Respiration (A-level only): Respiration produces ATP.

The idea

Respiration transfers energy from organic molecules to ATP. ATP is synthesised by substrate-level phosphorylation in glycolysis and the Krebs cycle, and by oxidative phosphorylation. In oxidative phosphorylation, electrons from reduced coenzymes pass along the electron transfer chain, releasing energy to actively transport protons. The resulting proton gradient drives ATP synthase to phosphorylate ADP. ATP is the immediate energy source for cells. It is hydrolysed by ATP hydrolase to ADP and Pi, releasing manageable energy for processes such as active transport, muscle contraction and protein synthesis. Glycolysis initially requires ATP to phosphorylate glucose, so respiration both consumes and produces ATP. Phosphorylation makes glucose more reactive and less stable, allowing it to be split more readily; this is accepted as lowering the activation energy of the reaction.

What earns the marks

• State that ATP is produced via substrate-level phosphorylation in glycolysis and the Krebs cycle.
• Explain that oxidative phosphorylation produces ATP using energy from the electron transfer chain and a proton gradient driving ATP synthase.
• Identify that ATP is hydrolysed to ADP and inorganic phosphate by ATP hydrolase to release energy.
• Provide a named use of ATP, such as active transport, muscle contraction, DNA replication or protein synthesis.
• Note that glycolysis requires an initial investment of ATP to phosphorylate glucose, making glucose more reactive and less stable, which is accepted as lowering the activation energy of the reaction.

Common mistakes and how to put them right

• Stating respiration 'produces energy' instead of transferring energy to ATP.

  Put it right: Energy cannot be created or destroyed.

• Claiming respiration does not use ATP, forgetting that glycolysis requires an initial investment of ATP to phosphorylate glucose.

• Treating ATP as a long-term energy store rather than an immediate energy source that is continuously synthesised.

• Naming 'energy' as the use of ATP rather than specifying a cellular process like active transport.

• Rejecting 'lowers activation energy' as a description of glucose phosphorylation.

  Put it right: This is accepted as equivalent to making glucose more reactive and less stable.

Exam tips

• Always name the specific process ATP powers (e.g., active transport or muscle contraction) rather than just stating it is used for energy.
• Remember to distinguish between substrate-level phosphorylation (glycolysis and Krebs cycle) and oxidative phosphorylation when describing ATP synthesis.