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Movement In and Out of Cells: Question 10

Syllabus 3.1

Multiple choice Core 1 mark

In the lungs, oxygen moves from the air inside the alveoli, where its concentration is high, into the blood in the surrounding capillaries, where its concentration is lower, passing across the thin cell membranes of the alveoli and the capillary walls. The cells of the alveoli use no energy from respiration to bring about this movement of oxygen.

Which term correctly describes this movement of oxygen from the alveoli into the blood?

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 the particles’ own random movement. It needs no energy input from the cells involved, and it can happen across a cell membrane as well as through open air or liquid.

Step 2: Match this to the scenario

The stem states that the concentration of oxygen in the alveoli is high and the concentration of oxygen already in the blood is lower, and that no energy from respiration is used by the alveoli cells to move the oxygen. Oxygen is therefore moving down its own concentration gradient, from a region of higher concentration to a region of lower concentration, using no energy, exactly the definition of diffusion.

Why the other options are wrong

  • A (osmosis): osmosis refers specifically to the net movement of water molecules through a partially permeable membrane. Oxygen is a gas, not water, so its movement here cannot be described as osmosis.
  • C (active transport, wrong reasoning): a substance does not need protein carriers to cross a cell membrane if it is moving down its own concentration gradient; carriers and energy are needed for active transport, which moves particles against a gradient, not for this passive movement of oxygen down its gradient.
  • D (active transport, wrong direction): the blood already containing some oxygen does not mean oxygen is moving against a gradient here. The stem states the oxygen concentration in the alveoli is higher than in the blood, so oxygen is still moving down its concentration gradient, not against it.

Final answer

Oxygen moves from the alveoli into the blood by diffusion, down its concentration gradient and without any energy input, option B.