Active transport: Sugar molecules are used for cell respiration.

The idea

Sugar molecules absorbed into the blood are transported to cells where they are used in cell respiration. Respiration releases energy from sugar, and this energy is used for life processes such as muscle contraction, active transport, cell division and maintaining body temperature. In cells, glucose is broken down in a series of reactions; aerobic respiration uses oxygen and produces carbon dioxide and water, while anaerobic respiration in animals produces lactic acid. The energy released is transferred to ATP, which powers cell activities. This links the absorption of sugar in the gut to the energy needs of the whole organism.

What earns the marks

• Sugar molecules are transported in the blood to body cells.
• Sugar is broken down in cell respiration to release energy.
• The energy released is used for life processes such as muscle contraction, active transport and cell division.
• Aerobic respiration uses oxygen and produces carbon dioxide and water.
• Anaerobic respiration in animals produces lactic acid and releases less energy per sugar molecule.

Common mistakes and how to put them right

• Saying respiration makes energy.

  Put it right by stating that respiration releases energy from sugar.

• Confusing respiration with breathing.

  Put it right by describing respiration as a chemical process in cells.

• Thinking sugar is used only for energy and not as a building material.

  Put it right by noting that sugar can also be converted into other molecules, but its main role here is respiration.

Exam tips

• Write a word equation for aerobic respiration to show the reactants and products.
• Link the energy released to a named life process rather than leaving it general.
• Distinguish aerobic from anaerobic respiration by naming the products and relative energy release.