Triglycerides, or Fats - Triglycerides consist of a glycerol (3-carbon alcohol) and three fatty acids. In saturated fats, all the carbon molecules in the carbon skeleton of the fatty acid are bonded with hydrogen atoms. In unsaturated fats, some carbon atoms in the carbon skeleton are double bonded to each other rather than to hydrogen. Triglycerides function in energy storage.
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Fats are composed only of atoms of carbon, hydrogen, and oxygen (C, H, and O), which are arranged into two subunits: a glycerol unit (which is a three-carbon alcohol) and a fatty acid unit. Glycerol is composed of three carbons, to which three alcohol (OH) groups are attached. Fatty acids consist of an unbranched carbon skeleton to which hydrogen and a carboxyl group (COOH), an organic acid functional group, is attached.
Fats are formed by the dehydration synthesis of fatty acid chains to a glycerol molecule. As many as three fatty acid chains can bond to glycerol. When three fatty acids bind with glycerol, the fat is called a triglyceride, since tri- means three. Since the type of fatty acids attached to the glycerol can differ from one triglyceride to another, there are many types of this polymer. In Figure 3.7, stearic, oleic, and palmitic acids form the fatty acid tails.
Fats contain fewer oxygen atoms than other macromolecules and have more hydrogen bonds. For this reason these compounds hold more energy than other molecules. (The reason for this will be clear when we discuss oxidation and reduction in Chapter 6.) More oxygen is required to break down fat molecules during energy-releasing activities in cells. Since fat molecules weigh less than carbohydrate molecules, fats are ideal for the efficient storage of energy.
Having fewer oxygen atoms, fats lack the electropositive regions that would attract the hydrogen atoms (electronegative regions) in water molecules. In other words, fat molecules are nonpolar and hydrophobic they do not dissolve in water, only in other nonpolar substances like ether or methane. For instance, did you know that the wax you apply to a kitchen floor is a fat? That's why you cannot remove it by simply washing the floor with water water molecules are polar. You have to use wax remover (which is nonpolar) to remove wax, since fats are hydrophobic. In general, we can say that like dissolves like. That is, polar substances dissolve polar substances, and nonpolar dissolve nonpolar. That's why wax, a fat and therefore nonpolar, will dissolve only in a nonpolar substance.
As we mentioned in Chapter 1, living things are constantly changing their chemical makeup in order to remain the same, to maintain homeostasis. Fats are constantly being turned over, or converted. They are hydrolyzed to fatty acids and glycerol, and when completely broken down they yield as much as 9 Calories per gram, almost twice as much energy as a gram of carbohydrate, which yields 3.5 to 4 Calories per gram. (A Calorie is the amount of heat energy required to raise the temperature of 1000 grams of water 1 degree Celsius; see page 125.) This high-energy yield is due to the fact that fats contain a higher proportion of carbon-hydrogen bonds, or CH, than any other class of molecules in living things.
Triglycerides contain large amounts of chemical energy and are stored in seeds or in the fatty tissue of animals. Triglycerides are also used for thermal insulation and protection in animals such as the arctic seal.
The fatty acid units in a triglyceride can be saturated or unsaturated. In a saturated fat, the carbon
skeleton of the fatty acid is saturated with hydrogens in other words, there are hydrogen atoms covalently bonded to all possible locations on the carbon atoms . Animal fats, such as butter, lard, and bacon fat, are saturated fats. Since saturated fats usually have fairly high melting temperatures, they are solid at room temperature.
In an unsaturated fat, on the other hand, some carbon atoms form double bonds with other carbon atoms , so that the molecule contains fewer hydrogen atoms than a saturated fat. The formation of these double bonds produces kinks in the fatty acid chains, preventing a tight fit between the fatty acid molecules (Figure 3.8c). The "looseness" of the molecule's structure affects its consistency. For example, plant fats (called oils) are usually unsaturated. This type of fat generally has a low melting point, which is why vegetable oils are usually liquid at room temperature.
When individuals, be they human or house pets, take in more energy in the form of food than they require, the excess energy is stored in fat, and excess fat can lead to obesity. Obesity is the most common chronic medical problem in the United States today. When individuals diet, they are living off their stored fat reserves. If they Use up these fat reserves faster than they replace them, they lose weight. As some of you may know, losing excess fat is not all that easy. In order to lose one pound of body weight, a human must decrease caloric consumption by 3500 Calories or exercise sufficiently to use up 3500 Calories. (Source: Avila, Vernon L. Biology : Investigating Life On Earth Jones and Bartlett/Bookmark Series in Biology Page 61-64)
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