A. Structure

The plasma membrane, also called the bacterial cytoplasmic membrane, is visible in electron micrographs of thin sections. It is a typical “unit membrane” composed of phospholipids and upward of 200 different proteins. Proteins account for approximately 70% of the mass of the membrane, which is a considerably higher proportion than that of mammalian cell membranes. Figure 1 illustrates a model of membrane organization. The membranes of prokaryotes are distinguished from those of eukaryotic cells by the absence of sterols (with some exceptions, eg, mycoplasmas, which also lack a cell wall, incorporate sterols, such as cholesterol, into their membranes when growing in sterol-containing media). However, many bacteria contain structurally related compounds called hopanoids, which likely fulfill the same function. Unlike eukaryotes, bacteria can have a wide variety of fatty acids within their mem branes. Along with the typical saturated and unsaturated fatty acids, bacterial membranes can contain fatty acids with additional methyl, hydroxy, or cyclic groups. The relative proportions of these fatty acids can be modulated by the bacterium to maintain the optimum fluidity of the membrane. For example, at least 50% of the cytoplasmic membrane must be in the semifluid state for cell growth to occur. At low temperatures, this is achieved by greatly increased syn thesis and incorporation of unsaturated fatty acids into the phospholipids of the cell membrane.

Fig1. Bacterial plasma membrane structure. This diagram of the fluid mosaic model of bacterial membrane structure shown the integral proteins (green and red) floating in a lipid bilayer. Peripheral proteins (yellow) are associated loosely with the inner membrane surface. Small spheres represent the hydrophilic ends of membrane phospholipids and wiggly tails, the hydrophobic fatty acid chains. Other membrane lipids such as hopanoids (purple) may be present. For the sake of clarity, phospholipids are shown proportionately much larger size than in real membranes. (Reproduced with permission from Willey JM, Sherwood LM, Woolverton CJ: Prescott, Harley, and Klein’s Microbiology, 7th ed. McGraw-Hill; 2008. © McGraw-Hill Education.)

The cell membranes of the Archaea differ from those of the Bacteria. Some Archaeal cell membranes contain unique lipids, isoprenoids, rather than fatty acids, linked to glycerol by ether rather than an ester linkage. Some of these lipids have no phosphate groups, and therefore, they are not phospholipids. In other species, the cell membrane is made up of a lipid monolayer consisting of long lipids (about twice as long as a phospholipid) with glycerol ethers at both ends (diglycerol tetraethers). The molecules orient themselves with the polar glycerol groups on the surfaces and the nonpolar hydrocarbon chain in the interior. These unusual lipids contribute to the ability of many Archaea to grow under environmental conditions such as high salt, low pH, or very high temperature.

B. Function

The major functions of the cytoplasmic membrane are (1) selective permeability and transport of solutes; (2) electron transport and oxidative phosphorylation in aerobic species; (3) excretion of hydrolytic exoenzymes; (4) contain the enzymes and carrier molecules that function in the biosynthesis of DNA, cell wall polymers, and membrane lipids; and (5) bear the receptors and other proteins of the chemotactic and other sensory transduction systems.

1. Permeability and transport—The cytoplasmic membrane forms a hydrophobic barrier impermeable to most hydrophilic molecules. However, several mechanisms (transport systems) exist that enable the cell to transport nutrients into and waste products out of the cell. These trans port systems work against a concentration gradient to increase the nutrient concentrations inside the cell, a function that requires energy in some form. There are three general trans port mechanisms involved in membrane transport: passive transport, active transport, and group translocation.

a. Passive transport—This mechanism relies on diffusion, uses no energy, and operates only when the solute is at higher concentration outside than inside the cell. Simple diffusion accounts for the entry of very few nutrients, including dis solved oxygen, carbon dioxide, and water itself. Simple diffusion provides neither speed nor selectivity. Facilitated diffusion also uses no energy, so the solute never achieves an internal concentration greater than what exists outside the cell. However, facilitated diffusion is selective. Channel proteins form selective channels that facilitate the pas sage of specific molecules. Facilitated diffusion is common in eukaryotic microorganisms (eg, yeast) but is rare in prokaryotes. Glycerol is one of the few compounds that enters prokaryotic cells by facilitated diffusion.

b. Active transport—Many nutrients are concentrated more than a thousandfold as a result of active transport. There are two types of active transport mechanisms depending on the source of energy used: ion-coupled transport and ATP binding cassette (ABC) transport.

