Triglycerides, also known as triacylglycerols, constitute the majority of the lipids in the body, and it is these molecules that are generally referred to simply as "fat." Triglycerides are formed by the linking together of glycerol, a three-carbon alcohol, with three fatty acids. Each of the three hydroxyl groups in glycerol is linked to the carboxyl group of a fatty acid by the removal of a molecule of water. The three fatty acids in a molecule of triglyceride need not be identical. Therefore, a variety of fats can be formed with fatty acids of different chain lengths and degrees of saturation. Animal fats generally contain a high proportion of saturated fatty acids, whereas vegetable fats contain more unsaturated fatty acids. Saturated fats tend to be solid at low temperatures.
Unsaturated fats, on the other hand, have a very low melting point, and thus they are liquids (oil) even at very low temperatures. Thus, heating a hamburger on the stove melts the saturated animal fats, resulting in grease appearing in the frying pan. When allowed to cool, however, the oily grease returns to its solid form. Hydrolysis of triglycerides releases the fatty acids from glycerol, and these products can then be metabolized to provide energy for cell functions. Thus, the storage of energy in the form of triglycerides and polysaccharides requires dehydration reactions (removal of a molecule of water). Similarly, both are broken down to usable forms of fuel by hydrolysis.