K.C.S.E Biology Q & A - MODEL 2006PP2QN08
Describe how water moves from the soil to the leaves in a tree.
Water exists as a thin film in the soil between soil particles. The concentration cell sap is greater than that of the surrounding solution in the soil; Thus drawing water molecules across the cell wall and membrane into the root hair cells; by osmosis; water drawn into the root hair cell dilutes the cell sap/ makes it less concentration than that in the adjacent cell into the cortex cells. (By osmosis); across the endosperm by active transport; into the xylem vessels (of the root); Then conduct the water up into the xylem (vessels) of the stem; into xylem of leaves. Water is pushed/ rises up the stem by root pressure ( in the xylem vessels) water would rise by capillary; cohesion, and adhesive forces; water moves as a continuous an uninterrupted water column in the xylem (vessel) up the tree to the leaves. As water vaporizes from the spongy mesophyll cells; their cells sap becomes more concentrated than adjacent water flows into the cells from other surroundings cells; which in turn takes in water from xylem vessels within the leaf veins. This creates a pull / suction force/ transpiration pull that pulls a stream of water from xylem vessel in the stem and roots; the transpiration pull maintains continuous column of water from the roots into the leaves (transpiration stream).
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