We now come to what is certainly the most functionally complex of the cell structures, the cell membrane. The cell membrane is responsi-ble for a broad range of physiological activities including solute transport, electron transport, photosynthetic electron transport, the estab-lishment of electrochemical gradients, ATP synthesis, biosynthesis of lipids, biosynthesis of cell wall polymers, secretion of proteins, the secretion and uptake of intercellular signals, and responses to environmental signals. To refer to the cell membrane simply as a lipopro-tein bilayer does not do justice to the machinery embedded in the lipid matrix, a complex mosaic of parts whose structure and interactions at the molecular level are not well understood.
As expected, the protein composition of cell membranes is complex. There can be more than 100 different proteins. Many of the proteins are clustered in functional aggregates (e.g., the proton translocating ATPase, the fl agella motor, electron transport complexes, certain of the solute transporters). At the molecular level, the membrane is certainly a complex and busy place. What follows is a general descrip-tion of the membrane, without reference to its microheterogeneity.
Bacterial cell membranes
Bacterial cell membranes consist primarily of phospholipids and protein in a fl uid mosaic structure in which the phosphlipids form a bilayer (Fig. 1.16). The structure is said to be fl uid because there is extensive lateral mobility of bulk proteins and phospholipids. Nevertheless, certain protein aggregates (e.g., complex solute transporters and electron transport aggregates) remain as aggregates within which the proteins interact to catalyze sequential reactions.
1. The lipids
The phospholipids are fatty acids esterifi ed to two of the hydroxyl groups of phosphoglycer-ides (Fig. 1.17). The structure and synthesis of phospholipids is described in detail in Section 10.1.2. The third hydroxyl group in the glyc-erol backbone of the phospholipid is covalently bound to a substituted phosphate group, which makes one end of the molecule very polar owing to a negative charge on the ionized phosphate group. Because the phospholipids are polar at one end and nonpolar at the other end (the end
across the membrane through or on special pro-tein transporters that bridge the phospholipid bilayer. These modes are discussed in the context of solute transport in Chapter 17. (However, the lipid bilayer is permeable to water mole-cules, gases, and small hydrophobic molecules.) An important consequence of the lipid matrix is that ions do not freely diffuse across the mem-brane unless they are carried on or through protein transporters. Because of this, the mem-brane is capable of holding a charge that is due to the unequal transmembrane distribution of ions. This is discussed in Chapter 5.
4. Aquaporins (water channels)
Although the lipid bilayer allows rapid equili-bration of water, there do exist in E. coli and other bacteria water channels that are similar to the aquaporins found in eukaryotes; these water channels, called aquaporins, enhance the rapid equilibration of water across the cell membrane. (Reviewed in ref. 105.) The gene coding for the water channel protein in E.
coli is aqpZ. The expression of the aqpZ gene under different extracellular osmolarity con-ditions and its requirement for viability have been investigated.106 Null mutants of aqpZ are viable, although the colonies are smaller than the wild-type strain. Interestingly, when E. coli is grown in media of high osmolarity, the synthesis of the aquaporin channels is repressed. This may help protect the cell from hypo- osmotic stress in the event of a sharp with the fatty acids), they are said to be
amphi-pathic, able to spontaneously aggregate while their nonpolar fatty acid regions interact with each other by hydrophobic bonding; their polar phosphorylated regions, on the other hand, face the aqueous phase, where ionic interactions occur with cations, water, and polar groups on proteins. Phospholipids accomplish all this by spontaneously forming lipid bilayers in water solutions or in cell membranes.
2. The proteins
There are two classes of proteins in membranes, integral and peripheral. Integral proteins are embedded in the membrane and bound to the fatty acids of the phospholipids via hydrophobic bonding. They can be removed only with deter-gents or solvents. Peripheral proteins, attached at membrane surfaces to the phospholipids by ionic interactions, can be removed by washing the membrane with salt solutions. The insertion of the proteins into the membrane during mem-brane synthesis is discussed in Section 18.2.
