Ecosystems — OCR GCSE Biology
Test yourself on Ecosystems with OCR GCSE practice questions.
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Ecosystems explained
Decomposition rate depends on environmental factors that affect microbial metabolism and extracellular enzyme activity.
Read the full explanation
Temperature affects kinetic energy: warming increases enzyme-substrate collisions and decay rate up to an optimum; above the optimum, microbial enzymes denature and decay slows markedly, though it does not necessarily stop completely. Water content is vital because decomposers need moisture for metabolism, enzyme secretion and nutrient diffusion; desiccation prevents microbial growth. Oxygen availability determines whether aerobic or anaerobic respiration occurs. Aerobic decomposition uses oxygen for efficient respiration and rapid breakdown of organic matter, releasing carbon dioxide. In waterlogged soils, oxygen is limited, so anaerobic decomposition proceeds more slowly and can produce methane and organic acids.
Your focus
- Explain how temperature, water availability and oxygen affect microbial decomposition rates.
- Contrast aerobic and anaerobic decomposition in terms of speed and products.
- Apply knowledge of decomposition factors to composting and biogas production.
Ecosystems exam tips
Marking Points
- Explain that increasing temperature increases kinetic energy and enzyme-substrate collisions, raising decomposition rate up to an optimum; above the optimum, enzymes denature and the rate falls.
- Explain that decomposers require moisture for metabolic processes, enzyme secretion and diffusion of soluble nutrients, so low water content reduces decomposition rate.
- Explain that aerobic decomposition requires oxygen for efficient respiration, giving rapid breakdown of organic matter and release of carbon dioxide.
- Describe anaerobic conditions, such as waterlogged soils or bogs, where lack of oxygen results in slower decomposition and production of methane and organic acids.
- Compare aerobic and anaerobic decomposition in terms of oxygen requirement, relative speed and products.
Examiner Tips
- 💡Explain temperature effects using enzyme kinetics: kinetic energy, collision frequency, optimum temperature and denaturation.
- 💡Contrast aerobic decomposition (fast, requires oxygen, releases carbon dioxide) with anaerobic decomposition (slower, no oxygen, releases methane and organic acids).
Common Mistakes
- Stating that bacteria die of heat rather than explaining that their enzymes denature at high temperatures. Correction: high temperature denatures enzymes, slowing or stopping reactions; the decomposers may survive but function poorly.
- Assuming decomposition ceases entirely in the absence of oxygen. Correction: anaerobic decomposers can still operate, but more slowly, producing methane and organic acids.
- Believing waterlogged soils speed up decay. Correction: excess water excludes oxygen, creating anaerobic conditions and slowing decomposition.
- Saying decay stops completely above the optimum temperature. Correction: decay slows markedly because enzymes denature, but it does not necessarily halt entirely.