Lipids are molecules whose main function in plants is to store energy and assist in the transmission of information between cells. Lipids include fat, sterols, waxes and fat-soluble vitamins, including A, D, E, and K. Plants transform carbohydrates into storage lipids or fat in the form of what are called triglycerides.
A triglyceride is produced when three fatty acids bond with a molecule of glycerol, a viscous liquid that has no color or odor. Fatty acids contain molecules of carbon, oxygen and hydrogen; there are three kinds of fatty acids: saturated, monounsaturated and polyunsaturated.
Fatty Acid Saturation
Fatty acids are commonly called fats in popular literature. The carbon atoms in saturated fatty acids have all the hydrogen they need, hence the term "saturated." They are stable, remaining solid at room temperature, and they don't combine easily with oxygen. The American Heart Association says that saturated fatty acids can collect in the arteries of the heart and have adverse effect on health; their intake in the diet should be limited.
Monounsaturated fats are liquid at room temperature; when they are refrigerated they begin to solidify. Dietitians recommend monounsaturated fats as being least harmful dietary fat.
Polyunsaturated fats are liquid both in the refrigerator and at room temperature. They can become rancid when they combine with oxygen in the air. They are preferred to saturated fats in the diet.
Hydrogen is sometimes added to unsaturated fats to saturate them and make them solid. These fats, found in hydrogenated vegetable oil, are called trans fats. They are used commercially to make packaged cookies and crackers and to deep fry potatoes and other foods. The American Heart Association recommends that trans fats be avoided.
Plants usually collect fats only when energy is needed in a small space, usually a seed. This is why cooking oils are usually obtained from seeds such as rape seeds used to make canola oil, soybeans or the corn kernels. Plants convert carbohydrates into triglycerides in seeds; when the seeds germinate, they turn the triglycerides back into carbohydrates.
Some vegetable oils come from seeds, others come from pulp, including avocado and olives.
The amount of fat in plants varies greatly. Lentils contain about 1 percent fat, while soybeans contain 20 percent fat, peanuts contain 44 percent, almonds 55 percent, and walnuts 65 percent.
Among vegetable oils, coconut and palm oil contain the highest percentage of saturated fat at 85.2 and 45.2 percent, respectively. Canola oil contains the lowest amount at 5.3 percent.
The highest percentage of monounsaturated fat are olive oil and canola oil at 69.7 and 64.3 percent, respectively.
Safflower oil and wheat germ oil contain the highest amounts of polyunsaturated fat at 72.1 and 60.7 percent, respectively.
Essential Fatty Acids
Essential fatty acids are polyunsaturated fats that the body needs but does not produce. Linoleic acid and alpha-linolenic acids are essential fatty acids that are found in flax and hemp oil and in fish oil.