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. Phospholipids 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
LDL and HDL Particles
Illustration of a low density lipoprotein (LDL), or bad cholesterol, molecule (left) and a high density lipoprotein (HDL), or good cholesterol, molecule (right), showing their comparative sizes. JUAN GAERTNER/Science Photo Library/Getty Images
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 composed 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.
