A student leaves a piece of bread in a warm, moist cupboard. After two weeks, the bread is covered in mould and has significantly decreased in mass. Explain why the bread decreases in mass over the two weeks.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Decomposers, including fungi and bacteria, break down organic material through a series of biological processes. The rate of decomposition is affected by temperature, moisture and oxygen availability.
Model answer (5 marks)
The mould and bacteria on the bread are decomposers that feed on the bread. They secrete extracellular enzymes that break down the complex carbohydrates, proteins and fats into soluble simple molecules. These products are absorbed by the decomposers for growth and respiration. In aerobic respiration the decomposers convert the sugars into energy, releasing carbon dioxide and water as waste. The loss of carbon as CO₂ and the removal of organic material by the decomposers means the bread loses mass over the two weeks.
Examiner tips
- Mention mould/bacteria as decomposers first
- Explain extracellular digestion and absorption
- State aerobic respiration releases CO₂ and water
- Link loss of CO₂ to mass loss
Common mistakes
- Forgetting to mention the role of enzymes
- Confusing intracellular with extracellular digestion
- Not linking CO₂ release to mass loss
Mark scheme (5 marks)
- The mould (fungi) / bacteria are decomposers that feed on / break down the bread
- Decomposers release enzymes onto / into the bread (extracellular digestion) to break down complex organic molecules
- The soluble products / simple molecules are absorbed into the decomposer for use in respiration / growth
- Carbon dioxide and water are released as waste products of (aerobic) respiration
- Loss of carbon (as CO₂) / loss of organic material to the decomposers means the mass of the bread decreases
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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