Please read the article below and write a detailed word summary of the article and its correlation with the concepts being studied.
cytoplasmic membrane (CM; cell membrane; plasma membrane; plasmalemma; protoplast membrane)
from Dictionary of Microbiology & Molecular Biology
The lipid- and protein-containing, selectively permeable membrane which encloses the cytoplasm in prokaryotic and eukaryotic cells; in most types of microbial cell the CM is bordered externally by the CELL WALL. In microbial cells the precise composition of the CM may depend on growth conditions and on the age of the cell.
The structure of all biological membranes appears to conform to the basic fluid mosaic model. In this model the lipid molecules form a bilayer within which the protein molecules are partly or wholly embedded—some spanning the entire width of the bilayer; the lipid molecules are orientated such that their polar groups form the outer, hydrophilic surfaces of the bilayer while their hydrocarbon chains form the hydrophobic interior of the bilayer. (cf. UNIT MEMBRANE.) The membrane proteins are sometimes categorized as either peripheral (= extrinsic) proteins (bound to the membrane e.g. by electrostatic forces, and easily removable by electrolytes or chelating agents) or integral (= intrinsic) proteins (bound more strongly by hydrophobic bonds and extractable, with difficulty, by detergents or organic solvents). Cytoplasmic membranes are characteristically asymmetrical, i.e. the components of the outer (externally facing) side of the membrane are not identical to those of the inner (cytoplasmic) side.
Membrane ‘fluidity’ primarily involves lateral and rotational motion of whole lipid molecules as well as motion of the hydrocarbon chains of the lipid molecules (rather than movement of whole lipid molecules from one layer of the membrane to the other layer). The hydrocarbon chains may be disordered and flexible (the α-conformation) or ordered, rigid and perpendicular to the plane of the bilayer (the β-conformation). Fluidity greater than a certain level is essential for the normal physiological role of the CM. The degree of fluidity is governed by temperature and e.g. by the length and structure of the hydrocarbon chains; membranes containing unsaturated chains are generally more ‘fluid’ than are membranes containing saturated chains of the same length.
Previously, it was thought that only rarely do lipid molecules move from one side of the bilayer to the other. It is now known that such transmembrane movement (flip-flop) occurs with a much greater frequency than was originally supposed; in at least one case evidence has been obtained for a ‘flippase’ which may facilitate such translocation [Cell (1985) 42 51-60].
Although it is believed that the membrane lipids are normally present as a bilayer, results of nuclear magnetic resonance (NMR) and other studies have indicated the presence of transient, temperature-dependent, localized non-bilayer lipid phases within biological membranes. In one of these phases, termed hexagonal II (or HII), the lipid molecules form an array of fine (∼20 Å diam.) water-filled cylinders whose walls are composed of the polar heads of lipid molecules; the array of cylinders is hexagonal in cross-section, the space between the cylinders containing the hydrocarbon chains of the lipid molecules. [Lipid structure of biological membranes: TIBS (1985) 10 418-421.]
The CM has various functions, one of which is to regulate the cytoplasmic milieu by controlling the inward and outward passage of ions and molecules. Some uncharged and/or lipophilic molecules can pass relatively freely through the CM; these include e.g. water, carbon dioxide, oxygen, ammonia (but not ammonium ions), acetic acid (undissociated form) and ethanol. However, ions and most molecules cannot pass freely through the CM, so that their transmembrane translocation requires more or less specific TRANSPORT SYSTEMS. (Specific mechanisms also exist for OSMOREGULATION.) Transport commonly occurs at the expense of metabolic energy, and some systems depend on the presence of a transmembrane electrochemical gradient—e.g. proton motive force (pmf: see CHEMIOSMOSIS) or SODIUM MOTIVE FORCE—such gradients being a general feature of CMs.
According to species, the CM is also the site of RESPIRATION (and, in some organisms, photosynthesis).
The CM is involved in the synthesis of external structures, such as the cell wall and capsule, and also in the synthesis of components of the CM.