3. Permeability
The phospholipid bilayer acts as a permeabil-ity barrier to virtually all water-soluble mol-ecules. Thus most solutes diffuse or are carried Fig. 1.16 Model of the cell membrane showing bimolecular lipid leafl ets and embedded proteins;
the phospholipid molecules are interacting with one another via their hydrophobic (apolar) “tails.” The hydrophilic (polar) “heads” of the phospholipids face the outside of the membrane, where they interact with proteins and ions. Proteins can span the membrane or be partially embedded. Source: Singer, S. J., and G. L. Nicolson. 1972. The fl uid mosaic model of the structure of cell membranes. Science 175:720–731.
Copyright 1972 by the Association of Academies of Science. Reprinted with permission from AAAS.
Fig. 1.17 Phospholipids have both a polar and a non-polar end. (A) Phospholipid with two fatty acids (R) esterifi ed to glycerol. The phosphate is conjugated to X, which determines the type of phospholipid. In bacteria, X is usually serine, ethanolamine, a deriva-tive of glycerol, or a carbohydrate derivaderiva-tive. See Section 10.1.3 for a more complete description of bacterial phospholipids. (B) Schematic drawing of a phospholipid showing the polar (circle) and nonpo-lar (straight lines) regions.
facilitating the uptake of organic osmolytes such as proline and betaine. Recently it was reported that ProP localizes to the membrane at the poles of cells because of the presence there of a lipid called cardiolipin, which is more highly concen-trated in the cytoplasmic membrane near the cell poles.111 This is very interesting because it points to an important role of certain lipids in localizing membrane proteins to specifi c mem-brane locations.
Archaeal cell membranes 1. The lipids
Archaeal membrane lipids differ from those found in bacterial membranes.112-114 The archaeal lipids consist of isoprenoid alcohols (either 20 or 40 carbons long), linked via ether bonds, in contrast to the ester linkages in eubac-teria, to one glycerol to form monoglycerol diethers or to two glycerols to form diglycerol tetraethers. These are illustrated in Fig. 1.18.
The synthesis of these lipids is described later, in Section 10.1.3. (Recall that bacterial glyc-erides are fatty acids esterifi ed to glycerol.
Refer to Fig. 1.16 for a comparison.) The C20 alcohol is a fully saturated hydrocarbon called phytanol. The C40 molecules are two phytanols linked together head to head in the diglycerol tetraether lipids. Thus, the lipids are either phytanylglycerol diethers or diphytanyl diglyc-erol tetraethers. The diethers and tetraethers occur in varying ratios depending upon the bac-terium. For example, there may be from 5 to 25 different lipids in any one cell. This is really quite a diverse mix, and it can be contrasted to the lipid complement of a typical bacterium, which has only four or fi ve different phospholipids.
The diversity of the archaeal lipids is due to the different polar head groups that exist, as well as to the mix of core lipids to which the head groups are attached (Fig. 1.18). Although the polar head group is responsible for the polarity of most phospholipids, there is some polarity at one end of the archaeal lipids without a polar head group because of the free hydroxyl group on the glycerol. (Recall that hydroxyl groups are capable of forming hydrogen bonds with water and proteins.) It is usually stated that the ether linkages, which are more stable to hydrolytic cleavage, are an advantage over ester linkages in the acidic and thermophilic environments in which many archaea live.
decrease in the osmolarity of the external medium.
5. Mechanosensitive (MS) channels
For reviews of mechanosensitive channels, read refs. 105 and 107 through 109. As will be explained in Section 16.2, bacteria adjust the internal osmotic pressure so that it is always higher than the external osmotic pressure. This keeps water fl owing into the cell via osmosis and maintains the high internal turgor pressure that is important for growth. The internal osmotic pressure is kept high by the accumulation of cer-tain solutes such as K+, glutamate, glutamine, proline, trehalose, and betaine.
