Membranes, cell bilayer structure

The most important bilayers are formed by the phospholipids, shown in Figure 12.1(a).They form the membrane that covers the surface of our blood cells. Bilayer structures are stabilized by solvation of the head groups by water and by the nearly total avoidance of water by the hydrophobic hydrocarbon tails. We observe the same pattern in many self-assemblies found in nature, such as micelles, reverse micelles, and microemulsions, discussed later. 

Fig. 7 Diagrammatic representation of the fluid mosaic model of the cell membrane. The basic structure of the membrane is that of a lipid bilayer in which the lipid portion (long tails) points inward and the polar portion (round head ) points outward. The membrane is penenetrated by transmembrane (or integral) proteins. Attached to the surface of the membrane are peripheral proteins (inner surface) and carbohydrates that bind to lipid and protein molecules (outer surface). (Modified from Ref. 14.)...

Exploration of the use of liposomes in wool processing stems from the similarity that exists between the bilayer structure of the cell membrane complex of wool and that of the liposomes. Merino wool contains about 1% by weight of lipids, these forming the hydrophobic barrier of the cell membrane complex. Cholesterol is one of the main lipid... 

Phospholipids are a major component of living cell membranes. Physical and chemical properties of bilayer structure composed of phospholipids have been well studied, (ij.2) One of the intriguing properties of phospholipids is that they form a closed structure hereafter referred to as vesicles. Vesicles have attracted much attention since they are considered to mimic biocelIs. 

Cell membranes are bilayers of amphipathic acids, for example phospholipids and sterols, which contain globular proteins. The structure is governed by the essential requirement for stability in an aqueous environment, that is, the hydrophobic tails of the lipid molecules point towards each other, leaving the outer surfaces composed of polar, hydrophilic groups. 

A cell is enclosed by a lipid bilayer known as the plasma membrane. In Vignette 1.2 in Chapter 1 we discussed an example of a membrane, a complex structure with a mosaic of embedded or adsorbed moieties such as proteins. It is these membranes that protect the intracellular contents from the exterior environment of the cells and regulate the transport of materials into and out of the cells. They can also act as signal transducers and control the electrical excitation in the nervous system by altering the (membrane) permeability to particular ions in response to stimuli. Such electrical activities can propagate over long distances and represent one of the most spectacular of the membrane functions. 

Figure 8.15 is a sketch of one possible relationship between the lipid bilayer and the membrane proteins. Molecules are free to move laterally in these membranes hence the structure pictured in Figure 8.15 is called the fluid mosaic model of a cell membrane. 

Phospholipids are ideal compounds for making membranes because of their amphipathic nature (see chapter 17). The polar head-groups of phospholipids prefer an aqueous environment, whereas the nonpolar acyl substituents do not. As a result, phospholipids spontaneously form bilayer structures (see fig. 17.6), which are a dominant feature of most membranes. The phospholipid bilayer is the barrier of the cell membrane that prevents the unrestricted transport of most molecules other than water into the cell. Entry of other molecules is allowed if a specific transport protein is present in the cell membrane. Similarly, the phospholipid bilayer prevents leakage of metabolites from the cell. The amphipathic nature of phospholipids has a great influence on the mode of their biosynthesis. Thus, most of the reactions involved in lipid synthesis occur on the surface of membrane structures catalyzed by enzymes that are themselves amphipathic. 

The existence of a forbidden water layer thickness range, which seems to be a general phenomenon with these gel phases, might be relevant to cell adhesion and equilibrium distances at cell contact. The gel represents one type of lipid bilayer structure that occurs in membranes (see below), and, because of the dominance of neutral lipid molecules, the... 

Fig. 4.13 Illustration of the bilayer structure of cell membranes showing phospholipid molecules and their opposite hydrophilic and hydrophobic sections...

Cholesterol - an essential component of mammalian cells - is important for the fluidity of membranes. With a single hydroxy group, cholesterol is only weakly am-phipathic. This can lead to its specific orientation within the phospholipid structure. Its influence on membrane fluidity has been studied most extensively in erythrocytes. It was found that increasing the cholesterol content restricts molecular motion in the hydrophobic portion of the membrane lipid bilayer. As the cholesterol content of membranes changes with age, this may affect drug transport and hence drug treatment. In lipid bilayers, there is an upper limit to the amount of cholesterol that can be taken up. The solubility limit has been determined by X-ray diffraction and is... 

FIGURE 2.12 (a) The structure of a bilayer lipid membrane (b) the structure of the glyceryl esters that are major components of bacterial and eukaryotic cell membranes and (c) the structure of the glyceryl ethers that are major components of archaean cell membranes. R is a hydrophilic chain of CH and CH2 atoms. 

Notably, all these characteristic alterations are conferred by cell membranes particularly the cytoplasmic membrane that is basically a lipid-bilayer structure imbedded with certain proteins. The lipid-bilayer forms a barrier to surround and protect the cell contents and the transmembrane proteins are responsible for the cell communications with the environment. For example, receptor proteins mediate the growth signals produced by growth factors and mitogens and any other stimuli. Ion channel proteins control the flux of ions across the cytoplasmic membrane to regulate membrane potential, osmolar-ity (or cell volume), etc. 

Liposomes, Vesicles and Cast Films - Supramolecular Assemblies Based on Lipid Bilayers Amphiphilic molecules or lipid molecules sometimes form double-layer structures. This structure is called a bilayer structure, and it can be used to model a cell membrane. 

Figure 4.22 shows examples of some hpid structures that are found in actual cell bilayer membranes. Some of them possess a phosphate moiety and are... 

The second example of a biological supermolecule is a cell membrane. As described in Chap. 4, a cell membrane consists mainly of a fluidic lipid bilayer containing proteins (Fig. 6.2). The Hpids are self-assembled into the bilayer structure and the proteins float within the Hpid bilayer. The whole structure is formed through self-assembly processes. 

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