This memory aid covers Memory Aids within Decomposition for GCSE Biology. Topic 4: Decomposition It is section 9 of 12 in this topic. Use it for quick recall, then test yourself straight afterwards so the memory aid becomes usable in an answer.
Topic position
Section 9 of 12
Practice
15 questions
Recall
20 flashcards
Memory Aids
Factors affecting decomposition rate — "TOMP" (Temperature, Oxygen, Moisture, pH):
- Temperature — warmer is faster (up to optimum); too hot = enzyme denaturation
- Oxygen — aerobic conditions needed for most decomposers; anaerobic = very slow
- Moisture — water needed for enzyme activity and decomposer survival
- pH — neutral optimum (~7); acid or alkaline conditions slow enzyme activity
How decomposers feed — "Secrete, Digest, Absorb" (SDA): Decomposers Secrete enzymes outside their body, enzymes Digest the organic material into small soluble molecules, then the decomposer Absorbs those molecules. This is different from animals, which ingest food first and then digest it internally.
Ideal compost conditions: Think of a composting heap as needing the opposite of a fridge — warm (not cold), moist (not dry), aerated (not compacted). These conditions maximise decomposer activity and produce finished compost fastest.
Quick Check: A student sets up four identical pots, each containing the same mass of leaf litter and soil. The pots are kept at four different temperatures: 5 degrees C, 20 degrees C, 37 degrees C, and 60 degrees C for four weeks. Predict and explain the results, including which pot would have the most decomposition and which would have the least.
Prediction: The 37 degrees C pot would show the most decomposition. The 5 degrees C pot would show very little decomposition, and the 60 degrees C pot would also show very little. The 20 degrees C pot would show intermediate decomposition. Explanation: Decomposition is carried out by bacteria and fungi using enzymes. At 5 degrees C, enzyme and substrate molecules have low kinetic energy, limiting collision frequency — decomposer metabolic rate is very low. At 20 degrees C, enzyme activity is higher — decomposer bacteria and fungi can grow and reproduce, increasing decomposition rate. At 37 degrees C, enzyme activity is at its optimum for most soil decomposers — maximum rate of decomposition. At 60 degrees C, the high temperature causes the active sites of decomposer enzymes to permanently change shape (denaturation). Enzymes can no longer bind their substrates, so decomposition almost stops despite the high temperature.
Quick Check: A biogas generator produces methane from food waste under anaerobic conditions. A student suggests that the biogas generator would work more efficiently if oxygen was added. Evaluate this suggestion.
The suggestion is incorrect. Biogas is produced specifically by anaerobic bacteria (methanogens) that carry out anaerobic respiration in the absence of oxygen. These bacteria use organic compounds as their substrate and produce methane as a metabolic by-product. Adding oxygen would allow aerobic bacteria to outcompete the anaerobic methanogens — aerobic bacteria grow faster and are more efficient at breaking down organic matter, but they produce CO2 and water, not methane. The methane-producing bacteria would be suppressed or eliminated. The result would be faster decomposition of the food waste, but little or no methane production. Biogas generators are deliberately maintained under anaerobic conditions to favour methane production. Adding oxygen would destroy the purpose of the generator.
Quick Check: A farmer wants to maximise the rate of decomposition in a compost heap. Using your knowledge of factors affecting decomposition, suggest three practical steps the farmer should take and explain the biological reason for each step.
Step 1 — Turn the heap regularly (introduce oxygen): Most decomposers are aerobic and require oxygen for respiration. Turning the heap breaks up compacted material, allowing air to penetrate throughout. This maintains aerobic conditions and maximises decomposer activity. Step 2 — Keep the heap moist (add water if dry): Decomposer bacteria and fungi need water as a solvent for enzyme secretion and for their own metabolic reactions. In dry conditions, decomposers become inactive or die. Maintaining optimal moisture (damp but not waterlogged) keeps decomposers active. Step 3 — Add materials that increase surface area (shred large items): Detritivores (earthworms, woodlice) and decomposers work more quickly when organic material has greater surface area exposed. Shredding leaves, cardboard, and food waste creates more surface for enzyme contact, increasing the rate of breakdown. Bonus: Ensuring the heap stays warm (insulating with a cover) keeps enzyme activity high, as warmer temperatures increase molecular kinetic energy and reaction rates up to the enzyme optimum.