One consequence of maintaining a high inter-nal osmotic pressure is that a sudden decrease in the external osmotic pressure can endanger the cell by promoting a sudden increase in the infl ux of water, leading to overexpansion of the cell wall and subsequent lysis of the cell. This is sometimes referred to as hypo-osmotic stress or hypo-osmotic shock. The cells are protected from this form of destruction by mechanosensi-tive (MS) channels that open under conditions of hypo-osmotic stress and provide a means for internal solutes to rapidly exit the cell, thus low-ering the internal osmotic pressure. (See note 110 for an explanation.) Mutants that do not have MS channels lyse when subjected to hypo-osmotic stress.
Mechanosensitive channels are present in most bacteria as well as archaea. E. coli has three such channels: a large channel called MscL, a small channel called MscS, and a
“mini” channel called MscM (L refers to large, S to small, and M to mini).
6. Osmosensory transporters
As explained in Chapter 16, bacteria adapt to high-osmolarity media by increasing the intracellular concentrations of certain solutes called osmolytes. This raises the cytoplasmic osmolarity so that water does not rush out of the cell into the more concentrated solution. As stated in Section 16.2.3 (Turgor pressure and its importance for growth), it is very important for bacteria to maintain an internal osmolar-ity higher than the external osmolarosmolar-ity so that turgor pressure within the cell is maintained.
E. coli has an osmosensory transporter called ProP; this membrane protein senses increas-ing osmolarity in the medium and responds by
Fig. 1.18 Major lipids of Methanobacterium thermoautotrophicum: (A) glycerol diether (archaeol), (B) diglycerol tetraether (caldarchaeol), (C) a glycolipid (gentiobiosyl archaeol), (E) a phospholipid (archaeti-dyl–X), where X can be inositol, serine, or ethanolamine, (F) a phospholipid (caldarchaeti(archaeti-dyl–X), (G) a phosphoglycolipid (gentiobiosyl caldarchatidyl–X). Source: Nishihara, M., H. Morii, and Y. Koga. 1989.
Heptads of polar ether lipids of an archaebacterium, Methanobacterium thermoautotrophicum: structure and biosynthetic relationship. Biochemistry 28:95–102. Reprinted with permission from Nishihara, M., H.
Morii, and Y. Koga. Copyright 1989 American Chemical Society.
generally believed to be derived from invagi-nations of chemically modifi ed areas of the cell membrane. Connections to the cell mem-brane are not always seen, however, and it is unknown whether the intracytoplasmic mem-branes are derived from an invagination of the cell membrane or are synthesized independently of the cell membrane (e.g., the thylakoids of cyanobacteria). A few prokaryotes with intra-cytoplasmic membranes and their physiological roles are listed.
1. Methanotrophs
Bacteria that grow on methane as their sole source of carbon (methanotrophs) possess intracytoplasmic membranes that are suggested to function in methane oxidation. Methane oxi-dation is discussed later, in Section 14.2.1.
2. Nitrogen fi xers
Bacteria for which nitrogen gas serves as a source of nitrogen use an oxygen-sensitive enzyme called nitrogenase to reduce the nitrogen to ammonia, which is subsequently incorporated into cell material. Many of these organisms have extensive intracytoplasmic membranes. One such nitrogen-fi xing bacterium is Azotobacter vinelandii, whose intracytoplasmic membranes increase with the degree of aeration of the 2. The proteins
There is little information regarding archaeal membrane proteins. It is known, however, that in bacteriorhodopsin and halorhodopsin in Halobacterium, the conformational array in the cell membrane is dependent upon interaction with polar membrane lipids.113 The functions of these two proteins are discussed in Sections 4.8.4 and 4.9.
3. The membrane
The thermoacidophilic archaea and some methanogens have tetraether glycerolipids in the cell membrane. These lipids have a polar head group at both ends and span the mem-brane, forming a lipid monolayer (Fig. 1.19).
