Cell membranes are composed primarily of lipids and proteins. The lipid component consists of phospholipids, cholesterol, and glycolipids and is responsible for the high permeability of cell membranes to lipid-soluble substances such as carbon dioxide, oxygen, fatty acids, and steroid hormones. The lipid component of cell membranes is also responsible for the low permeability of cell membranes to water-soluble substances such as ions, glucose, and amino acids. The protein component of the membrane consists of transporters, enzymes, hormone receptors, cell-surface antigens, and ion and water channels.
Phospholipid Component of Cell Membranes
Phospholipids consist of a phosphorylated glycerol backbone (“head”) and two fatty acid “tails” (Fig. 1-2). The glycerol backbone is hydrophilic (water soluble), and the fatty acid tails are hydrophobic(water insoluble). Thus, phospholipid molecules have both hydrophilic and hydrophobic properties and are called amphipathic. At an oil-water interface (see Fig. 1-2A), molecules of phospholipids form a monolayer and orient themselves so that the glycerol backbone dissolves in the water phase and the fatty acid tails dissolve in the oil phase. In cell membranes (see Fig. 1-2B), phospholipids orient so that the lipid-soluble fatty acid tails face each other and the water-soluble glycerol heads point away from each other, dissolving in the aqueous solutions of the ICF or ECF. This orientation creates a lipid bilayer.
Figure 1–2 Orientation of phospholipid molecules at oil and water interfaces. Depicted are the orientation of phospholipid at an oil-water interface (A) and the orientation of phospholipid in a bilayer, as occurs in the cell membrane (B).
Protein Component of Cell Membranes
Proteins in cell membranes may be either integral or peripheral, depending on whether they span the membrane or whether they are present on only one side. The distribution of proteins in a phospholipid bilayer is illustrated in the fluid mosaic model, shown in Figure 1-3.
Figure 1–3 Fluid mosaic model for cell membranes.
Integral membrane proteins are embedded in, and anchored to, the cell membrane by hydrophobic interactions. To remove an integral protein from the cell membrane, its attachments to the lipid bilayer must be disrupted (e.g., by detergents). Some integral proteins are transmembrane proteins, meaning they span the lipid bilayer one or more times; thus, transmembrane proteins are in contact with both ECF and ICF. Examples of transmembrane integral proteins are ligand-binding receptors (e.g., for hormones or neurotransmitters), transport proteins (e.g., Na+-K+ ATPase), pores, ion channels, cell adhesion molecules, and GTP-binding proteins (G proteins). Other integral proteins are embedded in the membrane but do not span it.
Peripheral membrane proteins are not embedded in the membrane and are not covalently bound to cell membrane components. They are loosely attached to either the intracellular or extracellular side of the cell membrane by electrostatic interactions (e.g., with integral proteins) and can be removed with mild treatments that disrupt ionic or hydrogen bonds. One example of a peripheral membrane protein is ankyrin, which “anchors” the cytoskeleton of red blood cells to an integral membrane transport protein, the Cl−-HCO3− exchanger (also called band 3 protein).