(a) Bacterial cytoplasmic membranes. In transmission electron micrographs the CM, ca. 7-8 nm thick, typically appears as a trilaminar structure: an electron-translucent layer sandwiched between two electron-dense layers. In Gram-negative bacteria, localized regions of the CM may be involved in ADHESION SITES. In many species of bacteria it has been demonstrated that the inner (cytoplasmic) face of the CM bears minute, spherical, ‘stalked’ particles (see PROTON ATPASE) which are involved in energy conversion. In many (not all) species, CYTOCHROMES and other components of an ELECTRON TRANSPORT CHAIN occur in the CM. (See also PURPLE MEMBRANE.)
Lipid components of bacterial CMs are mainly phospholipids; in many or all cases, these are synthesized within the membrane itself.
Phosphatidylglycerols (PG) appear to occur in all bacteria, while phosphatidylethanolamine (PE) is more common and more abundant in Gram-negative species; phosphatidylcholine is absent in Gram-positive cells and is rare in Gram-negative bacteria. Phosphatidylinositol occurs in some bacteria (e.g. Mycobacterium spp). PLASMALOGENS are found in the cytoplasmic membrane in some anaerobes. In Escherichia coli the main phospholipid is PE—PG and diphosphatidylglycerol (DPG, cardiolipin) being relatively minor components.
Glycolipids are common in small quantities; in some Gram-positive bacteria molecules of glycolipid may be covalently linked to glycerol TEICHOIC ACIDS, forming lipoteichoic acid.
Sphingolipids are rare in bacterial membranes.
Sterols are absent in most species (cf. MYCOPLASMATACEAE).
Hopanes (triterpene derivatives which resemble sterols in size, rigidity and amphiphilicity) are present in some bacteria [JGM (1985) 131 1363-1367].
Lipoamino acids (O-aminoacylphosphatidylglycerols) occur in the membranes of some Gram-positive bacteria (e.g. Bacillus spp, Clostridium spp, Staphylococcus aureus). These are esters of PG and basic amino acids such as arginine, lysine or ornithine; the proportion of lipoamino acids varies according to growth phase and to the pH of the medium.
The CM fatty acids may be straight-chained or branched (the latter more common in Gram-positive species), saturated or unsaturated; some contain a cyclopropane ring (which is formed by methylation at a double bond). Phospholipids often contain one saturated and one unsaturated fatty acid residue per molecule.
In E. coli the main saturated fatty acids are hexadecanoic (palmitic) and tetradecanoic (myristic) acids, minor components including e.g. octadecanoic (stearic) and dodecanoic (lauric) acids; the main unsaturated fatty acids (all of which are cis-monoenes) include e.g. cis-Δ9-hexadecenoic (palmitoleic) acid.
In general, the CM varies in its fatty acid composition according to growth conditions (e.g. temperature, pH). Thus, in some species the proportion of unsaturated fatty acids increases when the growth temperature decreases—e.g. in E. coli the proportion of unsaturated fatty acids increases from 16% to 49% when the growth temperature falls from 36°C to 25°C; such a change appears to be a compensatory response which helps to maintain optimum membrane fluidity. In contrast, almost no increase in monoenoic fatty acids occurs in the CM of Staphylococcus aureus when the growth temperature drops from 37°C to 25°C [Book ref. 44, p 402]. In some bacteria, adaptation to lower growth temperatures involves an increase in the length of fatty acid chains rather than an increase in the degree of unsaturation [JGM (1985) 131 2293-2302].
The lipids of the CM clearly contribute to the essential feature of selective permeability. However, the lipids also have other functions; for example, in E. coli phosphatidylethanolamine can behave as a ‘molecular chaperone’, being required for the correct folding (maturation) of the membrane protein LacY [EMBO (1998) 17 5255-5264].
Proteins in the bacterial CM include a variety of enzymes (involved e.g. in the synthesis of phospholipids and cell wall components); for example, penicillin-binding proteins (involved in the synthesis of PEPTIDOGLYCAN) may form part of a protein complex in the CM [FEMS Reviews (1994) 13 1-12]. There are also components of TRANSPORT SYSTEMS, energy-converting systems (see e.g. ELECTRON TRANSPORT CHAIN and EXTRACYTOPLASMIC OXIDATION) and sensing systems (see e.g. CHEMOTAXIS).
