Phosphoglycerides consist of glycerol esterified to two fatty acids and phosphoric acid. The phosphoglycerides are particularly important in membrane formation. These complex lipids contain a backbone of sphingosine rather than glycerol and only one fatty acid component. They are abundant in brain and nerve tissue. Membranes surround tissue cells as a selective barrier, and they encase the organelles found in eucaryotic cells.
Membranes contain both proteins and lipids.
According to the fluid mosaic model, the lipids are arranged in a bilayer fashion with the hydrophobic portions on the insider of the bilayer. Proteins float in the bilayer. Steroids are compounds that have four rings fused together in a specific way. The most abundant steroid in humans is cholesterol, which serves as a starting material for other important steroids such as the bile salts, adrenocorticoid hormones, and the sex hormones. Hormones are chemical messengers, synthesized by specific glands, that affect various target tissues in the body.