Active transport: describe how substances are transported into and out of cells by diffusion, osmosis and active transport

The idea

Substances cross cell membranes by three 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 is passive. Osmosis is the diffusion of water molecules across a partially permeable membrane from a dilute solution to a concentrated solution, also passive. Active transport moves substances from a lower concentration to a higher concentration, against the concentration gradient, using energy from respiration.

For example: Root hair cells use active transport to absorb mineral ions from soil, while oxygen enters cells by diffusion and water enters by osmosis.

What earns the marks

• Diffusion: net movement of particles from higher to lower concentration, down a concentration gradient, passive (no energy from respiration required).
• Osmosis: net movement of water molecules across a partially permeable membrane from a dilute solution to a concentrated solution, passive.
• Active transport: net movement of substances from lower to higher concentration, against a concentration gradient, requiring energy from respiration.
• Examples: oxygen and carbon dioxide exchange by diffusion; water uptake by root hair cells by osmosis; mineral ion uptake by root hair cells by active transport.
• Reference to partially permeable membranes is expected for osmosis, while concentration gradients should be mentioned for all three processes.

Common mistakes and how to put them right

• Stating that diffusion requires energy from respiration.

  Put it right: Diffusion is passive and relies on the random motion of particles; no metabolic energy is needed.

• Describing osmosis as the movement of any dissolved substance rather than water.

  Put it right: Osmosis specifically refers to the net movement of water molecules across a partially permeable membrane.

• Saying active transport moves substances down a concentration gradient.

  Put it right: Active transport moves substances against a concentration gradient, from lower to higher concentration, and requires energy.

Exam tips

• Use the phrase 'net movement' when defining diffusion and osmosis to show you understand it is an overall direction, not individual particle motion.
• When describing osmosis, always mention the partially permeable membrane and that water moves from a dilute to a concentrated solution.