Cell membranes are generally composed of proteins and fatty-acid-based lipids, according to the Encyclopedia Britannica. Membranes actively involved in metabolism have a higher proportion of protein. Mitochondria, the cell’s fastest metabolizing organelles, have a membrane that consists of 75 percent protein. The Schwann cell’s membrane only has 20 percent protein.
The Scitable explains that cellular membranes are mostly composed of glycerophospholipids, two fatty acid chains, a phosphate group and molecules comprised of glycerol, which is a three-carbon molecule functioning as the backbone of membrane lipids. The most abundant lipids in cell membranes are glycerophospholipids, which are insoluble in water. They possess a distinct geometry that enables them to form into bilayers without an energy input.
Lipids make up around half the mass of cell membranes, as Nature Education notes. In animal cell plasma membranes, around 20 percent of the lipids are composed of cholesterol molecules. However, these molecules do not occur in mitochondrial and bacterial membranes. They help regulate the membranes’ stiffness, while other, less prominent lipids facilitate cell recognition and cell signaling.
Aside from lipids, membranes also have plenty of proteins. Half the mass of most cellular membranes is composed of proteins. The transmembrane proteins stick out on both sides of the membrane to which they are embedded.