Osmosis is the process of movement of water molecules from its
- A.higher concentration to its lower concentration through a cell wall.
- B.lower concentration to its higher concentration through a fully permeable membrane.
- C.higher concentration to its lower concentration through a fully permeable membrane.
- D.higher concentration to its lower concentration through a semi-permeable membrane.
Correct answer
Explanation
The correct answer is D, higher concentration to its lower concentration through a semi-permeable membrane. Osmosis is the passage of water from where water is more concentrated (a dilute solution) to where it is less concentrated, across a membrane that lets water through but holds back solutes.
The cell membrane is selectively permeable, so when a cell sits in a dilute solution water enters it, and in a strong salt or sugar solution water leaves it. This is why raisins swell in water, why salted vegetables shrink and why root hair cells take up water from the soil. The pressure needed to stop this flow is called osmotic pressure.
A is wrong because the plant cell wall is fully permeable and lets both water and dissolved substances pass, so it does not cause osmosis. B is wrong on both counts: the direction is reversed and the membrane is wrong. C is wrong because movement through a fully permeable membrane would let solutes pass too, which is ordinary diffusion, not osmosis.
Exam tip: osmosis needs a semi-permeable membrane and moves water from dilute to concentrated solution.
Practice Questions
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