Oil. Does it mix with water? No. Biologically, why is this important?
Oil is a lipid. The property of chemically not being able to mix with water gives lipids some very important biological functions. Lipids form the outer membrane of cells. Why?
A lipid is an organic compound such as fat or oil. Organisms use lipids to store energy, but lipids have other important roles as well. Lipids consist of repeating units called fatty acids. Fatty acids are organic compounds that are made of long chains of carbon and hydrogen.
Types of Lipids
- triglycerides : the main form of stored energy in animals. The subunits of a triglyceride molecule are 3 fatty acids attached to a glycerol backbone. See the Figure below.
- phospholipids : the major components of cell membranes.
- steroids : serve as chemical messengers and have other roles.
Triglyceride Molecule. The left part of this triglyceride molecule represents glycerol. Each of the three long chains on the right represents a different fatty acid. From top to bottom, the fatty acids are palmitic acid, oleic acid, and alpha-linolenic acid. The chemical formula for this triglyceride is C 55 H 98 O 6 . KEY:H=hydrogen, C=carbon, O=oxygen
Lipids and Diet
Humans need lipids for many vital 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 in the diet for most of these functions. Although the human body can manufacture most of the lipids it needs, there are others, called essential fatty acids , that must be consumed in food. Essential fatty acids include omega-3 and omega-6 fatty acids. Both of these fatty acids are needed for important biological processes, not just for energy.
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 also increase lipid levels in the blood. This, in turn, can increase the risk for health problems such as cardiovascular disease. The dietary lipids of most concern are saturated fatty acids, trans fats, and cholesterol. For example, cholesterol is the lipid mainly responsible for narrowing arteries and causing the disease atherosclerosis.
- Organisms use lipids to store energy. There are two types of fatty acids: saturated fatty acids and unsaturated fatty acids.
- Animals use saturated fatty acids to store energy. Plants use unsaturated fatty acids to store energy.
- Phospholipids are the major components of cell membranes.
- Excess dietary lipids can be harmful.