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2018Q1(e)(iii)Absorption by Roots – The Processes Involved
Correct the statementHard1 mark
1(e)

Correct the following statements by changing the underlined words:

The cell sap of root hair is HYPOTONIC.

Topic: Cell sap of root hair

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Answer

Exam answer

The cell sap of root hair is HYPERTONIC (more concentrated) compared to the soil water.

Explanation

THE WHOLE MECHANISM OF ABSORPTION depends on this, which is why the word matters so much.

The cell sap of the root hair is MORE CONCENTRATED than the surrounding soil water. Water therefore moves from the DILUTE soil solution INTO the more concentrated cell sap by OSMOSIS — endosmosis — through the selectively permeable plasma membrane.

If the sap were HYPOTONIC, as the statement claims, water would move the other way and the plant could absorb nothing at all.

THE PRACTICAL CONSEQUENCE, tested in the 2019 paper: adding EXCESS FERTILISER makes the soil solution hypertonic instead, reversing the gradient, so water leaves the root hairs and the plant wilts — fertiliser burn.

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