Unlike water, all minerals cannot be passively absorbed by the roots.
Two factors account for this:
- minerals are present in the soil as charged particles (ions) which cannot move across cell membranes and
- The concentration of minerals in the soil is usually lower than the concentration of minerals in the root.
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. Ions are absorbed from the soil by both passive and active transport. Specific proteins in the membranes of root hair cells actively pump ions from the soil into the cytoplasms of the epidermal cells.
Like all cells, the endodermal cells have many transport proteins embedded in their plasma membrane; they let some solutes cross the membrane, but not others. Transport proteins of endodermal cells are control points, where a plant adjusts the quantity and types of solutes that reach the xylem.
Note that the root endodermis because of the layer of suberin can actively transport ions in one direction only.