Lipids Definition l Classification l Simple l compound l complex l Derived l Miscellaneous l Neutral

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Functions of Lipids
• Functions of Lipids l ...
lipid, any of a diverse group of organic compounds including fats, oils, hormones, and certain components of membranes that are grouped together because they do not interact appreciably with water. One type of lipid, the triglycerides, is sequestered as fat in adipose cells, which serve as the energy-storage depot for organisms and also provide thermal insulation. Some lipids such as steroid hormones serve as chemical messengers between cells, tissues, and organs, and others communicate signals between biochemical systems within a single cell. The membranes of cells and organelles (structures within cells) are microscopically thin structures formed from two layers of phospholipid molecules. Membranes function to separate individual cells from their environments and to compartmentalize the cell interior into structures that carry out special functions. So important is this compartmentalizing function that membranes, and the lipids that form them, must have been essential to the origin of life itself.
Water is the biological milieu-the substance that makes life possible-and almost all the molecular components of living cells, whether they be found in animals, plants, or microorganisms, are soluble in water. Molecules such as proteins, nucleic acids, and carbohydrates have an affinity for water and are called hydrophilic (“water-loving”). Lipids, however, are hydrophobic (“water-fearing”). Some lipids are amphipathic-part of their structure is hydrophilic and another part, usually a larger section, is hydrophobic. Amphipathic lipids exhibit a unique behaviour in water: they spontaneously form ordered molecular aggregates, with their hydrophilic ends on the outside, in contact with the water, and their hydrophobic parts on the inside, shielded from the water. This property is key to their role as the fundamental components of cellular and organelle membranes.
Fatty acids
Fatty acids rarely occur as free molecules in nature but are usually found as components of many complex lipid molecules such as fats (energy-storage compounds) and phospholipids (the primary lipid components of cellular membranes). This section describes the structure and physical and chemical properties of fatty acids. It also explains how living organisms obtain fatty acids, both from their diets and through metabolic breakdown of stored fats.
Structure
Biological fatty acids, members of the class of compounds known as carboxylic acids, are composed of a hydrocarbon chain with one terminal carboxyl group (COOH). The fragment of a carboxylic acid not including the hydroxyl (OH) group is called an acyl group. Under physiological conditions in water, this acidic group usually has lost a hydrogen ion (H+) to form a negatively charged carboxylate group (COO−). Most biological fatty acids contain an even number of carbon atoms because the biosynthetic pathway common to all organisms involves chemically linking two-carbon units together (although relatively small amounts of odd-number fatty acids do occur in some organisms). Although the molecule as a whole is water-insoluble by virtue of its hydrophobic hydrocarbon chain, the negatively charged carboxylate is hydrophilic. This common form for biological lipids-one that contains well-separated hydrophobic and hydrophilic parts-is called amphipathic.
Fatty acids containing more than one carbon-carbon double bond (polyunsaturated fatty acids) are found in relatively minor amounts. The multiple double bonds are almost always separated by a CH2 group (―CH2―CH=CH―CH2―CH=CH―CH2―), a regular spacing motif that is the result of the biosynthetic mechanism by which the double bonds are introduced into the hydrocarbon chain. The table lists the most common polyunsaturated fatty acids, linoleic and arachidonic, together with several that are less common. Arachidonic acid (C20) is of particular interest as the precursor of a family of molecules, known as eicosanoids (from Greek eikosi, “twenty”), that includes prostaglandins, thromboxanes, and leukotrienes. These compounds, produced by cells under certain conditions, have potent physiological properties, as explained in the section Intracellular and extracellular messengers. Animals cannot synthesize two important fatty acids, linoleic acid (an omega-6 fatty acid) and alpha-linolenic acid (an omega-3 fatty acid), that are the precursors of the eicosanoids and so must obtain them in the diet from plant sources. For this reason, these precursors are called essential fatty acids.
Lipids Definition l Classification l Simple l compound l complex l Derived l Miscellaneous l Neutral
Lipids Definition l Classification l Simple l compound l complex l Derived l Miscellaneous l Neutral
Lipids Definition l Classification l Simple l compound l complex l Derived l Miscellaneous l Neutral