Lipids are a group of organic compounds characterized by their presence in plants and animals, and their insolubility in water. They are extracted from cells using solvents such as ether or benzene that are non-polar. They are composed of mainly hydrogen and carbon and include materials such as fats, phospholipids, steroids, and waxes. Lipids have important functions in the body, being involved in such activities as energy storage and generation, the construction of cell membranes, and systemic chemical signaling. They also have a protective function in nature.
The most common type of lipids are fats. Fats are large molecules composed of two smaller types of molecules, fatty acids and glycerol. Glycerol is a polyhydroxy alcohol containing three carbon atoms and three hydroxyl groups. Fatty acids can be thought of as having two ends. One end is the "head" which contains a carboxylic acid group (COOH). The other end is the "tail" which is composed of a long carbon skeleton that can have anywhere from 6 to 20 or more atoms in length. It is this long tail group that makes fats insoluble in water. Nearly all naturally occurring fatty acids have an even number of carbon molecules.
A fat is an ester made when three fatty acids are joined on the glycerol molecule through an ester linkage. This is caused by a dehydration reaction between the hydroxyl group from the glycerol and the carboxyl groups of the fatty acids. Fats are also known as triglycerides reflecting the molecular composition. The fatty acids in a fat can each be the same or there can be two or three different types. The most common animal fat is tristearin, which is composed of glycerol and three stearic acid molecules.
Pure fats have no color, odor, or taste. However, impurities give fat a natural brownish color. Fats are slippery and less dense than water. When exposed to air for extended periods of time, fats become rancid and develop an unpleasant taste and odor.
Fatty acids are characterized by their carbon chain lengths and the number and location of their double bonds. Saturated fatty acids are those that contain no double bonds in their carbon chains. The term is used because all of the bonds that could possibly be occupied by hydrogen atoms are filled. Conversely, unsaturated fatty acids have one or more double bonds between carbon atoms in their chains. For this reason, the shape of an unsaturated fatty acid is slightly kinked wherever a double bond occurs. Some common saturated fatty acids include palmitic and stearic acid. Unsaturated fatty acids include palmitoleic, oleic, linoleic, and linolenic acid. Linoleic and linolenic acid are essential fatty acids that are needed to make critical compounds in the body. However, the body can not produce them so they must be included in a regular diet.
Animal fats such as bacon grease, butter, and lard tend to be saturated. This molecular structure tends to cause them to solidify at room temperature. In contrast, the fats from fish and plants are mostly unsaturated and liquid at room temperature. The kinks in the carbon chains do not allow the molecules to pack closely enough to form solids at these temperatures. Since these materials are liquid they are typically referred to as oils.
Unsaturated fats may be chemically modified by a process called hydrogenation. This involves the addition of hydrogen to the carbon chain thereby removing the double bonds and producing hydrogenated vegetable oils. Products such as peanut butter and margarine use hydrogenated vegetable oils to prevent lipids from separating out as liquids.
Saturated fats have been implicated as possible causes for atherosclerosis. A symptom of this cardiovascular disease is the formation of plaques on the internal linings of blood vessels. They impede the flow of blood and can lead to a heart attack.
Fats have an important role in biochemical metabolism. The primary function of fats is to store energy. One gram of fat contains almost twice as much energy as a gram of carbohydrate. Mammals store fats in specialized tissues composed of adipose cells. This tissue not only stores energy but also provides insulation and protection to vital organs in the body.
Fats can be reduced in a solution of sodium hydroxide to produce glycerol and another kind of lipid, soap. From a chemical standpoint, soap is a salt formed by the reaction of a base, such as potassium or sodium hydroxide, with fatty acids. In this reaction, the triglycerides are reduced to their component fatty acids. The base then neutralizes them into salts. A byproduct of this method of soap production is glycerin. This process, called saponification, is the basis for the modern day production of bar soap. Detergents are similar to soaps and are produced in an analogous reaction however, the starting material is derived from linear, alkyl compounds. The addition of a sulfate group helps prevent some of the drawbacks associated with soaps in hard water.
Since soaps and detergents are salts they separate into their component ions in a solution of water. One end is composed of an ion that has two ends with different solubility characteristics. The fatty, hydrocarbon portion or "tail" is hydrophobic (water hating) and associates with the oily particles in the solution. The carboxylate end or "head" is hydrophilic (water loving) and tends to associate with the aqueous phase. This unique structure is responsible for the cleansing ability of soaps and detergents. For this reason they are generally known as surface-acting agents, or surfactants.
Phospholipids are another type of lipid similar to fats in that they are composed of long chain fatty acids connected to a glycerol molecule. However, phospholipids have only two fatty acids attached to the bonding sites on the glycerol molecule. The third site is occupied by a phosphate group which has a negative electrical charge. This phosphate group makes phospholipids polar like surfactants. An additional smaller charged ion is typically linked with the phosphate group. A common phospholipid is phosphatidylcholine or lecithin.
The molecular structure of phospholipids alters their solubility in water. Since the phosphate group is water soluble, this part of the molecule tends to be hydrophilic. The fatty acid tails, however, remain hydrophobic. When put in a solution of water, phospholipids assemble into structures which minimize contact between the water and the hydrophobic tails. One structure, known as a micelle, is a small, spherical cluster of molecules that have the phosphate groups on the surface and the fatty acid tails on the interior. Lamellar sheets are another structure that phospholipids take on depending on the concentration of the phospholipid in the solution. Phospholipids are used by cells to produce the cell membrane. They are arranged in a double layer, or bilayer, with the phosphate groups on the outside and the fatty acids groups on the inside. This structure allows cells to be protected from the external environment.
Related to phospholipids are sphingolipids. They are composed of fatty acids bonded to the amino alcohol sphingosine. The most important sphingolipids are sphingomyelins. A common sphingomyelin is ceramide. It is present in most animal cell membranes and provides protection and insulation.
Another class of lipid are steroids. A steroid is characterized by a carbon skeleton made up of four connected rings. Steroids have various functions that are determined by the groups attached to their rings. They are found throughout the body.
Cholesterol is one of the most important types of steroids. It is a component of all membranes in animal cells. It is also a precursor from which nearly all other steroids are synthesized. Cholesterol is the starting material for bile acids, steroid hormones, and vitamin D. It is also the precursor for sex hormones such as estrogen, progesterone, and testosterone. Bile acids are produced from cholesterol in the liver. The most significant bile acid in humans is cholic acid. It aids in digestion by emulsifying fats in the small intestine. Vitamin D is a steroid produced in the skin through a reaction between cholesterol and sunlight. One problem with cholesterol is that high levels in the body have been found to contribute to heart disease so doctors suggest limiting dietary cholesterol.
Adrenocorticoid hormones are lipids formed in the adrenal gland. They are involved in the maintenance of water and electrolyte balance and in various metabolic reactions. An example of these compounds is cortisol.
Waxes are another type of lipid. They are similar to fats and phospholipids in that they are esters, however, they are based on monohydroxy alcohols instead of glycerol. Most waxes are solid at room temperature but have low- melting points. In nature, they are produced by plants and animals as waterproof coatings. This waterproofing characteristic makes them important industrial compounds. Waxes are primarily used to make candles, cosmetics, and coatings. A common example is beeswax that is composed primarily of myricyl palmitate, formed by the reaction of palmitic acid with an alcohol. Another important wax is lanolin that is derived from wool. It is used for a variety of medical and cosmetic purposes. Other important waxes include carnauba wax and spermaceti.
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