Explain why oxygen moves from the air sacs (alveoli) in the lungs into the blood in the surrounding capillaries.

Edexcel GCSE Biology (1BI0) — 1.5 Transport in cells · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

During gas exchange in the lungs, oxygen must cross from the alveoli into the blood to be transported around the body. Oxygen concentration in the alveoli is higher than in the blood arriving at the lungs.

Model answer (4 marks)

Oxygen moves by diffusion.
1. There is a concentration gradient – oxygen concentration is higher in the alveoli than in the blood.
2. Molecules move from the region of higher concentration to the region of lower concentration.
3. This movement continues until equilibrium is reached, or until the blood flow keeps the gradient.
4. The continuous flow of blood removes oxygen from the capillaries, maintaining the gradient and allowing further diffusion.

Examiner tips

Use the word "diffusion" first. Mention the concentration gradient. Explain direction of movement. Link to equilibrium or blood flow keeping the gradient.

Common mistakes

Not naming diffusion. Failing to mention the concentration gradient. Saying oxygen moves "into the blood" without explaining the direction. Omitting the role of blood flow or equilibrium.

Mark scheme (4 marks)

Names the correct process as diffusion
States that there is a concentration gradient / higher concentration of oxygen in the alveoli than in the blood
States that particles / molecules move from a region of higher concentration to a region of lower concentration
States that this continues until equilibrium is reached / until the concentration gradient no longer exists, OR links the continuous movement of blood to maintaining the concentration gradient

Key terms in this question

alveoli