Membranes are an absolute requirement for all living organisms. Cells must interact during a selective fashion with their environment, acquire nutrients, and eliminate waste. They even have to take care of their interior during a constant, highly organized state within the face of external changes. The cell wall encompasses the cytoplasm of both prokaryotic and eukaryotic cells. It's the chief point of contact with the cell’s environment and thus is liable for much of its relationship with the surface world. The plasma membranes of procaryotic cells are particularly important because they need to fill a fantastic sort of role. Additionally, to retaining the cytoplasm, the cell wall also is a selectively permeable barrier: it allows particular ions and molecules to pass, either into or out of the cell, while preventing the movement of others. Thus the membrane prevents the loss of essential components through leakage while allowing the movement of other molecules. Because many substances cannot cross the cell wall without assistance, they must aid such movement when necessary. Transport systems are used for such tasks as nutrient uptake, waste excretion, and protein secretion. The procaryotic cell wall is also the situation of a spread of crucial metabolic processes: respiration, photosynthesis, and therefore the synthesis of lipids and cell membrane constituents. Finally, the membrane contains special receptor molecules that help procaryotes detect and answer chemicals in their surroundings. Clearly, the cell wall is important to the survival of microorganisms.
All membranes have a standard, basic design. However, procaryotic membranes can differ dramatically in terms of the lipids they contain. Indeed, membrane chemistry is often wont to identify particular procaryotic species. To know these chemical differences and therefore the many functions of the cell wall, it's necessary to become conversant in membrane structure.
Fluid Mosaic Model of Membrane Structure
The most widely accepted model for membrane structure is that the fluid mosaic model of Singer and Nicholson, which proposes that membranes are lipid bilayers within which proteins float The model is predicated on studies of eucaryotic and bacterial membranes, and a spread of experimental approaches was wont to establish it. Transmission microscopy (TEM) studies were particularly important. When cell membranes are stained and examined by TEM, they're revealed as very thin structures, about 5 to 10 nm thick, that appear as two dark lines on either side of a lightweight interior. This characteristic appearance has been interpreted to mean that the membrane is organized in two sheets of molecules arranged end-to-end. When membranes are cleaved by the freeze-etching technique, they will be reverse split the middle of the lipid bilayer, exposing the complex internal structure. Within the lipid bilayer, small globular particles are visible; these are suggested to be membrane proteins lying within the membrane lipid bilayer. the utilization of atomic force microscopy has provided powerful images to support this interpretation.
The chemical nature of membrane lipids is critical to their ability to make bilayers. Most membrane-associated lipids are structurally asymmetric, with polar and nonpolar ends, and are called amphipathic. The polar ends interact with water and are hydrophilic; the nonpolar hydrophobic ends are insoluble in water and have a tendency to accompany each other. In aqueous environments, amphipathic lipids can interact to make a bilayer. The outer surfaces of the bilayer membrane are hydrophilic, whereas hydrophobic ends are buried within the interior far away from the encompassing water.
Integral proteins can diffuse laterally within the membrane to new locations but don't flip-flop or rotate through the lipid layer. Carbohydrates often are attached to the outer surface of cell wall proteins, where they need important functions.
Bacterial Membranes
Bacterial membranes are almost like eucaryotic membranes therein many of their amphipathic lipids are phospholipids, but they typically differ from eucaryotic membranes in lacking sterols (steroid-containing lipids) like cholesterol. However, many bacterial membranes contain sterol-like molecules called hopanoids.
Hopanoids are synthesized from equivalent precursors as steroids, and just like the sterols in eucaryotic membranes, they probably stabilize the membrane. Hopanoids also are of interest to ecologists and geologists: the entire mass of hopanoids in sediments is estimated to be around 10 11‒12 tons about the maximum amount because the total mass of organic carbon altogether living organisms (10 12 tons)—and evidence exists that hopanoids have contributed significantly to the formation of petroleum.
The emerging picture of bacterial plasma membranes is one among a highly organized and asymmetric system that is also flexible and dynamic. Numerous studies have demonstrated that lipids aren't homogeneously distributed within the cell wall. Rather, there are domains during which particular lipids are concentrated. It's also been demonstrated that the lipid composition of bacterial membranes varies with environmental temperature in such how that the membrane remains fluid during growth. For instance, bacteria growing at lower temperatures have more unsaturated fatty acids in their membrane phospholipids—that is, there are one or more double covalent bonds within the long hydrocarbon chain. At higher temperatures, their phospholipids have more saturated fatty acids—those during which the carbon atoms are connected only with single covalent bonds.
Although prokaryotes don't contain complex membranous organelles like mitochondria or chloroplasts, internal membranous structures are observed in some bacteria. These are often extensive and sophisticated in photosynthetic bacteria and in bacteria with very high respiratory activity, like the nitrifying bacteria. the interior membranes of cyanobacteria are called thylakoids and are analogous to the thylakoids of chloroplasts. They contain chlorophyll and therefore the photosynthetic reaction centres liable for converting light energy into ATP, the energy currency employed by cells. The interior membranous structures observed in bacteria could also be aggregates of spherical vesicles, flattened vesicles, or tubular membranes. Their function could also be to supply a bigger membrane surface for greater metabolic activity.
Archaeal Membranes
One of the foremost distinctive features of the Archaea is that the nature of their membrane lipids. They differ from both Bacteria and Eucarya in two ways. First, they contain hydrocarbons derived from isoprene units—five-carbon, branched molecules. Thus the hydrocarbons are branched. Second, the hydrocarbons are attached to glycerol by ether links instead of ester links. When two hydrocarbons are attached to glycerol, the lipids are called diether lipids. Usually, the diether hydrocarbon chains are 20 carbons long. Sometimes tetraether lipids are formed when two glycerol residues are linked by two long hydrocarbons that are 40 carbons long. Cells can adjust the general length of the tetraethers by cyclizing the chains to make pentacyclic rings. Phosphate-, sulfur-, and sugar-containing groups are often attached to the third carbons of the glycerol moieties within the diethers and tetraethers, making them polar lipids. Seventy to 93% of archaeal membrane lipids are polar. The remaining lipids are nonpolar and are usually derivatives of squalene.
Despite these significant differences in membrane lipids, the essential design of archaeal membranes is analogous thereto of Bacteria and eucaryotes—there are two hydrophilic surfaces and a hydrophobic core. When C 20 diethers are used, a daily bilayer membrane is made. When the membrane is made of C 40 tetraethers, a monolayer membrane with far more rigidity is made. As could be expected from their need for stability, the membranes of utmost thermophiles like Thermoplasma and Sulfolobus, which grow best at temperatures over 85°C, are almost completely tetraether monolayers. Archaea that sleep in moderately hot environments have membranes containing some regions with monolayers and a few with bilayers.
References :
Prescott's Principles of Microbiology by Joanne M. Willey, Linda M. Sherwood, and Christopher J. Woolverton.
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