Membrane, biological cell phospholipids

Myo-inositol is one of the most biologically active forms of inositol. It exists in several isomeric forms, the most common being the constituent of phospholipids in biological cell membranes. It also occurs as free inositol and as inositol hexaphosphate (IP6) also known as phytate which is a major source from food. Rice bran is one of the richest sources of IP6 as well as free inositol. Inositol is considered to belong to the B-complex vitamins. It is released in the gastrointestinal tract of humans and animals by the dephosphorylation of IP6 (phytate) by the intestinal enzyme phytase. Phytase also releases intermediate products as inositol triphosphate and inositol pentaphosphate. Inositol triphosphate in cellular membrane functions as an important intra- and intercellular messenger, that merits its value as a nutritional therapy for cancer. 

Lyotropic liquid-crystalline nanostructures are abundant in living systems. Accordingly, lyotropic LC have been of much interest in such fields as biomimetic chemistry. In fact, biological membranes and cell membranes are a form of LC. Their constituent rod-like molecules (e.g., phospholipids) are organized perpendicularly to the membrane surface yet, the membrane is fluid and elastic. The constituent molecules can flow in plane quite easily but tend not to leave the membrane, and can flip from one side of the membrane to the other with some difficulty. These LC membrane phases can also host important proteins such as receptors freely floating inside, or partly outside, the membrane. 

Bishop, W.R. Bell, R.M. (1988) Assembly of phospholipids into cellular membranes biosynthesis, transmembrane movement and intracellular translocation. Ararat. Rev. Cell Biol. 4, 579-610. Advanced review of the enzymology and cell biology of phospholipid synthesis and targeting. 

The key structural component of the cell membrane is a phospholipid bilayer in which proteins are embedded. Some of the proteins protrude from the outer membrane surface. These proteins typically serve as receptor sites for endogenous substances such as hormones or neurotransmitters, for purposes of eliciting a biological response from the tissue to which the cell belongs. Other proteins traverse the lipid bilayer, forming aqueous pores or channels that connect the exterior of the cell with its interior. These pores vary from about 4 to 10 A in diameter, being predominantly about 7 A. 

Figure 4.28 Schematic representation of a biological cell membrane. A bimolecular layer of phospholipid with hydrocarbon chains orientated to the interior and hydrophilic groups on the outside is penetrated by protein (shaded areas). Protein is also found adsorbed at the membrane surface...

Lipids play a number of important biological roles. Phospholipids are triesters of glycerol in which one ester is derived from a phosphatidylamine. They are important structural units in cell membranes. Prostaglandins are 20-carbon cyclopentane derivatives of arachidonic acid that have profound biological effects, even in minute quantities. Waxes are monoesters of long-chain acids and alcohols. 

Membranes, which are the subject of this section, can be relatively thick (0.1 mm) if made chemically (see their use in the PEM fuel cell, (Section 13.7.3). Biological membranes are very much thinner (50-100 A), of the same (3-5 nm) range as that of passive oxides (Section 12.5). Of what do biological membranes consist Figure 14.6 shows the essential constituents. They are lipids and proteins. How much there is of one and how much of the other varies widely. Thus, in a myelin membrane the lipid content is 80% while at the other end of the range, in mitochondria, there is an inner membrane containing only about 20% lipid. There are many kinds of lipids (as well as very many kinds of proteins), but those in membranes are usually phospholipids and are represented in Fig. 14.7. The structure often contains an H atom and this allows... 

Current theories propose that the development of cell membranes accelerated early life. Enclosing chemicals responsible for replication, protein synthesis and energy generation within a confined space allows these essential life processes to occur far more frequently than they would do otherwise. Biological membranes comprise two phospholipid layers held together by interdigitation of their hydrophobic alkyl tails as shown in Fig. 5.1. They form a lamellar structure with their polar internal and external surfaces separated by a non-polar region. In their extended form each... 

Surfactant Effects on Microbial Membranes and Proteins. Two major factors in the consideration of surfactant toxicity or inhibition of microbial processes are the disruption of cellular membranes b) interaction with lipid structural components and reaction of the surfactant with the enzymes and other proteins essential to the proper functioning of the bacterial cell (61). The basic structural unit of virtually all biological membranes is the phospholipid bilayer (62, 63). Phospholipids are amphiphilic and resemble the simpler nonbiological molecules of commercially available surfactants (i.e., they contain a strongly hydrophilic head group, whereas two hydrocarbon chains constitute their hydrophobic moieties). Phospholipid molecules form micellar double layers. Biological membranes also contain membrane-associated proteins that may be involved in transport mechanisms across cell membranes. 

Conversely, membranes composed of phospholipids with highly unsaturated acyl chains are less rigid, because of interactions between such bulky acyl chains. This state of membrane disorder produces a physical fluidity and it is proposed that such membranes offer minimal hindrance lo the function of proteins (e.g., receptors or channels) that lie within the membrane. Metabolically active membranes, such as those found in neuron cell bodies, are comprised of phospholipids containing unsaturated sn-2 chains. Work by Brown (1994), Dratz (Dratz Holte, 1993), and Litman (Littman Mitchell, 1996) support this notion, as recombinant membranes that comprise higher levels of unsaturated PUFA, particularly the long-chain n-3, DHA, mimic, more closely, biological activity when measured in vitro. 

Phosphatidylcholine, a glycerolphosphatediester drawn in Figure 9.5, is the constituent molecule of phospholipidic membranes , which form the envelopes of biological cells, cell... 

Asymmetry. Biological membranes are asymmetric that is, the lipid composition of each half of a bilayer is different. For example, the human red blood cell membrane possesses substantially more phosphatidylcholine and sphingomyelin on its outside surface. Most of the membrane s phosphatidylserine and phos-phatidylethanolamine are on the inner side. Membrane asymmetry is not unexpected, because each side of a membrane is exposed to a different environment. Asymmetry originates during membrane synthesis, because phospholipid biosynthesis occurs on only one side of a membrane (Special Interest Box 12.3). The protein components of membranes (discussed below) also exhibit considerable asymmetry with distinctly different functional domains within membrane and on the cytoplasmic and extracellular faces of membrane. 

The membranes of cells are largely composed on of a phospholipid bilayer and proteins. Most of our current information concerning biological membranes is summarized by the fluid mosaic model proposed by S. J. Singer and G. L. Nicholson in 1972. This is the model depicted in Figure 10.10. 

This is all fairly interesting, but you may start wondering what does it have to do with biology Here is the answer Molecules of phospholipids have the shape of a tadpole , although normally with two, or sometimes even three, tails. They are the chief constituent of membranes that separate biological cells from the outside world and divide the cells into compartments. The considerable thickness of the double tails prevents phospholipids from clumping into micelles, hence they form into layered walls. 

Lipids are a diverse class of nonpolar biological molecules used by organisms for longterm energy storage (fats, oils) and as elements of biological structures (phospholipids, cell membranes, waxes). 

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