Explain how substances are transported into and out of cells, including by…

The idea

Substances cross cell membranes by three named processes. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, and it is passive because no energy is supplied by the cell; oxygen diffuses into a respiring cell and carbon dioxide diffuses out. Osmosis is the diffusion of water across a partially permeable membrane from a dilute solution (higher water potential) to a more concentrated solution (lower water potential); it is also passive. Active transport moves particles against a concentration gradient, from lower to higher concentration, using energy released by respiration and carrier proteins in the membrane; root hair cells absorbing mineral ions are the standard example. For each process, identify the direction of movement, whether energy is needed and the role of the membrane.

What earns the marks

• Diffusion: net movement of particles from higher to lower concentration down a concentration gradient; passive, no energy from the cell.
• Osmosis: movement of water through a partially permeable membrane from a dilute solution (higher water potential) to a more concentrated solution (lower water potential); passive.
• Active transport: movement of particles against a concentration gradient (lower to higher) using energy from respiration and carrier proteins.
• Correctly distinguishes which process applies to a given example, e.g. oxygen into a cell by diffusion, water into a root hair cell by osmosis, mineral ions into a root hair cell by active transport.
• Recognises that diffusion and osmosis are passive while active transport requires energy released by respiration.

Common mistakes and how to put them right

• Saying osmosis moves solutes rather than water.

  Put it right by stating osmosis is the movement of water molecules only.

• Describing active transport as moving substances down a concentration gradient.

  Put it right by stating it moves particles against the gradient and needs energy.

• Claiming diffusion requires energy from respiration.

  Put it right by stating diffusion is passive and needs no energy input.

Exam tips

• For each option, check the direction of movement (down or against the gradient) before deciding.
• Link the process to a named example such as oxygen uptake or mineral ion absorption to test the definition.
• Remember the membrane in osmosis is partially permeable, not fully permeable.