Cell membranes amphipathic lipids

Triacylglycerols are the major energy-storing lipids, whereas phosphoglycerols, sphingomyelin, and gly-cosphingolipids are amphipathic and have structural functions in cell membranes as well as other specialized roles. 

The nonpolar lipid core consists of mainly triacylglycerol and cholesteryl ester and is surrounded by a single surface layer of amphipathic phospholipid and cholesterol molecules (Figure 25-1). These are oriented so that their polar groups face outward to the aqueous medium, as in the cell membrane (Chapter 14). The protein moiety of a lipoprotein is known as an apo-lipoprotein or apoprotein, constituting nearly 70% of some HDL and as litde as 1% of chylomicrons. Some apolipoproteins are integral and cannot be removed, whereas others are free to transfer to other hpoproteins. 

The cells of all contemporary living organisms are surrounded by cell membranes, which normally consist of a phospholipid bilayer, consisting of two layers of lipid molecules, into which various amounts of proteins are incorporated. The basis for the formation of mono- or bilayers is the physicochemical character of the molecules involved these are amphipathic (bifunctional) molecules, i.e., molecules which have both a polar and also a non-polar group of atoms. Examples are the amino acid phenylalanine (a) or the phospholipid phosphatidylcholine (b), which is important in membrane formation. In each case, the polar group leads to hydrophilic, and the non-polar group to hydrophobic character. 

Amphipathic molecules can form bilayered lamellar structures spontaneously if they have an appropriate geometry. Most of the major cell membrane lipids have a polar head, most commonly a glycerophosphorylester moiety, and a hydrocarbon tail, usually consisting of two... 

Nutrients. Amphipathic lipids are used by cells to build membranes (see p. 214). Typical membrane lipids include phospholipids, glycolipids, and cholesterol. Fats are only weakly amphiphilic and are therefore not suitable as membrane components. 

All biological membranes are constructed according to a standard pattern. They consist of a continuous bilayer of amphipathic lipids approximately 5 nm thick, into which proteins are embedded, in addition, some membranes also carry carbohydrates (mono- and oligosaccharides) on their exterior, which are bound to lipids and proteins. The proportions of lipids, proteins, and carbohydrates differ markedly depending on the type of cell and membrane (see p. 216). 

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. 

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. 

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 plasma membrane of epithelial cells, in common with other cell types, is selectively permeable, allowing the penetration of some substances but not others. The construction of the membrane from amphipathic lipid molecules forms a highly impermeable barrier to most polar and charged molecules, thereby preventing the loss of most water-soluble contents of the cell. This selective permeability presents a physical barrier to drag absorption, limiting absorption to specific routes and mechanisms, as described below (see Section 1.3.3). 

Phospholipids and sphingolipids are major components of cell membranes. They are amphipathic molecules that is, one portion of the molecule is hydrophilic and associates with H20, and another portion contains the hydrocarbon chains derived from fatty acids, which are hydrophobic and associate with lipids (see Figure 6-4). 

Phospholipids and glycosphingolipids are amphipathic lipid constituents of membranes (Chapter 10). They play an essential role in the synthesis of plasma lipoproteins (Chapter 20) and eicosanoids (Chapter 18). They function in transduction of messages from cell surface receptors to second messengers that control cellular processes (Chapter 30) and as surfactants. Cholesterol is mainly of animal origin and is an essential constituent of biomembranes (Chapter 10). In plasma, cholesterol is associated with lipoproteins (Chapter 20). Cholesterol is a precursor of bile acids formed in the liver of steroid hormones secreted by adrenals, gonads, and placenta and 7-dehydrocholesterol of vitamin D formed in the skin. In tissues, cholesterol exists primarily in the unesterified form (e.g., brain and erythrocytes), although appreciable quantities are esterified with fatty acids in liver, skin, adrenal cortex, and plasma lipoproteins. 

Phospholipids comprise the outer monolayer of lesional lipoproteins and the membranes of lesional cells. In lipoproteins, the phospholipid monolayer provides an amphipathic interface between the neutral lipid core and the aqueous external environment, and provides the structural foundation for the various apolipoproteins (Chapter 18). In the specific context of atherosclerosis, the phospholipids of lesional lipoproteins are modified by various oxidative reactions that could have important pathological consequences. In lesional cells, membrane phospholipids not only play structural roles but also are precursors to important phospholipase-generated signaling molecules that may participate in atherogenesis. 

Cell membranes are constructed principally of phospholipids, sphingolipids, and proteins, together with small amounts of cholesterol. The common feature of the membrane lipids is their similarity to soap and detergent molecules—the presence of both hydrophilic and hydrophobic portions in the same molecule. Such molecules are referred to as amphipathic molecules. 

Lipids are amphipathic inolecules composed of a polar, hydrophilic head connected to a nonpolar, hydrophobic hydrocarbon tail. When in an aqueous environment, lipids tend to associate noncovalently. There are two driving forces for this association the hydrophobic effect due to the nonpolar tails, and the van der Waals interactions between the hydrocarbon portions of the molecules. This behavior in water can cause lipids to spontaneously form surface monolayers, bilayers, micelles, or vesicles, depending on the structures of the head and tail of the lipid molecule. We shall direct our attention here to the cell membrane bilayer, the most important of these biological assemblies. 

Cell membranes are composed of a double layer (or bilayer) of amphipathic lipids with which a variety of proteins are associated. Amphipathic lipids are thus called because they contain both a hydrophobic and a hydrophilic moiety. Lipids in membrane bilayers are oriented in such a way that their hydrophobic (lipophilic)... 

Phospholipids are important components of cell membranes and lipoproteins (Chapter 37). They are amphipathic compounds, i.e. they have an affinity for both aqueous and non-aqueous environments. The hydrophobic part of the molecule associates with hydrophobic lipid molecules, while the hydrophilic part of the molecule associates with water. In this way, phospholipids are compounds that form bridges between water and lipids. 

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