CM proteins also include the molecular water channels called aquaporins (see MIP CHANNEL). (See also MECHANOSENSITIVE CHANNEL.)
[Structural dynamics of the CM of E. coli: Book ref. 122, pp 121-160.]
The bacterial CM is the target for a variety of ANTISEPTICS and DISINFECTANTS (see e.g. QUATERNARY AMMONIUM COMPOUNDS) and ANTIBIOTICS (see e.g. DEPSIPEPTIDE ANTIBIOTICS, GRAMICIDINS, POLYMYXINS and TYROCIDINS).
(b) Fungal cytoplasmic membranes. The CMs in fungi (and in other eukaryotes) differ significantly from those in bacteria. In fungal CMs the major lipids typically include phosphatidylcholine and phosphatidylethanolamine (with smaller amounts of phosphatidylinositol and phosphatidylserine) together with SPHINGOLIPIDS; as a rough generalization, the fatty acids of phospholipids in higher fungi tend to contain even numbers of carbon atoms and to be saturated or mono-unsaturated, while those of lower fungi tend to have odd numbers of carbon atoms and to be polyunsaturated. (Changes in the degree of fatty acid saturation can have interesting physiological effects; for example, when Saccharomyces cerevisiae strain Y185 becomes de-repressed for general amino acid permease, a higher degree of fatty acid unsaturation in the CM correlates with a more rapid expression of the permease [JGM (1985) 131 57-65].) Most fungal membranes also contain sterols (e.g. ergosterol), so that fungi are generally susceptible to POLYENE ANTIBIOTICS (see also AZOLE ANTIFUNGAL AGENTS); vegetative cells of members of the Pythiaceae have been reported to lack sterols, although sterols are required during the reproductive phase.
Proteins in fungal CMs include those involved in transport processes, components of ATPase complexes, and various enzymes—such as those needed for the synthesis of walls and membranes.
In addition to lipids and proteins, fungal CMs contain small amounts of carbohydrate. In e.g. some slime moulds, certain CM carbohydrates are important LECTIN receptors and are involved in cell-cell recognition and/or adhesion.
(c) Archaeal cytoplasmic membranes. In members of the ARCHAEA the CM contains lipids of a kind which do not occur in bacteria. Unlike the ester-linked glycerol-fatty acid bacterial lipids, archaeal lipids are characteristically ether-linked molecules that contain e.g. isoprenoid or hydro-isoprenoid components. Some of the archaeal lipids are structurally analogous to those of bacteria; for example, the di-ether and di-ester lipids both have a single polar end. However, some archaeal lipids (e.g. tetra-O-di(biphytanyl) diglycerol) contain one ether-linked glycerol residue at each end of the molecule; having two polar ends, such molecules may span the width of the CM. Given the exteme habitats typically occupied by these organisms, it seems likely that some or all of the archaeans will be found to contain membrane components whose characteristics reflect an adaptation to the environment.
Some archaeans contain MIP CHANNELS.
Copyright © 2006 John Wiley & Sons Ltd.
We are a professional custom writing website. If you have searched a question and bumped into our website just know you are in the right place to get help in your coursework.
Yes. We have posted over our previous orders to display our experience. Since we have done this question before, we can also do it for you. To make sure we do it perfectly, please fill our Order Form. Filling the order form correctly will assist our team in referencing, specifications and future communication.
1. Click on the “Place order tab at the top menu or “Order Now” icon at the bottom and a new page will appear with an order form to be filled.
2. Fill in your paper’s requirements in the "PAPER INFORMATION" section and click “PRICE CALCULATION” at the bottom to calculate your order price.
3. Fill in your paper’s academic level, deadline and the required number of pages from the drop-down menus.
4. Click “FINAL STEP” to enter your registration details and get an account with us for record keeping and then, click on “PROCEED TO CHECKOUT” at the bottom of the page.
5. From there, the payment sections will show, follow the guided payment process and your order will be available for our writing team to work on it.
Need help with this assignment?
Order it here claim 25% discount
Discount Code: SAVE25