Diffusion: Substances may move into and out of cells across the cell membranes via diffusion.

The idea

Cell membranes are partially permeable barriers that allow some substances to pass through by diffusion. Diffusion is the passive, net movement of particles from a region of higher concentration to a region of lower concentration, so substances can enter or leave a cell without the cell using energy. For example, oxygen diffuses from a higher concentration outside a respiring cell into the lower concentration inside, while carbon dioxide diffuses in the opposite direction. In a solution, dissolved particles such as glucose or mineral ions may also diffuse across the membrane if they can pass through it. The direction of net movement depends on concentration gradients, and the process continues until concentrations become equal or the gradient is removed. Because diffusion is passive, it does not require ATP from respiration, distinguishing it from active transport.

What earns the marks

• States that diffusion is a passive process that does not require energy from respiration.
• Describes net movement of particles from an area of higher concentration to an area of lower concentration.
• Applies the idea to a named substance entering or leaving a cell, such as oxygen entering a respiring cell or carbon dioxide leaving it.
• Recognises that cell membranes are partially permeable and that only certain substances can diffuse across them.
• Explains that diffusion continues until the concentration gradient is reduced or removed, leading to equal concentrations.
• Distinguishes diffusion from active transport, which requires energy and can move substances against a concentration gradient.

Common mistakes and how to put them right

• Saying that diffusion requires energy from respiration.

  Put it right: Diffusion is passive and does not need ATP; active transport is the energy-requiring process.

• Describing movement as from low to high concentration.

  Put it right: Net diffusion is from higher to lower concentration down a concentration gradient.

• Claiming that all substances can freely cross any cell membrane.

  Put it right: Membranes are partially permeable, so only some particles can diffuse through.

• Confusing diffusion with osmosis.

  Put it right: Osmosis is the diffusion of water specifically across a partially permeable membrane, whereas diffusion can involve any substance in solution or gas.

Exam tips

• When asked how a substance enters or leaves a cell, name the substance, state the direction of its concentration gradient and link this to diffusion across the partially permeable membrane.
• Use the phrase net movement from higher to lower concentration to show understanding that individual particles move randomly but the overall effect is directional.
• If a question asks why diffusion is important, give a concrete example such as oxygen entering a muscle cell for respiration or carbon dioxide leaving a leaf cell.
• Avoid saying particles want to move or try to spread out; describe the random motion and the resulting net movement instead.