Cell membrane complex composition

Medullary cells are loosely packed, and during dehydration (formation), they leave a series of vacuoles along the fiber axis. Medullary cells are spherical and hollow inside and are bound together by a cell membrane complex type material (see Figure 1-43). Because the medulla is believed to contribute negligibly to the chemical and mechanical properties of human hair fibers [131] and is difficult to isolate [122,134], it has received comparatively little scientific attention. The chemical composition of medullary protein derived from African porcupine quill has been reported by Rogers [118] and is described in Chapter 2. 

Lipid extracted from human hair is similar in composition to scalp lipid [134]. Thus, the bulk of the extractable lipid in hair is free lipid however, cell membrane complex lipid is also partially removed by extraction of hair with lipid solvents or surfactants. In a sense, the scalp serves as a lipid supply system for the hair, with sebum being produced continuously by the sebaceous glands [135]. Sebum production is controlled hormonally by androgens that increase cell proliferation in the sebaceous glands, and this in turn increases sebum production [135,136], although seasonal and even daily variations in the rate of sebum production do occur [137]. 

Leeder [16] has shown that the composition of the cell membrane complex (see Section 5.2.5), of which the lipid fraction is one component, has a dramatic influence on fiber and fabric properties. The composition of the internal lipid fractions of a number of specialty animal fibers has been the subject of detailed study [309,310]. Wool, cashmere, cashgora, and mohair contain free cholesterol and desmosterol in the ratio of 1.7-2.6 1 [309]. By comparison, llama, camel, and alpaca fibers contain virtually no free cholesterol or desmosterol. The results for yak vary widely [309,310]. Rabbit and dog hairs have distinctive sterol compositions, which are unlike each other and different from that of wool and goat fibers. 

Figure 2 Composite structure of hair at various length-scales (a) filament protein with alternating helical/linker sections (helical section is shown atttie bottom) (b) coiled coil of filament proteins (in a polar environment two polypeptide chains naturally coil around each other in parallel, as this leaves the hydrophobic groups shielded in the centre) (c) intermediate filament with 16 coils (d) filament embedded in matrix (e) macrofibril (f) cortical call enclosed by cell-membrane complex.

Wool fibers contain two types of cells, viz. cuticle cells and cortical cells. The cuticle cells consist of external epicuticle, exocuticle, and endocuticle. The cortical cells are divided into two different types of cells termed as orthocortical and paracortical cells which occupy about 90% of the wool fibers. They are separated from one another by a cell membrane complex with three layer structure. The cortex structure is constituted from the crystalline microfibril of the a-helical aggregate embedded in a matrix of high sulfur content. Wool fiber is thus a composite material with a variety of function on mechanical, chemical, and physical properties. 

From the information given above it is obvious that cell surfaces display an enormous complexity. A perfect model to study the interaction of a peptide with a biological membrane would require knowledge about the cell membrane composition in that particular tissue. Even if such information were available it will most probably not be possible to fully mimic the biological environment. However, some important aspects may still be studied with the available models. Whenever possible, one should try to relate the information derived from such a model to information gained from biological data taken on real cells (cell-lines) such as binding affinities etc. in order to prove the validity of the model for the study of a particular aspect. 

Most lipids are barely soluble in water, and many have amphipathic properties. In the blood, free triacylglycerols would coalesce into drops that could cause fat embolisms. By contrast, amphipathic lipids would be deposited in the blood cells membranes and would dissolve them. Special precautions are therefore needed for lipid transport in the blood. While long-chain fatty acids are bound to albumin and short-chain ones are dissolved in the plasma (see p. 276), other lipids are transported in lipoprotein complexes, of which there several types in the blood plasma, with different sizes and composition. 

In general the sphingolipids are located on the exterior face of a membrane while the phospholipids make up the inner face. This is understandable when we recall that sphingolipids include the gangliosides and cerebrosides whose polar ends contain carbohydrates or complex carbohydrate derivatives. The large number of chiral centers in carbohydrate molecules offer a complex pattern on the surface of the membrane which can impart a large degree of specificity for a particular cell type. The composition and limited fluidity of the bilayer make the entire membrane asymmetric, that is, different on the inner and outer layers or leaflets. 

Thus, the fat globules are surrounded, at least initially, by a membrane typical of eukaryotic cells. Membranes are a conspicuous feature of all cells and may represent 80% of the dry weight of some cells. They serve as barriers separating aqueous compartments with different solute composition and as the structural base on which many enzymes and transport systems are located. Although there is considerable variation, the typical composition of membranes is about 40% lipid and 60% protein. The lipids are mostly polar (nearly all the polar lipids in cells are located in the membranes), principally phospholipids and cholesterol in varying proportions. Membranes contain several proteins, perhaps up to 100 in complex membranes. Some of the proteins, referred to as extrinsic or peripheral, are loosely attached to the membrane surface and are easily removed by mild extraction procedures. The intrinsic or integral proteins, about 70% of the total protein, are tightly bound to the lipid portion and are removed only by severe treatment, e.g. by SDS or urea. 

Complexes of alkali metals and alkaline-earth metals with carbohydrates have been reviewed in this Series,134 and the interaction of alkaline-earth metals with maltose has been described.135 Standard procedures for the preparation of adducts of D-glucose and maltose with the hydroxides of barium, calcium, and strontium have been established. The medium most suitable for the preparation of the adduct was found to be 80% methanol. It is of interest that the composition of the adducts, from D-glucose, maltose, sucrose, and a,a-trehalose was the same, namely, 1 1, in all cases. The value of such complex-forming reactions in the recovery of metals from industrial wastes has been recognized. Metal hydroxide-sugar complexes may also play an important biological role in the transport of metal hydroxides across cell membranes. 

Another important parameter is the surface charge associated with microbial particles. This reflects the composition of the cell wall surrounding the cell membranes it is complex and dependent on environmental conditions and on the stages of the life cycle 7 . 

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

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