stem and roots along the gradient turgor pressure. In the cells of stem and roots, the organic solutes are either consumed or converted into insoluble form and the excess water is released into xylem (by turgor pressure gradient) through cambium. Merits: i. When a woody or herbaceous plant is girdled, the sap contains high sugar containing exudates from cut end.
ii. Positive concentration gradient disappears when plants are defoliated. Objections: i. This hypothesis explains the unidirectional movement of solute only.
However, bidirectional movement of solute is commonly observed in plants. ii. Osmotic pressure of mesophyll cells and that of root hair do not confirm the requirements. iii.
This theory gives passive role to sieve tube and protoplasm, while some workers demonstrated the involvement of ATP. . Mineral Absorption Minerals in soil exist in two forms, either dissolved in soil solution or adsorbed by colloidal clay particle. Previously, it was mistakenly assumed that absorption of mineral salts from soil took place along with absorption of water.
But absorption of minerals and ascent of sap are identified as two independent processes. Minerals are absorbed not only by root hairs but also by the cells of epiblema. Plasma membrane of root cells are not permeable to all ions and also all ions of same salt are not absorbed in equal rate. Penetration and accumulation of ions into living cells or tissues from surrounding medium by crossing membrane is called mineral absorption .
Movement of ions into and out of cells or tissues is termed as transport or flux . Entry of the ion into cell is called influx and exit is called efflux . Various theories have been put forward to explain this mechanism. They are categorized under passive mechanisms (without the involvement of metabolic energy) and active mechanisms (involvement of metabolic energy).
. . Passive Absorption . Ion-Exchange: Ions of external soil solution were exchanged with same charged (anion for anion or cation for cation) ions of the root cells.
There are two theories explaining this process of ion exchange namely: - - . . PM i. Contact exchange and ii.