can be regulated, changing the rate of osmosis across the membrane. However, water
movement through aquaporins is always passive, so the direction of water movement is
unchanged by alterations in aquaporin permeability.
Mineral ions, which carry electric charges, generally cannot move across a membrane
unless they are aided by transport proteins, including ion channels and carrier proteins (see
Section 5.3.4). The ions would otherwise be blocked by the hydrophobic interior of the
membrane, and they are too large to pass through aquaporins.
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 therefore requires energy. The combination of concentration and electrical
gradients is called an electrochemical gradient. Uptake against an electrochemical gradient
involves active transport, which is fueled by ATP generated by cellular respiration. Active
transport requires specific transport proteins.
Unlike animals, plants do not have a sodium–potassium pump (see Section 5.4.1) 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 (Figure 35.3, step 1).
Because protons (H+) are positively charged, their accumulation outside the cell has two
* An electrical gradient is created such that the region outside the cell becomes positively
charged with respect to the region inside.
* A proton concentration gradient develops, with more protons outside the cell than
Each of these results has consequences for the movement of other ions. Because the inside
of the cell is now more negative than the outside, cations (positively charged ions) such as
potassium (K+) move into the cell by facilitated diffusion through their specific membrane
channels (Figure 35.3, step 2). 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 35.3, step 3). 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 negatively charged with respect to the exterior; that is,
they build up a significant membrane potential. Biologists can measure the membrane
potential of a plant cell with microelectrodes, just as they can measure similar charge