Substances may move into and out of cells across the cell membranes via…

The idea

Every cell is surrounded by a cell membrane that controls what enters and leaves. Diffusion is one way substances cross this membrane. Particles in a gas or in solution move randomly, and if there is a higher concentration of a substance on one side of the membrane than the other, the random movement produces a net transfer towards the lower concentration. The membrane does not need to provide energy for this movement, so diffusion is a passive process. For example, oxygen diffuses from the air in the lungs into the blood, and carbon dioxide diffuses from the blood into the lungs. The statement is assessed by asking you to identify diffusion as a movement mechanism across cell membranes and to relate it to concentration differences.

What earns the marks

• Substances can enter cells across the cell membrane by diffusion.
• Substances can leave cells across the cell membrane by diffusion.
• Diffusion across a membrane depends on a concentration difference between the two sides.
• The net movement is from higher concentration to lower concentration until the concentrations become equal.
• Diffusion is passive and does not require energy from respiration.
• Examples include oxygen entering cells and carbon dioxide leaving cells.

Common mistakes and how to put them right

• Saying that diffusion requires energy.

  Put it right: Diffusion is passive and relies on the random movement of particles.

• Describing movement only into cells.

  Put it right: Substances move both into and out of cells by diffusion.

• Confusing diffusion with active transport.

  Put it right: Active transport moves substances against a concentration gradient and requires energy.

Exam tips

• Always name the membrane as the barrier crossed and state the direction of net movement.
• Use the phrase concentration gradient to explain why diffusion happens.
• When giving an example, name the substance and the two regions it moves between.