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Biology HLTheme B: Form and function (HL)
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Explain how the cohesion-tension theory accounts for the rise of water through the xylem of a tall tree.
States that water evaporates from the mesophyll cells and leaves the stomata during transpiration, lowering water potential in the leaf.
States that water molecules are cohesive, forming hydrogen bonds with each other, so a continuous unbroken column of water is pulled up the xylem.
States that water molecules are adhesive to the walls of the narrow xylem vessels, helping counteract gravity.
States that this creates tension (negative pressure) that is transmitted down the column, drawing water up from the roots.
Check your reasoning:Cohesion, adhesion and tension must all be named explicitly — a vague description of 'water being pulled up' without these terms loses marks.
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