This is the only known example of a membrane having no midplane region. Since there is no midplane region, the lipid monolayer is more resistant to levels of heat that would disrupt the hydrophobic bonds holding the two lipids in the lipid bilayer together. The increased resis-tance to heat of the lipid monolayer may confer an advantage to organisms living at high tem-peratures. However, it cannot be claimed that diether lipids or tetraether lipids are a specifi c adaptation to high temperatures, although they may be advantageous in these environments.
This is because some mesophilic methanogens have tetraether lipids, whereas two extremely thermophilic archaea, Methanopyrus kandleri and Thermococcus celer, do not have tetraether lipids.
1.2.6 Cytoplasm
The cytoplasm is defi ned as everything enclosed by the cell membrane. Cytoplasm is a viscous material containing a heavy concentration of protein (100–300 mg/mL),115 salts, and metab-olites. In addition, there are large aggregates of protein complexes designed for specifi c meta-bolic functions, various inclusions, and highly condensed DNA. Intracytoplasmic membranes are also present in many prokaryotes. The soluble part of the cytoplasm is called the cyto-sol. We will begin with the intracytoplasmic membranes.
Intracytoplasmic membranes
Many prokaryotes have intracytoplasmic membranes that have specialized physiological functions.116 Intracytoplasmic membranes are often connected to the cell membrane and are
Fig. 1.19 Lipid layer with membrane proteins (shaded areas) in archaebacteria membranes. The glycerol diethers form a lipid bilayer, and the tetraethers form a monolayer. Some archaea (e.g., the extreme halophiles) contain only the diethers. Most of the sulfur-dependent thermophiles have primarily the tetraethers, with only trace amounts of the diethers.
Many methanogens have signifi cant amounts of both the di- and tetraethers.
organisms to fl oat in lakes and ponds at depths that support growth because of favorable light, temperature, or nutrients. For example, the green sulfur bacterium Pelodictyon phaeoclath-ratiforme forms gas vesicles only at low light intensities.117 Perhaps this allows the bacteria to fl oat at depths where the light is optimal for photosynthesis. Many bacteria and cyanobac-teria with gas vesicles are plentiful in stratifi ed freshwater lakes, but they are not as abundant in isothermally mixed waters. Other prokary-otes containing gas vesicles (e.g., the halophilic archaeon Halobacterium) live in hypersaline waters, and a few marine species of cyanobacte-ria belonging to the genus Trichodesmium have gas vesicles. When gas vesicles are collapsed by experimentally subjecting cells to high hydro-static pressure or turgor pressure, the cells are no longer buoyant and sink. Collapsed vesicles do not recover, and the cells acquire gas-fi lled vesicles only by de novo synthesis of new vesi-cles. Thus, during synthesis of the vesicles, water is somehow excluded, presumably because of the hydrophobic nature of the inner protein surface.
2. Carboxysomes
Bacteria that obligately grow on CO2 as their sole or major source of carbon (strict autotro-phs) sometimes have large (100 nm) polyhedral protein-walled microcompartments called car-boxysomes.118, 119 These inclusions have been observed in nitrifying bacteria, sulfur oxidizers, and cyanobacteria. The distribution appears to be species specifi c (e.g., not all sulfur oxidizers have carboxysomes). However, it should be pointed out that most oceanic microorganisms that fi x carbon dioxide, including all cyanobac-teria, do so with carboxysomes. In these organ-isms ribulose-1,5-bisphosphate carboxylase (RuBP carboxylase), the enzyme in the Calvin cycle that incorporates CO2 into organic car-bon, is stored in carboxysomes. The enzyme is discussed in Section 14.1.1. Although many autotrophs lack carboxysomes, it has been suggested that an advantage to having them is that the RuBP carboxylase is sequestered there, where the concentration of CO2 is kept high.
This may be due to the carbonic anhydrase associated with the shell of the carboxysome.