1) Ion-coupled transport—These systems move a molecule across the cell membrane at the expense of a previously established ion gradient such as proton- or sodium-motive force. There are three basic types: uniport, symport, and antiport (Figure 2). Ion-coupled transport is particularly common in aerobic organisms, which have an easier time generating an ion-motive force than do anaerobes. Uniporters catalyze the transport of a substrate independent of any coupled ion. Symporters catalyze the simultaneous transport of two substrates in the same direction by a single carrier; for example, an H+ gradient can permit symport of an oppositely charged ion (eg, glycine) or a neutral molecule (eg, galactose). Antiport ers catalyze the simultaneous transport of two like-charged compounds in opposite directions by a common carrier (eg, H+:Na+). Approximately, 40% of the substrates transported by E. coli use this mechanism.

2) ABC transport—This mechanism uses ATP directly to transport solutes into the cell. In Gram-negative bacteria, the transport of many nutrients is facilitated by specific binding proteins located in the periplasmic space; in Gram-positive cells, the binding proteins are attached to the outer surface of the cell membrane. These proteins function by transferring the bound substrate to a membrane-bound protein complex. Hydrolysis of ATP is then triggered, and the energy is used to open the membrane pore and allow the unidirectional movement of the substrate into the cell. Approximately 40% of the substrates transported by E. coli use this mechanism.

c. Group translocation—In addition to true transport, in which a solute is moved across the membrane without change in structure, bacteria use a process called group translocation (vectorial metabolism) to effect the net uptake of certain sugars (eg, glucose and mannose), the substrate becoming phosphorylated during the transport process. In a strict sense, group translocation is not active transport because no concentration gradient is involved. This process allows bacteria to use their energy resources efficiently by coupling transport with metabolism. In this process, a membrane carrier protein is first phosphorylated in the cytoplasm at the expense of phosphoenolpyruvate; the phosphorylated carrier protein then binds the free sugar at the exterior mem brane face and transports it into the cytoplasm, releasing it as a sugar phosphate. Such systems of sugar transport are called phosphotransferase systems. Phosphotransferase systems are also involved in movement toward these carbon sources (chemotaxis) and in the regulation of several other metabolic pathways (catabolite repression).

d. Special transport processes—Iron (Fe) is an essential nutrient for the growth of almost all bacteria. Under anaerobic conditions, Fe is generally in the +2 oxidation state and soluble. However, under aerobic conditions, Fe is generally in the +3 oxidation state and insoluble. The internal compartments of animals contain virtually no free Fe; it is sequestered in complexes with such proteins as transferrin and lactoferrin. Some bacteria solve this problem by secreting siderophores— compounds that chelate Fe and promote its transport as a soluble complex. One major group of siderophores consists of derivatives of hydroxamic acid (−CONH2 OH), which chelate Fe3+ very strongly. The iron–hydroxamate complex is actively transported into the cell by the cooperative action of a group of proteins that span the outer membrane, periplasm, and inner membrane. The iron is released, and the hydroxamate can exit the cell and be used again for iron transport.

Some pathogenic bacteria use a fundamentally different mechanism involving specific receptors that bind host transferrin and lactoferrin (as well as other iron-containing host proteins). The Fe is removed and transported into the cell using an ABC transporter.

Fig2. Three types of porters: A: uniporters, B: symporters, and C: antiporters. Uniporters catalyze the transport of a single species independently of any other, symporters catalyze the cotransport of two dissimilar species (usually a solute and a positively charged ion, H+) in the same direction, and antiporters catalyze the exchange transport of two similar solutes in opposite directions. A single transport protein may catalyze just one of these processes, two of these processes, or even all three of these processes, depending on conditions. Uniporters, symporters, and antiporters have been found to be structurally similar and evolutionarily related, and they function by similar mechanisms. (Reproduced with permission from Saier MH Jr: Peter Mitchell and his chemiosmotic theories. ASM News 1997;63:13.)

2. Electron transport and oxidative phosphorylation—The cytochromes and other enzymes and components of the respiratory chain, including certain dehydrogenases, are located in the cytoplasmic membrane. The bacterial cytoplasmic membrane is thus a functional analog of the mitochondrial membrane—a relationship which has been taken by many biologists to support the theory that mitochondria have evolved from symbiotic bacteria. The mechanism by which ATP generation is coupled to electron transport is discussed in Chapter 6.

