Reading: Lipids
A lipid is an organic macromolecule such as fat or oil. Organisms use lipids to store energy. Lipids consist of repeating units called fatty acids. There are two types of fatty acids: saturated fatty acids and unsaturated fatty acids.
Saturated Fatty Acids
In saturated fatty acids, carbon atoms are bonded to as many hydrogen atoms as possible. This causes the molecules to form straight chains, as shown in Figure 2 below. The straight chains can be packed together very tightly, allowing them to store energy in a compact form. This explains why saturated fatty acids are solids at room temperature. Animals use saturated fatty acids to store energy.
Unsaturated Fatty Acids
In unsaturated fatty acids, some carbon atoms are not bonded to as many hydrogen atoms as possible. Instead, they are bonded to other groups of atoms. Plants use unsaturated fatty acids to store energy. Some examples of foods that contain unsaturated fats are shown in Figure 3 below.
Types of Lipids
triglycerides: the main form of stored energy in animals.
phospholipids: the major components of cell membranes.
steroids: serve as chemical messengers and have other roles.
Lipids and Diet
Humans need lipids for many important functions, such as storing energy and forming cell membranes. Lipids can also supply cells with energy. In fact, a gram of lipids supplies more than twice as much energy as a gram of carbohydrates or proteins. Lipids are necessary for the diet for most of these functions.
Although some lipids in the diet are essential, excess dietary lipids can be harmful. Because lipids are very high in energy, eating too many may lead to unhealthy weight gain. A high-fat diet may increase the risk of health problems such as cardiovascular disease. The dietary lipids of most concern are saturated fats, trans fats, and cholesterol. For example, cholesterol is the lipid mainly responsible for narrowing the heart arteries