The carbonic anhydrase catalyzes the conver-sion of HCO–3 to CO2.120
culture. Since respiratory activity is localized in the membranes, it is probable that an important role for Azotobacter intracytoplasmic mem-branes is to increase the cellular respiratory activity, to provide more ATP for nitrogen fi xa-tion and to remove oxygen from the vicinity of the nitrogenase. Nitrogen fi xation is discussed in Section 13.3.
3. Nitrifi ers
Intracellular membranes are also found in nitri-fying bacteria (i.e., bacteria that oxidize ammo-nia and nitrite as the sole source of electrons:
Nitrosomonas, Nitrobacter, Nitrococcus).
Several of the enzymes that catalyze ammonia and nitrite oxidation are in the membranes.
This is discussed in Section 13.4.
4. Phototrophs
In bacteria that use light as a source of energy for growth (phototrophs), the intracytoplasmic membranes are the sites of the photosynthetic apparatus. The membrane structure varies: fl at membranes, vesicles, fl at sacs (thylakoids in cyanobacteria), and tubular invaginations of the cell membrane (photosynthetic bacteria).
See the discussion of photosynthesis and pho-tosynthetic membranes in Chapter 6, especially Fig. 6.16.
Inclusion bodies, multienzyme aggregates, and granules
Certain bacteria contain specialized compart-ments in the cytoplasm. Some researchers refer to these entities as inclusion bodies. They are not surrounded by a lipid bilayer–protein membrane as are organelles in eukaryotic cells, although they can have a membrane or coat. In addition, there are numerous large aggregates and multienzyme complexes in all bacteria.
1. Gas vesicles
Aquatic bacteria such as cyanobacteria, certain photosynthetic bacteria, some nonphotosyn-thetic bacteria, and certain archaea have gas vesicles surrounded by a simple protein coat consisting primarily of gas vesicle protein A (GsvA), a small hydrophobic protein that is highly conserved among the diverse groups of organisms. Gas vesicles are hollow, spindle-shaped structures about 100 nm long, fi lled with gas in equilibrium with the gases dissolved in the cytoplasm. The gas vesicles allow the
Magnetosomes should be thought of as a navigational device or a magnetic compass that orients the bacteria with the Earth’s magnetic fi eld so that they swim in a particular direction, a behavior called magnetotaxis. In the Northern Hemisphere the geomagnetic north points down at an angle, and magnetotactic bacteria in the Northern Hemisphere that swim toward the geomagnetic north swim deeper into the water.
In the Southern Hemisphere the geomagnetic north points up at an angle, and magnetotactic bacteria in the Southern Hemisphere are “south seeking.” (At the equator, north-seeking and south-seeking magnetotactic bacteria coexist.) One way to demonstrate this in the laboratory is to place a small drop of water on a microscope coverslip and place the south pole of a small bar magnet 1–2 cm away from the drop. The south pole of the magnet corresponds to geomagnetic north. It is clear that swimming is important for this to occur, because dead cells are not pulled by a magnetic fi eld. A generally accepted model is that the earth’s magnetic fi eld is such that when the bacteria are swimming, the magnetosomes allow orientation in the magnetic fi eld and guide them to swim downward in their natural aque-ous habitat. All the magnetotactic bacteria are microaerophilic or anaerobic, and it is thought that magnetotaxis to lower levels is benefi cial because there is less oxygen at greater depths.
However, although this is the case for most MB that have been observed, exceptions do occur.
There has been a report of a population of mag-netotactic bacteria in the Northern Hemisphere that responds to high oxygen levels by swim-ming toward geomagnetic south in the drop assay just described, rather than in the direction of geomagnetic north. Bacteria displaying south polarity co-occurred in water samples from the
There has been a report of a population of mag-netotactic bacteria in the Northern Hemisphere that responds to high oxygen levels by swim-ming toward geomagnetic south in the drop assay just described, rather than in the direction of geomagnetic north. Bacteria displaying south polarity co-occurred in water samples from the