Fats, Steroids, and Other Lipids

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Lipid Types

Oil & Water
Oil is a type of lipid known as a triglyceride. Solid triglycerides are called fats and liquid triglycerides are called oils. Credit: Thomas Vogel/E+/Getty Images

Lipids are very diverse in both their respective structures and functions. These diverse compounds that make up the lipid family are so grouped because they are insoluble in water. They are however soluble in other organic solvents such as ether, acetone, and other lipids. Lipids serve a variety of important functions in living organisms. They act as chemical messengers, serve as valuable energy sources, provide insulation, and are the main components of membranes. Major lipid groups include fats, phospholipids, steroids, and waxes.

Fats

Fats are composed of three fatty acids and glycerol. These so called triglycerides can be solid or liquid at room temperature. Those that are solid are classified as fats, while those that are liquid are known as oils. Fatty acids consist of a long chain of carbons with a carboxyl group at one end. Depending on their structure, fatty acids can be saturated or unsaturated. Saturated fats raise LDL (low density lipoprotein) cholesterol levels in the blood. This increases the chances for developing cardiovascular disease. Unsaturated fats lower LDL levels and reduce the risk for disease. While fats have been denigrated to the point that many believe that fat should be eliminated from the diet, fat serves many useful purposes. Fats are stored for energy in adipose tissue, help to insulate the body, and cushion and protect organs.

Phospholipids

A phospholipid is composed of two fatty acids, a glycerol unit, a phosphate group and a polar molecule. The phosphate group and polar head region of the molecule is hydrophillic (attracted to water), while the fatty acid tail is hydrophobic (repelled by water). When placed in water, phospholipids will orient themselves into a bilayer in which the nonpolar tail region faces the inner area of the bilayer. The polar head region faces outward and interacts with the water. Phospholipids are a major component of cell membranes, which enclose and protect the cytoplasm and other contents of a cell. Phosopholipids are also a major component of myelin, a fatty substance that is important for insulating nerves and speeding up electrical impulses in the brain. It is the high composition of myelinated nerve fibers that causes white matter in the brain to appear white.

Steroids and Waxes

Steroids have a carbon backbone that consists of four fused ring-like structures. Steroids include cholesterol, sex hormones (progesterone, estrogen and testosterone) produced by gonads, and cortisone. Waxes are comprised of an ester of a long-chain alcohol and a fatty acid. Many plants have leaves and fruits with wax coatings to help prevent water loss. Some animals also have wax-coated fur or feathers to repel water. Unlike most waxes, ear wax is composed of phospholipids and esters of cholesterol.

Lipid Soluble Vitamins

Fat-soluble vitamins are stored in adipose tissue and in the liver. They are eliminated from the body more slowly than water-soluble vitamins. Fat-soluble vitamins include vitamins A, D, E, and K. Vitamin A is important for vision as well as skin, teeth, and bone health. Vitamin D aids in the absorption of other nutrients including calcium and iron. Vitamin E acts as an antioxidant and also aids in immune function. Vitamin K aids in the blood clotting process and maintaining strong bones.

Organic Polymers

Biological polymers are vital to the existence of all living organisms. In addition to lipids, other organic molecules include:

  • Carbohydrates - biomolecules that include sugars and sugar derivatives. They not only provide energy but are also important for energy storage.
  • Proteins - composed of amino acids, proteins provide structural support for tissues, act as chemical messengers, move muscles, and much more.
  • Nucleic Acids - biological polymers composed of nucleotides and important for gene inheritance. DNA and RNA are two types of nucleic acids.