3. Excretion of hydrolytic exoenzymes and pathogenicity proteins—All organisms that rely on macromolecular organic polymers as a source of nutrients (eg, proteins, polysaccharides, and lipids) excrete hydrolytic enzymes that degrade these polymers to subunits small enough to penetrate the cell membrane. Higher animals secrete such enzymes into the lumen of the digestive tract; bacteria (both Gram-positive and Gram-negative) secrete them directly into the external medium or into the periplasmic space between the peptidoglycan layer and the outer membrane of the cell wall in the case of Gram-negative bacteria (see The Cell Wall).

In Gram-positive bacteria, proteins are secreted directly across the cytoplasmic membrane, but in Gram-negative bacteria, secreted proteins must traverse the outer membrane as well. At least six pathways of protein secretion have been described in bacteria: the type I, type II, type III, type IV, type V, and type VI secretion systems. A schematic over view of the type I to V systems is presented in Figure 3. The type I and IV secretion systems have been described in both Gram-negative and Gram-positive bacteria, but the type II, III, V, and VI secretion systems have been found only in Gram-negative bacteria. Proteins secreted by the type I and III pathways traverse the inner (cytoplasmic) membrane (IM) and outer membrane (OM) in one step, but proteins secreted by the type II and V pathways cross the IM and OM in separate steps. Proteins secreted by the type II and V pathways are synthesized on cytoplasmic ribosomes as preproteins containing an extra leader or signal sequence of 15–40 amino acids—most commonly about 30 amino acids—at the amino terminal and require the sec system for transport across the IM. In E. coli, the sec pathway comprises a number of IM proteins (SecD to SecF, SecY), a cell membrane-associated ATPase (SecA) that provides energy for export, a chaperone (SecB) that binds to the preprotein, and the periplasmic signal peptidase. After translocation, the leader sequence is cleaved off by the membrane-bound signal peptidase, and the mature protein is released into the periplasmic space. In contrast, proteins secreted by the type I and III systems do not have a leader sequence and are exported intact.

Fig3. The protein secretion systems of Gram-negative bacteria. Five secretion systems of Gram-negative bacteria are shown. The Sec-dependent and Tat pathways deliver proteins from the cytoplasm to the periplasmic space. The type II, type V, and sometimes type IV systems complete the secretion process begun by the Sec-dependent pathway. The Tat system appears to deliver proteins only to the type II pathway. The type I and III systems bypass the Sec-dependent and Tat pathways, moving proteins directly from the cytoplasm, through the outer membrane, to the extracellular space. The type IV system can work either with the Sec-dependent pathway or can work alone to transport proteins to the extracellular space. Proteins translocated by the Sec-dependent pathway and the type III pathway are delivered to those systems by chaperone proteins. ADP, adenosine diphosphate; ATP, adenosine triphosphate; EFGY; PuIS; SecD; TolC; Yop. (Reproduced with permission from Willey JM, Sherwood LM, Woolverton CJ: Prescott, Harley, and Klein’s Microbiology, 7th ed. McGraw-Hill; 2008. © McGraw-Hill Education.)

In Gram-negative and Gram-positive bacteria, another cytoplasmic membrane system that uses the twin-arginine targeting translocase (tat pathway) can move proteins across the IM. In Gram-negative bacteria, these proteins are then delivered to the type II system (Figure 3). The tat pathway is distinct from the sec system in that it translocates already folded proteins.

Although proteins secreted by the type II and V systems are similar in the mechanism by which they cross the IM, differences exist in how they traverse the OM. Proteins secreted by the type II system are transported across the OM by a multiprotein complex (see Figure 3). This is the primary path way for the secretion of extracellular degradative enzymes by Gram-negative bacteria. Elastase, phospholipase C, and exotoxin A are secreted by this system in Pseudomonas aeruginosa. However, proteins secreted by the type V system autotransport across the outer membrane by virtue of a carboxyl terminal sequence, which is enzymatically removed upon release of the protein from the OM. Some extracellular proteins—eg, the IgA protease of Neisseria gonorrhoeae and the vacuolating cytotoxin of Helicobacter pylori—are secreted by this system.

