Biology · Ch 6 — Plant Water Relation
Transport of Mineral Ions
Transport of Mineral Ions
Soil is the plant's main source of mineral nutrition. Minerals themselves are the solid, crystalline inorganic materials naturally present in the earth's crust, and plants absorb them from the soil in their dissolved, ionic form — chiefly through the roots — after which they are transported to wherever in the plant they are required.
Do you know?
- Minerals that play an important role in the day-to-day life of the plant are called essential elements. About 36 to 40 elements are incorporated in the plant's life.
- Some minerals like C, H, O, P, N, S and Mg, required in large quantity, are called macro elements, while minerals like Cu, Co, Mn, B and Zn, required in small quantity, are called micro elements.
Minerals are absorbed by root cells through two general routes: a passive route, in which ions move by simple diffusion or facilitated diffusion in the vicinity of the cells, along their own concentration gradient; and an active route, in which cells take up ions by active, energy-requiring uptake. Chemical analysis of ash left after burning plant tissue confirms that minerals really are drawn from the soil and surroundings in this way, and also confirms that mineral absorption is an independent process from water absorption, even though the two end up sharing the same upward transport pathway.
Once absorbed into the root, mineral ions are pulled upward together with the ascending xylem sap, driven by the same transpiration pull that moves water (section 6.9c); this can be confirmed directly by chemically analysing the composition of ascending xylem sap. The minerals are carried to wherever the plant needs them — apical growing regions, lateral buds, young leaves, developing flowers, fruits, seeds and storage organs — moving from the source (the root) upward through the sap and being unloaded, near their destination, into the fine veins of the tissue by diffusion, before being taken up actively by the surrounding cells. Analysis of xylem exudate shows that nitrogen, for example, travels partly as simple inorganic ions but largely in organic form, as amino acids and related compounds, and small amounts of inorganic phosphorus and sulphur compounds are carried too; it was once believed that the xylem transports only inorganic material while the phloem transports only organic material, but this strict division is now known to be inaccurate, since some exchange of materials does occur between the xylem and the phloem. …