6.4.1Unit 6: Practical Skills in Biology II
The Role of Microorganisms — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 16 of 48 Unit quiz
Section 1 of 5Decomposition
Decomposers are the link that returns dead organic matter to the nutrient and carbon cycles.
Decomposers
- Bacteria and fungi break down dead plants and animals and are called decomposers.
Extracellular digestion
- They secrete enzymes onto the dead material, which break large organic molecules such as cellulose into smaller ones.
Absorption
- The small molecules, such as glucose, are absorbed and used as respiratory substrates or to make new cell material.
Waste
- Decomposition releases waste products and mineral nutrients that can be taken up by plants.
Saprotrophs
- Organisms that feed in this way on dead material, using extracellular digestion, are described as saprotrophs.
Section 2 of 5Returning Carbon to the Cycle
1
Dead tissue contains carbon in carbohydrates, proteins and lipids.
2
Decomposers digest the tissue and respire some of the small molecules.
3
Respiration releases carbon dioxide into the air or soil.
4
Under anaerobic conditions some microorganisms release methane instead.
5
Green plants take in carbon dioxide and fix the carbon again in carbohydrates by photosynthesis.
6
Without this return of carbon dioxide, plants would eventually run short of the raw material for photosynthesis.
Section 3 of 5Importance for Ecosystems
Importance for Ecosystems
Recycling
- Without decomposers, nutrients would stay locked in dead material and be unavailable to producers.
Soil fertility
- Minerals released from decaying matter are absorbed by plant roots to build new tissue.
Energy
- Decomposers use energy stored in dead material to power their own metabolism.
Gas balance
- Carbon dioxide and methane released affect the composition of the atmosphere.
Mineral ions
- Decay releases mineral ions, including those containing nitrogen, which roots absorb from the soil.
Section 4 of 5Factors Affecting Decay
Decomposition is a series of enzyme-controlled reactions, so it responds to the same factors as other enzyme reactions.
Temperature
- Warmer conditions generally speed enzyme activity and decomposition, whereas cold conditions slow it down.
Oxygen
- Aerobic decomposition is faster than decay in anaerobic conditions.
Moisture
- Microorganisms need water, so very dry material decays slowly.
Forensic use
- The rate and stage of decomposition give clues to the time since death.
Preservation
- Cold, dry or oxygen-free conditions slow decay, which is why some bodies are preserved for long periods.
Section 5 of 5Exam Checklist & Tips
6.4.1 – Key Points for the Exam
- Decomposer A bacterium or fungus that breaks down dead organic matter.
- Extracellular enzyme An enzyme secreted outside a cell to digest material before the products are absorbed.
- Methane A gas, CH₄, produced by some microorganisms in anaerobic conditions.
- Nutrient recycling The return of elements from dead organisms to forms that producers can use.
- Saprotroph An organism that feeds on dead organic matter by secreting enzymes and absorbing the products.
Common Mistakes
- Stating that decomposers digest food inside the cell first; they secrete enzymes onto it.
- Forgetting that decomposers respire, which is how they release carbon dioxide.
- Saying that dead material disappears without explanation, instead of naming decomposers and their enzymes.
- Writing that decomposition is the same in all conditions, when temperature, oxygen and moisture matter.
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