Aerobic and anaerobic respiration: glucose lactic acid

The idea

This statement gives the two key substances in the anaerobic respiration equation for muscles: glucose is the reactant and lactic acid is the product. In words, glucose → lactic acid. Anaerobic means without oxygen, so this pathway happens when muscle cells cannot get enough oxygen, such as during a sprint. Glucose is partially broken down, releasing some energy for muscle contraction but less than aerobic respiration. Lactic acid accumulates in the muscles and blood, contributing to fatigue. After exercise, extra oxygen is taken in to break down the lactic acid, which is the oxygen debt. For example, a runner sprinting 100 m relies on anaerobic respiration in leg muscles and then breathes heavily afterwards. The equation shows only one product, so carbon dioxide and water are not formed in this pathway.

What earns the marks

• Identifies glucose as the reactant in anaerobic respiration in muscles.
• Identifies lactic acid as the product of anaerobic respiration in muscles.
• Writes or completes the word equation glucose → lactic acid.
• Explains that this pathway occurs without oxygen and releases less energy than aerobic respiration.
• Relates lactic acid to muscle fatigue and to the oxygen debt repaid after exercise.

Common mistakes and how to put them right

• Adding carbon dioxide and water to the products.

  Put it right: Those are products of aerobic respiration, not anaerobic respiration in muscles.

• Writing lactic acid as the reactant and glucose as the product.

  Put it right: Glucose is broken down, so glucose is the reactant and lactic acid is the product.

• Using ethanol and carbon dioxide instead of lactic acid.

  Put it right: Ethanol and carbon dioxide are produced by anaerobic respiration in yeast, not in muscles.

Exam tips

• Use the arrow direction correctly: glucose → lactic acid, not lactic acid → glucose.
• Check spelling of lactic acid and glucose, as unclear spelling can lose credit in word-equation questions.
• If asked why the equation has only one product, link this to the absence of oxygen and incomplete breakdown of glucose.