Lipids are organic compounds that are mostly insoluble in water. They are composed of fats and oils, yield high energy and have a chemical composition of carbon, hydrogen and oxygen. Lipids perform three primary biological functions within the body which are as energy stores, structural components of cell membranes and as intracellular and intercellular signaling molecules. Lipids can be further classified into different categories which are triacylglycerols (triglycerides), phospholipids, steroids, terpenes, eicosanoids and lipid soluble vitamins.
Triacylglycerols or triglycerides are the simplest form of lipids formed by fatty acid and is made up of three fatty acids ester linked to a single glycerol. Due to the ester linked between the polar hydroxyls of glycerol and the polar carboxylates of fatty acids, triacylglycerols have characteristics such as nonpolar, hydrophobic and insoluble in water. Triacylglycerols are commonly found in vegetable oils, animal fats and dairy products. Triacylglycerols are stored as fat droplets in large amounts in vertebrate fat cells and as oils in plant seeds. They are better energy source compared to carbohydrates because triacylglycerols produces more than twice as much energy compared to oxidation of carbohydrates. The fat content of normal humans allows them to survive starvation for 2 to 3 months. Besides that, triacylglycerol under the skin can function as thermal insulation and energy reserves. Triacylglycerols are also involved in the saponification which is the formation of soaps from triacylglycerols by converting triacylglycerols into fatty acid salts (soap) and glycerol.
Phospholipids are the amphipathic lipids or lipid bilayers commonly found in membranes. Due to their amphipathic nature, they are effective as emulsifying agents, compounds that make or stabilizes emulsion. For example, the lecithin in egg whites keeps mayonnaise, an oil-water emulsion from separating. There are two classes of phospholipids which are glycerophospholipids and sphingolipids. Glycerophospholipids are amphiphilic molecules with non-polar aliphatic tails and polar phosphoryl-X heads. One of the main functions of glycerophospholipids is the formation of cellular membranes of all organisms and organelles in cells. Their amphipathic nature drives the formation of lipid bilayer structure of membranes. Sphingolipids consists of a polar head group and two non-polar tails with an amino alcohol called sphingosine as its core. Sphingolipids are major membrane components, have a structural function and protect the cell surface from harmful environmental factors. They also serve as adhesion sites for extracellular proteins and play an important role in signal transmission and cell recognition.
Steroids are biologically active organic compounds with three 6-membered rings and one 5-membered rings arranged in a specific molecular configuration made naturally in the human body. Cholesterol is the most common steroid and is mainly synthesized in the liver and a precursor to all other steroid hormones such as testosterone which are secreted by the gonads. Cholesterol is also the precursor to vitamin D and bile salts, which help in the emulsification of fats and their subsequent absorption by cells. Cholesterol is a necessary for proper functioning of the body as it is a component of the plasma membrane found within the phospholipid bilayer. Steroid hormones serve many functions including salt balance, metabolic and sexual functions.
Terpenes are major biosynthetic building blocks within nearly every living creature. Some examples of terpenes are vitamin A and steroids which are derivatives of the triterpene squalene. Terpenes are derived biosynthetically from units of isoprene, which has the molecular formula C5H8. Its activated forms, isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) are the building blocks. The functions of terpenes include producing color and odors associated with plants (essential oil), as vitamin and precursors, in visual pigments and chloroplast pigments.
Eicosanoids are signaling molecules made by oxidation of C20 essential fatty acids (EFAs) which are produced and used locally or on site. Eicosanoids derive from either omega-3 or omega-6. There are four families of eicosanoids which are prostaglandins, prostacyclins, thromboxanes and leukotrienes. These eicosanoids are derived from membrane lipids, with arachidonic acid being the most important precursor in humans. Eicosanoids exert complex control over many bodily systems, mainly in inflammation or immunity, and as messengers in the central nervous system. It acts at low concentration and are involved in the production of pain and fever, regulation of blood pressure, blood coagulation and reproduction.
Some vitamins are lipid soluble. These vitamins are known as vitamin A, D, E and K. Vitamin A is a group of unsaturated organic compounds that includes retinol, retinal, retinoic acid, and several provitamin A carotenoids and beta-carotene. Vitamin A serves as the site of the primary photochemical reaction in vision. The active molecule is retinal which is a vitamin A aldehyde. Retinal forms an imine with an -NH2 group of the protein opsin to form visual pigment called rhodopsin. The primary chemical event of vision in rod cells is absorption of light by rhodopsin followed by isomerization of the 11-cis double bond to the 11-trans double bond. Vitamin D is a group of structurally related compounds that play a role in the regulation of calcium and phosphorus metabolism. The most abundant form in the circulatory system is vitamin D3. Vitamin E is a group of compounds of similar structure and the most active is α-tocopherol. Vitamin E is an important antioxidant, necessary for reproduction in rats and may be necessary for reproduction in humans that functions to trap HOO• and ROO• radicals formed as a result of oxidation by oxygen of unsaturated hydrocarbon chains in membrane phospholipids. Meanwhile, vitamin K has a regulatory function in blood clotting.
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