Active transport: It also allows sugar molecules to be absorbed from lower concentrations in the gut into the blood which has a higher sugar concentration. Sugar molecules are used for cell respiration.

The idea

Active transport moves substances against a concentration gradient, from a lower concentration to a higher concentration, using energy from respiration. In the gut, sugar molecules may be at a lower concentration in the gut lumen than in the blood, so diffusion cannot supply them. Carrier proteins in the cell membrane of gut lining cells bind the sugar and use energy from ATP to change shape and transfer it into the blood. This allows sugar to be absorbed even when gut concentration is low, so respiration can continue. The sugar is then delivered to cells, where it is broken down in respiration to release energy for processes such as muscle contraction and active transport itself.

What earns the marks

• Active transport moves sugar from a lower concentration in the gut to a higher concentration in the blood, against the concentration gradient.
• Energy is required because the movement is against the gradient; this energy comes from respiration, often as ATP.
• Carrier proteins in the membrane of gut lining cells bind the sugar and change shape to transfer it across the membrane.
• The sugar absorbed into the blood is transported to cells and used in cell respiration to release energy.
• Without active transport, sugar would not be absorbed when gut concentration is lower than blood concentration, so respiration could be limited.

Common mistakes and how to put them right

• Thinking that diffusion can absorb sugar from a lower concentration in the gut into a higher concentration in the blood.

  Put it right: Diffusion only moves substances down a concentration gradient, so active transport is needed.

• Stating that active transport does not need energy.

  Put it right: Active transport requires energy from respiration because it moves substances against a concentration gradient.

• Confusing the role of sugar with the role of oxygen in respiration.

  Put it right: Sugar is a reactant broken down in respiration, while oxygen is also a reactant, and both are needed for aerobic respiration.

Exam tips

• Always name the direction of movement: from a lower concentration in the gut to a higher concentration in the blood, and state that this is against the concentration gradient.
• Link the need for energy to respiration explicitly, for example 'energy from respiration is used by carrier proteins to change shape'.
• When asked why sugar is absorbed, connect it to cell respiration and the release of energy for the organism, not just to 'making energy'.