| Cells are surrounded by membranes, thin films about 50 Å in width, composed of proteins and lipids, including both glycoproteins and glycolipids. Intracellular organelles are also compartmentalized by membranes. Biomembranes are not rigid or impermeable but highly mobile and dynamic structures. The plasma membrane is the gatekeeper of the cell. It controls not only the access of inorganic ions, vitamins and nutrients, but also the entry of drugs and the exit of waste products. Integral transmembrane proteins have important roles in transporting these molecules through the membrane and often maintain concentration gradients across the membranes. K+, Na+, and Ca2+ concentrations in the cytoplasm are maintained at ∼140, 10, and 10-4mmol/L (546, 23, and 0.0007 mg/dL), respectively, by the transporter proteins, whereas those outside (in the blood) are ∼5, 145, and 1-2 mmol/L (20, 333, and 7-14 mg/dL), respectively. The driving force for transport of ions and maintenance of ion gradients is directly or indirectly provided by ATP. We will begin this chapter with a description of the composition, structure and properties of lipids, and then explain how lipids and proteins interact to form biomembranes. The transport properties of membranes will be illustrated by several important examples.
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