The type I and III secretion pathways are sec independent and thus do not involve amino terminal processing of the secreted proteins. Protein secretion by these pathways occurs in a continuous process without the presence of a cytoplasmic intermediate. Type I secretion is exemplified by the α-hemolysin of E. coli and the adenylyl cyclase of Bordetella pertussis. Type I secretion requires three secretory proteins: an IM ATP-binding cassette (ABC transporter), which provides energy for protein secretion; an OM protein; and a membrane fusion protein, which is anchored in the inner membrane and spans the periplasmic space (see Figure3). Instead of a signal peptide, the information is located within the carboxyl terminal 60 amino acids of the secreted protein.

The type III secretion pathway is a contact-dependent system. It is activated by contact with a host cell, and then injects a toxin protein into the host cell directly. The type III secretion apparatus is composed of approximately 20 proteins, most of which are located in the IM. Many of these IM components are homologous to the flagellar biosynthesis apparatus of both Gram-negative and Gram-positive bacteria. As in type I secretion, the proteins secreted via the type III pathway are not subject to amino terminal processing during secretion.

Type IV pathways secrete either polypeptide toxins (directed against eukaryotic cells) or protein–DNA complexes either between two bacterial cells or between a bacterial and a eukaryotic cell. Type IV secretion is exemplified by the protein DNA complex delivered by Agrobacterium tumefaciens into a plant cell. Additionally, B. pertussis and H. pylori possess type IV secretion systems that mediate secretion of pertussis toxin and interleukin-8–inducing factor, respectively. The sec-independent type VI secretion was recently described in P. aeruginosa, where it contributes to pathogenicity in patients with cystic fibrosis. This secretion system is composed of 15–20 proteins whose biochemical functions are not well understood. However, recent studies suggest that some of these proteins share homology with bacteriophage tail proteins.

4. Biosynthetic functions—The cell membrane is the site of the carrier lipids on which the subunits of the cell wall are assembled as well as of the enzymes of cell wall biosynthesis. The enzymes of phospholipid synthesis are also localized in the cell membrane.

5. Chemotactic systems—Attractants and repellents bind to specific receptors in the bacterial membrane (see Flagella). There are at least 20 different chemoreceptors in the membrane of E. coli, some of which also function as a first step in the transport process.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم الشؤون الخدمية يواصل جهوده في توزيع الماء لزائري مرقد أبي الفضل العباس (عليه السلام)

قسم شؤون المعارف يجري القرعة الإلكترونية الخاصة بمشاهدي (مسابقة معارف التراث)

قسم الشؤون الفكرية يواصل برنامجه الصحي التوعوي في مدارس بغداد

من خدمة الزائرين إلى إغاثة المنكوبين.. العتبة العباسية المقدسة حاضرة في كل الميادين

المزيد

الآخبار الصحية

كوب قبل النوم.. مشروب طبيعي يحارب الأرق

دون أدوية.. طريقة طبيعية لخفض ضغط الدم بفعالية مذهلة

المعكرونة ليست عدوك.. كيف تصبح جزءا من وجبة صحية متوازنة؟

دراسة: تناول القهوة يوميا يخفض خطر الإصابة بالاضطرابات النفسية

المزيد

الآخبار التكنلوجية

علماء روس يخططون لبناء منشأة للكشف عن المادة المظلمة

ثاني أكسيد الكربون ليس تهديدا للمناخ فقط.. بل لصحة الإنسان أيضا!

ديدان الأرض تكشف تلوث التربة بالمعادن الثقيلة

خبير أرصاد جوية: ظاهرة النينيو القادمة قد تكون الأقوى خلال 10 سنوات

المزيد

مواضيع ذات صلة

Regulation of Cellular and Systemic Iron Homeostasis

Hemoglobin Function

Hemoglobin Structure

Phagocytes Ingest Target Cells

Cellular and Molecular Mechanisms in Development: Morphogens and Cell-to-Cell Signaling

Thrombopoietin Signaling

Mechanisms of Endomitosis in Megakaryocytes

Megakaryocytopoiesis

Ontogeny of Erythropoiesis

Cellular Dynamics in Erythropoiesis: Steady State and Stress Erythropoiesis

Erythropoiesis and Iron Regulation

Hematopoietic Microenvironment