Chapter 11 - Membranes

Abstract

The basic structure of all biological membranes is a double lipid layer, which contains phospholipids, glycolipids, and cholesterol. These lipids make a fluid phase to which proteins are associated. Proteins can be peripheral, sometimes loosely attached to the membrane, or integral, which cross the bilayer one or several times. Often carbohydrates are also bound to the membrane lipids and proteins. The cell membrane is a barrier with selective permeability. Transport across membranes is carried out via different mechanisms, including: (1) Simple diffusion, which is driven by concentration or electrical gradients of the transported solute. (2) Passive transport or facilitated diffusion, which also follows concentration or electrical gradients, but it is mediated via carriers and channels. These are integral membrane proteins, with specificity and saturability for the transported solute. Carriers can be divided into uniporters, which bind and transport only one type of solute; cotransporters or symporters, which move two solutes in the same direction; countertransporters, antiporters, or exchangers, which transport solutes in opposite directions. Channels are hydrophilic pores in the membrane, which allow the passage of ions with high specificity. They contain a gate mechanism regulated by different stimuli. Aquaporins are channels involved in water transport. (3) Active transporters carry out the movement of solutes against their electrochemical gradient; they require energy provided by the hydrolysis of ATP. They are grouped in several classes: P, V, F, and ABC transporters. Class P transporters include several ion transport ATPases. Class V or proton pumps are found in lysosomes and the plasma membrane of some cells. Class F comprises the ATP synthase of the mitochondrial inner membrane. Class ABC groups a family of proteins that move carbohydrates, lipids, peptides, biliary salts, and antibiotics out of the cell. (4) Secondary active transporters use the electrochemical gradient created by primary active transport systems like the sodium pump. (5) Endocytosis (phagocytosis and pinocytosis), a process by which substances are incorporated into the cell. This is mediated by vesicles, which contain the material to be endocytosed surrounded by plasma membrane. Endocytosis can be clathrin- and caveolin-mediated. The substance to be internalized is first bound by a receptor in the surface of the plasma membrane. Invagination of the membrane forms a clathrin- or caveolin-coated vesicle that is internalized into the cell. (6) Exocytosis, the process by which cells moves substance to the medium. This requires that the molecules to be exported be enclosed in vesicles formed in the trans Golgi and then trafficked to the plasma membrane, with which they fuse and release their content outside the cell.