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2017Q3(a)(ii)Absorption by Roots – The Processes Involved
Short answerHard1 mark
3(a)

The diagram given below represents a plant cell after being placed in a strong sugar solution. Study the diagram and answer the questions that follow: The diagram given below represents a plant cell after being placed in a strong sugar solution. Study the diagram and answer the questions that follow:

Figure for this question

Name the structure that acts as a selectively permeable membrane.

Topic: Selectively permeable membrane

Previous-year question practice database

Answer

Exam answer

The PLASMA MEMBRANE (cell membrane).

Explanation

THE DISTINCTION THAT MATTERS, and it is examined repeatedly: the CELL WALL is FREELY PERMEABLE — it lets everything through, solutes included the PLASMA MEMBRANE is SELECTIVELY PERMEABLE — it lets water through but holds back the solute

THE PROOF IS IN THIS VERY DIAGRAM: the gap between the shrunken protoplasm and the wall is filled with the STRONG SUGAR SOLUTION, which passed straight through the wall and was stopped only at the membrane. If the wall were selective, that gap would be empty.

This is also why OSMOSIS is defined as requiring a selectively permeable membrane — without selectivity there would be no osmosis at all, only diffusion.

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