Unlike water, all minerals cannot be passively absorbed by the roots. Therefore most minerals must enter the root by active absorption into the cytoplasm of epidermal cells. This needs energy in the form of ATP. The active uptake of ions is partly responsible for the water potential gradient in roots, and therefore for the uptake of water by osmosis. Some ions also move into the epidermal cells passively.
Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasms of the epidermal cells. Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem. It is important to note that the root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.
