Functions of lipids

The major roles of lipids can be described conveniently as structural, storage and metabolic, although individual lipids may have several different roles at different times or even at one and the same time. 

1 Structural lipids these are important at surfaces and in membranes - barriers between one environment and another 

Lipids play an important part in biological structures whose purpose is to provide barriers that protect organisms against their environment. The simplest type of barrier is simply a layer of lipids on the surface of the skin or fur of animals, the surface of leaves in plants, or associated with the walls of microorganisms. 

Lipids also form an integral part of biological membranes. All living cells are surrounded by a membrane that provides a barrier between the cell and its environment. They also occur within the cell, providing a structure in which many metabolic reactions take place. In mammals, the lipids involved in membrane structures are mainly the glycerophospholipids and unesterified (free) cholesterol, while in plants, the glycosylglycerides are predominant, especially in the chloroplasts and -sitosterol is the most abundant sterol rather than cholesterol. The chemistry of these structural lipids and their role in membrane architecture are described in Chapter 6, while their biochemistry forms the basis for Chapter 7. 

The importance of these compounds lies in their possession of chemical groupings that associate with water (hydrophilic groups) in juxtaposition with hydrophobic moieties. These sorts of lipids are often called polar lipids, or more technically amphiphilic (Greek liking both) and this amphiphilic nature is of immense importance in respect of their properties in membranes and in foods. (In contrast, hydrophobic fats, without polar groups, such as triacylglycerols, wax esters and sterols are often called neutral, apolar or non-polar lipids, but these are imprecise terms and best avoided.) 

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Figure 4. Half-life Ti/2of pH gradient decay in the dark as a function of lipid concentration for Mab(Phyt)2 or EPC vesicles after light-induced proton translocation by BR at 2S°C,...

VIII. Synopsis The Structure, Activity, and Function of Lipid A... 

The main function of lipidation is to promote membrane association of signaling proteins. Lipid anchors target proteins to the membrane, as is the case for the cytoplasmic protein tyrosine kinases, so that they can participate in membrane-associated signaling pathways. Furthermore, protein lipidation is thought to mediate protein-protein association and/or stabilize protein conformations (review Casey, 1995). 

Brown, D.A. London, E. (1998) Functions of lipid rafts in biological membranes. Annu. Rev. Cell Dev. Biol. 14, 111-136. 

Christie, W. W. 1979A. The composition, structure and function of lipids in the tissues of ruminant animals. Prog. Lipid Res. 17, 111-205. 

Describe two or more principal functions of proteins within cells, one function of DNA, two or more functions of RNA, and one function of lipids. 

Describe the structure of biological membranes and the characteristic functions of lipid-, protein-, and carbohydrate-containing components. Describe the differences between inner and outer membrane surfaces. 

Figure 10.4 (A) The DNA interaxial distance dDNA and the interlayer distance d in the L°a phase (Figure 10.2) plotted as a function of Lipid/DNA (UD) (wt/wt) ratio at the isoelectric point of the0 complex DOTAP/DNA=2.2. dDNA is seen to expand from 24.5 A to 57.1 A. The solid line through the data is the prediction of a packing calculation where the DNA chains form a space rilling one-dimensional lattice. (B) Schematic drawing of DNA-membrane multilayers showing the increase in distance between DNA chains as the membrane charge density is decreased (i.e., as DOpc increases) at the isoelectric point (Adapted from Raedler et al., 1997 Kohover etal., 1999).

Chesnoy, S. and Huang, L. (2000) Structure and function of lipid-DNA complexes for gene delivery. Annu. Rev. Biophys. Biomol. Struct., 29, 27-47. 

The lipid constituents in the membrane permit considerable movement of macromolecules, and membrane constituents may move appreciably within membranes. Membrane fluidity, a function of lipid composition, can be altered by temperature and chemicals... 

In most cases we can assume that the equilibrium distribution and partitioning of organic chemicals in both mammalian and nonmammalian systems is a function of lipid content in the animal and that the lipid-water partition coefficient (AW) is equal to K0w Instances where this is not the case include specific binding sites (e.g., kepone in the liver) and nonequilibrium conditions caused by slow elimination rates of higher level organisms or structured lipid phases that sterically hinder accumulation of very hydrophobic chemicals. For aquatic organisms in constant contact with water, the bioconcentration factor or fish-water partition coefficient (AW) s simply ... 

Sargent, J.R. (1976). The structure, metabolism and function of lipids in marine organisms. In Biochemical and Biophysical Perspectives in Marine Biology (D.C. Malms and J.R. Sargent, eds). Vol. 3, pp. 149-212. Academic Press, London. 

The biological function of lipid asymmetry and of proteins involved in the transmembrane traffic of lipids is multiple. Rapid reorientation of phospholipids in ery-... 

Fluorescence techniques have also been used to determine the localization of molecules in membranes. Using this technique, the localization of the linear dye molecule 3,3 -diethyloxadicarboxyamine iodide (DODCI) in lipid bilayer vesides was determined as a function of lipid chain length and unsaturation. It was found that the fraction of the dye in the interior region of the membrane was decreased as a function of chain length in the order C12 > C14 > C16 > C18. In unsaturated lipids it was Ci4 i > C14 0 > C16 1 > C16 0, which is in agreement with the general observation that the penetration of amphiphilic molecules into the interior of membranes increases with an increase in the fluidity of the membrane structure [59]. 

Murphy, D.J. 2001. The biogenesis and functions of lipid bodies in animals, plants, and microorganisms. Progr. Lipid Res. 40, 325-438. 

One of the main functions of lipids is to serve as the main structural components of cell membranes (7). Membranes resemble thin elastic sheets with a total thickness of about 5 nm. The membrane is composed of two lipid monolayers. The lipids in a membrane typically include two nonpolar and hydrophobic (water hating) acyl chains coimected to one another close to the head group, which in turn is usually polar and hydrophilic (water loving) and therefore can form hydrogen bonds with neighboring water molecules. This schizophrenic nature of lipid molecules causes them to self-assemble as closed objects such as liposomes, such that the head groups face water molecules while the hydrophobic hydrocarbon chains are protected from the water phase. 

In this section we will discuss the three major classes of the various constituents of the thylakoid membrane, namely lipids, proteins and electron carriers. All the essential constituents are listed in Tables 1 to 5. Since most ofthese constituents or complexes will be discussed in more detail in later chapters, the tables simply serve as a convenient listing ofthese components for easy reference. A new volume by Siegenthaler and Murata" should be consulted for the structure and function of lipids. 

Describe the functions of lipids and their solubility properties. Are animal lipids or plant lipids generally more saturated ... 

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