Figure 1 Solute / H+ cotransport at the plasma membrane. The plasma membrane H+ -ATPase pumps H+ against an electrochemical gradient out of the cell. H+ enters the cell along its electrochemical gradient, providing energy for co-transport. For example, Cl-/H+ symport, where Cl- is taken up by the cell against its electrochemical gradient; Na+/H+ antiport, where Na+ is pumped out of the cell against its electrochemical gradient. K+ can be co-transported via a K+/H+ symport (high affinity); uptake can also be substantial through K+ channels (low affinity). The membrane potential across the tonoplast is about 30 mV more positive than that across the plasma membrane (Plants in Action Chapter 3.6.3) because of transport processes across this membrane that are not discussed here.