Explain why glucose is described as the primary source of energy for cells, and describe how cells release energy from glucose.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Glucose is the main energy source because it is the simplest carbohydrate that cells can readily oxidise to release ATP. In aerobic respiration, glucose is broken down in the presence of oxygen to give carbon dioxide, water and a large amount of ATP. When oxygen is lacking, cells use anaerobic respiration: in animals this produces lactic acid, while in yeast and plants it produces ethanol and CO₂. Glucose is also the monomer of storage polysaccharides such as starch and glycogen, so it is quickly liberated from these reserves during digestion and can be used immediately for energy.
Examiner tips
- Use the word "aerobic" and "anaerobic" to show understanding of oxygen use. Mention the products (CO₂, H₂O, lactic acid, ethanol) to hit the mark. Show that glucose is a monomer of starch/glycogen to explain its availability.
- common_mistakes
- :
- Confusing glucose with other sugars (e.g. sucrose) as the energy source. Forgetting to mention the role of oxygen in aerobic respiration. Not specifying the products of anaerobic respiration (lactic acid or ethanol).
Mark scheme (5 marks)
- Glucose is broken down (in respiration) to release energy
- Glucose is used in aerobic respiration, which requires oxygen
- Products of aerobic respiration are carbon dioxide and water
- When oxygen is not available, anaerobic respiration occurs (in animals, producing lactic acid; in yeast/plants, producing ethanol and carbon dioxide)
- Glucose is the monomer / building block of polysaccharides (such as starch and glycogen), meaning it is readily available from digestion of these storage carbohydrates
Key terms in this question
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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