Cambridge International AS Level Biology
Facilitated diffusion
82
Large polar molecules, such as glucose and amino acids, cannot diffuse through the phospholipid bilayer. Nor can ions such as sodium (Na+) or chloride (Cl−). These can only cross the membrane with the help of certain protein molecules. Diffusion that takes place in this way is called facilitated diffusion. ‘Facilitated’ means made easy or made possible, and this is what the proteins do. There are two types of protein involved, namely channel proteins and carrier proteins. Each is highly specific, allowing only one type of molecule or ion to pass through it. Channel proteins are water-filled pores. They allow charged substances, usually ions, to diffuse through the membrane. Most channel proteins are ‘gated’. This means that part of the protein molecule on the inside surface of the membrane can move to close or open the pore, like a gate. This allows control of ion exchange. Two examples are the gated proteins found in nerve cell surface membranes. One type allows entry of sodium ions, which happens during the production of an action potential (page 335). Another allows exit of potassium ions (K+) during the recovery phase, known as repolarisation. Some channels occur in a single protein; others are formed by several proteins combined. Whereas channel proteins have a fixed shape, carrier proteins can flip between two shapes (Figure 4.9). As a result, the binding site is alternately open to one side of the membrane, then the other. If the molecules are diffusing across the membrane, then the direction of movement will normally depend on their relative concentration on each side of the membrane. They will move down
polar molecule or ion
phospholipid bilayer of membrane
a concentration gradient from a higher to a lower concentration. However, the rate at which this diffusion takes place is affected by how many channel or carrier protein molecules there are in the membrane, and, in the case of channel proteins, on whether they are open or not. For example, the disease cystic fibrosis is caused by a defect in a channel protein that should be present in the cell surface membranes of certain cells, including those lining the lungs. This protein normally allows chloride ions to move out of the cells. If the channel protein is not correctly positioned in the membrane, or if it does not open the chloride channel as and when it should, then the chloride ions cannot move out. Facilitated diffusion is the diffusion of a substance through transport proteins in a cell membrane; the proteins provide hydrophilic areas that allow the molecules or ions to pass through the membrane which would otherwise be less permeable to them.
Osmosis
Osmosis is a special type of diffusion involving water molecules only. In the explanations that follow, remember that: solute + solvent = solution In a sugar solution, for example, the solute is sugar and the solvent is water. In Figure 4.10 there are two solutions separated by a partially permeable membrane. This is a membrane that allows only certain molecules through, just like membranes in living cells. In the situation shown in Figure 4.10a, solution B has a higher concentration of
exterior
carrier protein with specific shape
cytoplasm
Figure 4.9 Changes in the shape of a carrier protein during facilitated diffusion. Here, there is a net diffusion of molecules or ions into the cell down a concentration gradient.