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Lipid

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   Lipids are a class of hydrocarbon-containing organic compounds. Lipids
   are categorized by the fact that they are soluble in nonpolar solvents
   (such as ether and chloroform) and are relatively insoluble in water.
   Lipid molecules have these properties because they consist of mainly
   carbon. Some lipids are used for energy storage, others serve as
   structural components of cell membranes, and some are important
   hormones or contain essential fatty acids. Although the term lipid is
   often used as a synonym for fat, the latter is in fact a subgroup of
   lipids called triglycerides.

Lipid function

     * Cell membrane structure
          + Constitutes a barrier for the cell (the phospholipid bilayer).
          + Controls membrane fluidity.
          + Controls the flow of material in and out of the cell membrane.
     * Energy storage (for instance, fats stored in adipose tissue)
          + Lipids are effective at storing energy. They contain more
            energy than, for example, carbohydrates, because of their high
            carbon to oxygen ratio (less oxygen than in carbohydrates),
            which means that each bond broken will take place in an
            oxidation reaction; but the catabolism will be greater because
            more oxygen will be used from the environment as less is
            stored.
     * Lipid hormones like steroids and eicosanoids mediate communication
       between cells.
     * Signal transduction - function in the transmission of information
       in cells.
     * Lipid vitamins - required for metabolism

Structure

Fatty acids and glycerides

   Chemically, fatty acids can be described as long-chain monocarboxylic
   acids and have a general structure of CH[3](CH[2])[n]COOH. The length
   of the chain usually ranges from 12 to 24, always with an even number
   of carbons. When the carbon chain contains no double bonds, it is
   called saturated. If it contains one or more such bonds, it is
   unsaturated. The presence of double bonds generally reduces the melting
   point of fatty acids. Furthermore, unsaturated fatty acids can occur
   either in cis or trans geometric isomers. In most naturally occurring
   fatty acids, the double bonds are in the cis configuration.

   Glycerides are lipids possessing a glycerol (propan-1, 2, 3-triol) core
   structure with one or more fatty acyl groups, which are fatty
   acid-derived chains attached to the glycerol backbone by ester
   linkages. Glycerides with three acyl groups ( triglycerides or neutral
   fats) are the main storage form of fat in animals and plants.

   An important type of glyceride-based molecule found in biological
   membranes, such as the cell's plasma membrane and the intracellular
   membranes of organelles, are the phosphoglycerides or
   glycerophospholipids. These are phospholipids that contain a glycerol
   core linked to two fatty acid-derived "tails" by ester or, more rarely,
   ether linkages and to one "head" group by a phosphate ester linkage.
   The head groups of the phospholipids found in biological membranes are
   phosphatidylcholine ( lecithin), phosphatidylethanolamine,
   phosphatidylserine and phosphatidylinositol. These phospholipids are
   subject to a variety of reactions in the cell: for instance, polar head
   groups or fatty acid tails can be released from specific phospholipids
   through enzyme-catalyze hydrolysis to generate second messengers
   involved in signal transduction. In the case of phosphatidylinositol,
   the head group can be enzymatically modified by the addition of one to
   three phosphate molecules, and this constitutes another mechanism of
   cell signaling. While phospholipids are the major component of
   biological membranes, other non-glyceride lipid components like
   sphingolipids and sterols (such as cholesterol in animal cell
   membranes) are also found in biological membranes.

   A biological membrane is a form of lipid bilayer, as is a liposome.
   Formation of lipid bilayers is a spontaneous process when the
   glycerophospholipids described above are placed in water. In an aqueous
   milieu, the polar heads of lipids tend to oriente toward the polar,
   aqueous environment, while the hydrophobic tails tend to minimize their
   contact with water. The nonpolar tails of lipids (U) tend to cluster
   together, forming a lipid bilayer (1) or a micelle (2). The polar heads
   (P) face the aqueous environment. Micelles form when single-tailed
   amphiphilic lipids are placed in a polar milieu, while lipid bilayers
   form when two-tailed phospholipids are placed in a polar environment
   (Fig. 2). Micelles are "monolayer" spheres and can only reach a certain
   size, whereas bilayers can be considerably larger. They can also form
   tubules. Bilayers that fold back upon themselves form a hollow sphere,
   enclosing a separate aqueous compartment, which is the basis of
   biological membranes.

   Micelles and bilayers separate out from the polar milieu by a process
   known as the "hydrophobic effect." When dissolving a nonpolar substance
   in a polar environment, the polar molecules (i.e. water in an aqueous
   solution) become more ordered around the dissolved nonpolar substance,
   since the polar molecules cannot form hydrogen bonds to the nonpolar
   molecule. Therefore, in an aqueous environment, the polar water
   molecules form an ordered " clathrate" cage around the dissolved
   nonpolar molecule. However, when the nonpolar molecules separate out
   from the polar liquid, the entropy (state of disorder) of the polar
   molecules in the liquid increases. This is essentially a form of phase
   separation, similar to the spontaneous separation of oil and water into
   two separate phases when one puts them together.
   Figure 2: Self-organization of lipids. A lipid bilayer is shown on the
   left and a micelle on the right.
   Enlarge
   Figure 2: Self-organization of lipids. A lipid bilayer is shown on the
   left and a micelle on the right.

   The self-organisation depends on the concentration of the lipid present
   in solution. Below the critical micelle concentration, the lipids form
   a single layer on the liquid surface and are dispersed in solution. At
   the first critical micelle concentration (CMC-I), the lipids organise
   in spherical micelles, at the second critical micelle concentration
   (CMC-II) into elongated pipes, and at the lamellar point (LM or
   CMC-III) into stacked lamellae of pipes. The CMC depends on the
   chemical composition, mainly on the ratio of the head area and the tail
   length.

Nutrition and health

   Lipids play diverse and important roles in nutrition and health. Many
   lipids are absolutely essential for life. However, there is also
   considerable awareness that abnormal levels of certain lipids,
   particularly cholesterol (in hypercholesterolemia) and, more recently,
   trans fatty acids, are risk factors for heart disease and other
   diseases.

   The daily requirement of lipids is 382 grams per day.
   Retrieved from " http://en.wikipedia.org/wiki/Lipid"
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