Active transport: Active transport moves substances from a more dilute solution to a more concentrated solution (against a concentration gradient). This requires energy from respiration.

The idea

Active transport is the carrier-mediated movement of a substance across a membrane from a region where it is more dilute to a region where it is more concentrated, so it runs against the concentration gradient. Because the substance is being moved away from equilibrium, the process cannot happen by diffusion and the cell must supply energy. That energy comes from respiration, which releases energy from glucose; the energy is used by carrier proteins in the membrane to change shape and transfer the substance. A concrete example is a root hair cell absorbing nitrate ions from soil solution that is more dilute than the cell contents. The rate of active transport depends on the number of carrier proteins and on the rate of respiration, so it stops if respiration is inhibited or oxygen is unavailable.

What earns the marks

• Active transport moves a substance from a more dilute solution to a more concentrated solution, which is against the concentration gradient.
• Movement against a concentration gradient cannot occur by diffusion alone and requires an input of energy.
• The energy for active transport is released by respiration, so the process depends on a supply of glucose and, in aerobic cells, oxygen.
• Carrier proteins in the cell membrane bind the substance and use energy to change shape, transferring the substance across the membrane.
• The rate of active transport is limited by the number of carrier proteins and by the rate of respiration, so it falls when respiration is inhibited.
• Active transport differs from diffusion because diffusion is passive and down a concentration gradient, whereas active transport is energy-requiring and up a concentration gradient.

Common mistakes and how to put them right

• Saying that active transport moves substances from a concentrated solution to a dilute solution.

  Put it right: The correct direction is from a more dilute solution to a more concentrated solution, against the concentration gradient.

• Stating that active transport needs energy but not naming respiration as the source.

  Put it right: The energy is released by respiration, so always link the process to respiration.

• Confusing active transport with osmosis.

  Put it right: Osmosis is the passive movement of water across a partially permeable membrane down a water potential gradient, whereas active transport uses energy and carrier proteins to move solutes against a concentration gradient.

Exam tips

• Define the direction of movement precisely using the words more dilute and more concentrated, then state that this is against the concentration gradient.
• When asked why a cell needs energy, link the answer to respiration releasing energy from glucose rather than simply writing that energy is needed.
• Use a named example, such as a root hair cell absorbing mineral ions, to show that you can apply the definition rather than only recall it.