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Movement In and Out of Cells: Question 1

Syllabus 3.1

Multiple choice Core 1 mark

A student opens a small bottle of nail varnish remover in one corner of a still, closed classroom with no fans or open windows. Within a few minutes, students sitting on the far side of the room notice the smell, even though the air in the room is not being stirred or blown in any way.

Which statement correctly explains why the vapour particles from the nail varnish remover spread across the room?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall what diffusion is

Diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration, down a concentration gradient, as a result of their own random movement. It needs no energy input from an outside source and does not require a membrane.

Step 2: Match this to the scenario

Near the open bottle, the concentration of nail varnish remover vapour particles in the air is high. Further away, across the room, the concentration is much lower (essentially zero at first). Because the room is still, with no fans or draughts, the only reason the vapour reaches the far side is that its particles are constantly moving at random, and this random movement produces a net movement from the high-concentration region to the low-concentration region. This is exactly the definition of diffusion.

Why the other options are wrong

  • A (convection): convection would require moving currents of air, but the room is described as still, with no fans and no air currents mentioned. It is the vapour particles’ own random movement, not bulk air movement, that spreads the smell here.
  • C (active transport): active transport moves particles against a concentration gradient, through a cell membrane, using energy from respiration. Vapour spreading through open air is not moving against a gradient and is not crossing any cell membrane, so this cannot be active transport.
  • D (osmosis): osmosis refers specifically to the net movement of water molecules through a partially permeable membrane. Vapour particles moving through open air, with no membrane involved, cannot be described as osmosis.

Final answer

The vapour spreads because its particles diffuse, moving randomly from the region of higher concentration near the bottle to the region of lower concentration across the room, option B.