Uptake and transport of mineral ions
- Nutritional requirements of plants are obtained from minerals and water for hydrogen in the soil.
- All minerals cannot be passively absorbed by the roots because
- minerals are present in the soil as charged particles, 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.
- Most minerals must enter the root by active absorption into the cytoplasm of epidermal cells, which needs energy in the form of ATP.
- 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.
- The root endodermis because of the layer of suberin has the ability to actively transport ions in one direction only.
- After the ions have reached xylem, further transport up the stem to all parts of the plant is through the transpiration stream.
- Unloading of mineral ions occurs at the fine vein endings through diffusion and active uptake by these cells.
- Mineral ions are frequently remobilised, particularly from older, senescing parts.
- older dying leaves export much of their mineral content to younger leaves.
- before leaf fall in decidous plants, minerals are removed to other parts.
- Some of the nitrogen travels as inorganic ions, much of it is carried in the organic form as amino acids and related compounds.
- Small amount of exchange of materials does take place between xylem and phloem, hence it cannot be said that xylem transports only inorganic nutrients while phloem transports only organic materials.
Fig. Transpiration stream