Studies of K_m values have shown a feedback inhibition (see Chapter 2 on this website) of carrier-mediated transport that allows the cell to maintain stable cytoplasmic ion concentrations over a wide range of external concentrations. For example, the apparent K_m value of the high-affinity K⁺ carrier in barley roots changes with the plant's demand for K⁺. Barley plants starved for K⁺ have high-affinity K_m values in the range of 0.02 to 0.03 mM, whereas well-fertilized plants have K_m values that are four- to fivefold greater (Web Figure 6.4.A). As explained in textbook Chapter 6, an increase in K_m reflects a decrease in the carrier's affinity for K⁺ (Epstein 1972).

Web Figure 6.4.A   The transport of potassium into barley roots shows two different phases. The biphasic kinetics of potassium uptake, accentuated in this figure by the change of scale at around 1 mM, suggests the presence of different types of transport systems for potassium. The high-affinity transport system, having a K_m value of 0.02 to 0.03 mM, is attributed to active transport by symporters; the low-affinity system (which may or may not show saturation) is attributed to diffusion through K⁺ channels. (After Epstein 1972.)

The change in apparent K_m probably reflects the expression of different members of a gene family, with their gene products thereby matching cellular needs to environmental ionic conditions (Schachtman 2000; Maser et al. 2001). If the plants’ adjustments were merely a matter of adding more transporters to the plasma membrane, there would be change in V_max without a change in K_m. This is not what is observed for K⁺. However, plants that have been starved and then resupplied with sulfate or phosphate often respond by an increase in V_max. As the internal sulfate and phosphate levels recover, the V_max values for their uptake drop to the usual values as excess carriers are removed from the membrane (Glass 1983).