Mineral ions, which carry electric charges, generally cannot move across a membrane unless they are aided by transport proteins (explained in Chapter 5). When the concentration of these charged ions in the soil is greater than that in the plant, ion channels and carrier proteins can move them into the plant by facilitated diffusion, which is a passive process. The concentrations of most ions in the soil solution, however, are lower than those required inside the plant. Thus the plant must take up these ions against a concentration gradient—a process that requires energy.
Electric charge differences also play a role in the uptake of mineral ions. Movement of a negatively charged ion into a negatively charged region is movement against an electrical gradient and requires energy. The combination of concentration and electrical gradients is called an electrochemical gradient. Uptake against an electrochemical gradient is active transport, an energy-requiring process, which depends on cellular respiration for a supply of ATP. Active transport, of course, requires specific transport proteins.
Unlike animals, plants do not have a sodium-potassium pump for active transport. Rather, plants have a proton pump, which uses energy obtained from ATP to move protons out of the cell against a proton concentration gradient
36.3 The Proton Pump in Active Transport of K+
and Cl- The buildup of hydrogen ions (H+) transported outside the cell by the proton pump (a) drives the movement of both cations (b) and anions (c) into the cell.
Inside of cell
Inside of cell
(Figure 36.3a). Because protons (H+) are positively charged, their accumulation outside the cell has two results:
► The region outside the cell becomes positively charged with respect to the region inside.
► A proton concentration gradient develops across the plasma membrane.
Each of these results has consequences for the movement of other ions. Because of the charge difference across the membrane, there is increased movement of cations (positively charged ions), such as potassium (K+), into the cell through their membrane channels. These ions move into the now more negatively charged interior of the cell by facilitated diffusion (Figure 36.3b). In addition, the proton concentration gradient can be harnessed to drive secondary active transport, in which anions (negatively charged ions) such as chloride (Cl) are moved into the cell against an electrochemical gradient by a symport protein that couples their movement with that of H+ (Figure 36.3c). In sum, there is a vigorous traffic of ions across plant cell membranes, involving specific membrane transport proteins and both active and passive processes.
The proton pump and the coordinated activities of other membrane transport proteins cause the interior of a plant cell to be very negative with respect to the exterior. Such a difference in charge across a membrane is called a membrane potential. Biologists can measure the membrane potential of a plant cell with microelectrodes, just as they can measure similar charge differences in nerve cells and other animal cells (see Chapter 44). Most plant cells maintain a membrane potential of at least -120 millivolts (mV).
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