lactamases (e.g. penicillinase)
    Aminoglycoside-phosphorylating enzymes





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