Figure 11–6 The kinetics of simple diffusion compared with transporter-mediated diffusion. Whereas the rate of diffusion and channel-mediated transport is directly proportional to the solute concentration (within the physical limits imposed by total surface area or total channels available), the rate of transporter-mediated diffusion reaches a maximum (Vmax) when the transporter is saturated. The solute concentration when the transport rate is at half its maximal value approximates the binding constant (Km) of the transporter for the solute and is analogous to the Km of an enzyme for its substrate. The graph applies to a transporter moving a single solute; the kinetics of coupled transport of two or more solutes is more complex and exhibits cooperative behavior.
Km; they are called uniporters. Others function as coupled transporters, in which the transfer of one solute strictly depends on the transport of a second. Coupled transport involves either the simultaneous transfer of a second solute in the same direction, performed by symporters (also called co-transporters), or the transfer of a second solute in the opposite direction, performed by antiporters (also called exchangers) (Figure 11–8).
The tight coupling between the transfer of two solutes allows the coupled transporters to harvest the energy stored in the electrochemical gradient of one solute, typically an inorganic ion, to transport the other. In this way, the free energy released during the movement of an inorganic ion down an electrochemical gradient is used as the driving force to pump other solutes uphill, against their electrochemical gradient. This strategy can work in either direction; some coupled transporters function as symporters, others as antiporters. In the plasma membrane of animal cells, Na+ is the usual co-transported ion because its electrochemical gradient provides a large driving force for the active transport of a second molecule. The Na+ that enters the cell during coupled transport is subsequently pumped out by an ATP-driven Na+-K+ pump in the plasma membrane (as we discuss later), which, by maintaining the Na+ gradient, indirectly drives the coupled transport. Such ion-driven coupled transporters as just described are said to mediate secondary active transport. In contrast, ATP-driven pumps are said to mediate primary active transport because in these the free energy of ATP hydrolysis is used to directly drive the transport of a solute against its concentration gradient.
Intestinal and kidney epithelial cells contain a variety of symporters that are driven by the Na+ gradient across the plasma membrane. Each Na+-driven symporter is specific for importing a small group of related sugars or amino acids into the cell. Because the Na+ tends to move into the cell down its electrochemical gradient, the sugar or amino acid is, in a sense, “dragged” into the cell with it. The greater the electrochemical gradient for Na+, the more solute is pumped
Figure 11–7 Three ways of driving active transport. The actively transported molecule is shown in orange, and the energy source is shown in red. Redox driven active transport is discussed in Chapter 14 (see Figures 14–18 and 14–19).
Figure 11–8 This schematic diagram shows transporters functioning as uniporters, symporters, and antiporters coupled transport (Movie 11.1).