DESCRIBE THE MOVEMENT OF WATER FROM THE ROOTS TO THE LEAVES. [20mks]
Water is drawn into the root hair cells by osmosis; due to the presence of dissolved substances in the cell sap of root hairs.
The concentration of cell sap is greater than that of the surrounding solution in the soil hence a concentration gradient occurs;
This exerts a high osmotic pressure; thus drawing the water molecules from the solution in the soil across the cell wall and cell membrane into the root hair cells;
More water drawn into the root hair cells dilutes the cell sap making it less concentrated than that in the adjacent cortex of the root.
Due to osmotic gradient water moves to the adjacent cells and to the next by osmosis; until it enters into xylem vessels located in the centre of the root;
These xylem vessels of the root then conduct the water up into the xylem vessels of the stem into the leaves.
There is a force in the roots called root pressure which pushes water up the stem; Energy is essential in this process;
In the xylem vessels, water rise by capillarity because the vessels are very narrow and there is a higher attractive force between the water molecules themselves and the water molecules-cell walls; These cohesive and adhesive forces; They are important in the maintenance of continuous and an uninterrupted water column in the xylem vessels up the tree to the leaves;
Water vapourises from the spongy mesophyll cells their cell sap becomes concentrated than adjacent cells; This increases the osmotic pressure of the spongy mesophyll cells; as a result water flows into the cell from other surrounding cells; which in turn take in water from xylem vessels in the stem and roots;
The transpiration pull maintains continuous column of water from the roots to the leaves. (maximum 20 Marks)
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