Active transport: explain the differences between the three processes.

The idea

Diffusion, osmosis and active transport differ in what moves, which direction relative to the concentration gradient, and whether energy is needed. Diffusion moves any dissolved or gaseous particles down a concentration gradient and is passive. Osmosis moves only water molecules across a partially permeable membrane, from a dilute solution to a concentrated solution, and is also passive. Active transport moves substances against a concentration gradient, from lower to higher concentration, using energy from respiration and carrier proteins.

For example: Oxygen enters cells by diffusion, water enters root hair cells by osmosis, and mineral ions enter root hair cells by active transport.

What earns the marks

• Diffusion and osmosis are passive and do not require energy from respiration; active transport requires energy from respiration.
• Diffusion moves any particles down a concentration gradient; osmosis moves only water molecules down a water potential gradient across a partially permeable membrane.
• Active transport moves substances against a concentration gradient, from lower to higher concentration, using carrier proteins.
• Osmosis involves a partially permeable membrane; diffusion may occur across a membrane or in a fluid without one; active transport always involves carrier proteins in a membrane.
• Comparisons should be made explicitly, for example using 'whereas' or 'unlike', to show the differences between the processes.

Common mistakes and how to put them right

• Saying diffusion and osmosis both require energy.

  Put it right: Both are passive processes; only active transport requires energy from respiration.

• Stating that osmosis moves solutes rather than water.

  Put it right: Osmosis is the movement of water molecules only, across a partially permeable membrane.

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

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

Exam tips

• Structure comparison answers by addressing one difference at a time, such as direction of movement, then energy requirement, then substances involved.
• Use comparative connectives like 'whereas' and 'unlike' to make the differences explicit and avoid simply describing each process separately.
• Refer to specific examples, such as oxygen by diffusion, water by osmosis and mineral ions by active transport, to support each comparison.