What you need to know
- 6.16Microorganisms decompose organic matter and recycle carbon.
The nitrogen cycle is not this point. Time of death is 6C 2 and 6C 3. A vulture or an earthworm is not a microorganism.
By the end, you should be able to…
- Describe extracellular digestionDigestion outside the cells: enzymes are secreted onto food and the products are absorbed. by saprobiontic nutritionFeeding by secreting enzymes onto dead organic matter and absorbing the soluble products. bacteria and fungi.
- Explain why decomposition recycles carbon as carbon dioxide.
- Separate microorganisms from detritivoreAn animal, such as an earthworm, that feeds on dead organic matter (detritus)..
Build your understanding
Saprobionts feed on the dead
Decomposition is the breakdown of dead organic matter and wastes. The microorganisms responsible are saprobiontAn organism, such as a fungus or bacterium, that feeds on dead organic matter by extracellular digestion., mainly bacteria and fungi. They do not swallow the material. They secrete enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. onto it. Those extracellular enzymes hydrolyse proteins, lipids, starchThe storage polysaccharide in plants, made of amylose and amylopectin. It is compact and insoluble, so it does not affect water potential., celluloseA structural polysaccharide of β-glucose. Straight chains are linked by hydrogen bonds into microfibrils that strengthen plant cell walls. and nucleic acids into soluble molecules. The cells then absorb the products across their membranes.
A detritivoreAn animal, such as an earthworm, that feeds on dead organic matter (detritus). such as an earthworm or a maggot bites matter into smaller pieces and increases the surface areaThe total area of a surface. A larger surface area gives a faster rate of diffusion.. That helps the enzymes. The detritivore is not the microorganism in 6.16. A scavenger that eats a carcass is not performing this extracellular digestionDigestion outside the cells: enzymes are secreted onto food and the products are absorbed. either.
Carbon leaves as carbon dioxide
The absorbed sugars, amino acidThe monomer of proteins: a central carbon with an amine group, a carboxyl group, a hydrogen and an R group. and fatty acids are respiratory substrateAn organic molecule that is broken down in respiration to release energy, such as glucose, fatty acids or amino acids.. aerobic respirationRespiration using oxygen: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. It releases much more ATP than anaerobic respiration. of decomposers releases carbon dioxide. That is the recycling: carbon that was locked in a body or a leaf becomes CO₂ again and can be fixed by photosynthesisThe process in which plants use light energy to make glucose from carbon dioxide and water, releasing oxygen.. Without decomposition, carbon would stay in dead remains and unavailable to producerAn organism, usually a plant, that makes its own organic molecules by photosynthesis. Producers form the first trophic level..
Anaerobic conditions, waterlogging, low temperature and acidity slow the microbes, so peat and landfill can store carbon for a long time. Warm, moist, oxygenated soil speeds the same organisms. The carbon cycleThe movement of carbon between the atmosphere, organisms, oceans and rocks by photosynthesis, respiration, decomposition and combustion. lesson is the wider map. This page is the microbial step on that map.
What you do not need to draw
Do not add a full nitrogen cycle unless a question asks for nitrogen. 6.16 names carbon. ureaThe main nitrogenous waste product made in the liver from ammonia, and excreted in urine. and protein do contain nitrogen, and bacteria are involved in that element too, but that is a different account.
Compost heaps are a domestic version of the same biology. They are not a forensic clock and not a pure culture. Several speciesA group of organisms that can interbreed to produce fertile offspring. work at once.
NOT: decomposition is not an animal eating the body, and the carbon does not disappear.
- Enzymes act on proteins, lipids, starch, cellulose and nucleic acids
- Cold, acidic, waterlogged or anaerobic conditions slow the microbes
- Peat can therefore store carbon for a long time
- Warm, moist, oxygenated soil speeds the same organisms
- A nitrogen cycle is a different account and is not required here
Two traps this diagram blocks
- Do not say the carbon is destroyed.
- Do not draw a full nitrogen cycle on this figure.
From the diagram alone
Question
Explain how fungi return carbon from a dead leaf to the atmosphere. [4]
Show a model answer
Fungi secrete enzymes onto the leaf. The enzymes hydrolyse large carbon compounds into soluble sugars, which the fungus absorbs. Aerobic respiration of those sugars releases carbon dioxide. The carbon can then be fixed again by photosynthesis.
| Organism | What it does | Is it the 6.16 microbe? |
|---|---|---|
| Bacterium or fungus | Extracellular enzymes, then respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP. | Yes |
| Earthworm or maggot | Breaks matter into pieces | No, it helps them |
| Producer | Fixes CO₂ in photosynthesis | No, it uses the recycled gas |
Make your reasoning visible
Enzyme, then CO₂
Secretion and hydrolysis first. Carbon dioxide comes from the decomposer’s respirationThe breakdown of respiratory substrates such as glucose in cells to release energy, which is used to make ATP., not from the dead leaf by itself.
Name the conditions
Warm, moist and oxygenated speeds aerobic decomposers. Say which factor changed.
Avoid this misconception
The claim: Decomposition is animals eating the body, and the carbon disappears.
The correction: Bacteria and fungi digest it with extracellular enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction.. Carbon is released as carbon dioxide and can be fixed again. It is recycled, not destroyed.
Order decomposition and carbon recycling
Saprobionts recycle the carbon in dead organisms. Order the chain from death to carbon dioxide in the air.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
An organism dies. Put the steps in order, until its carbon returns to the atmosphere.
0 of 6 in the right place
A detritivore is not the microorganism in 6.16; it only speeds things up. Decomposition can happen without detritivores, just more slowly.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
How do fungi and bacteria obtain food from a dead leaf?
Show suggested answer
They secrete extracellular enzymes that hydrolyse the polymers. Soluble products are then absorbed.
Practice question
Retrieval and explanation · suggested answer
How does this process recycle carbon?
Show suggested answer
Decomposers respire the absorbed products and release carbon dioxide, which producers can use in photosynthesis.
Practice question
Retrieval and explanation · suggested answer
Why is an earthworm not a sufficient answer to 6.16?
Show suggested answer
An earthworm is a detritivore. The specification point is microorganisms, which digest the matter chemically and release CO₂.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A waterlogged peat soil stores a large mass of dead plant carbon. Temperature is low and oxygen is scarce. Explain the storage using 6.16. What would you expect if the peat is drained and warmed? Assume the same plant material is still there.
Show a hint
The microbes need workable conditions.
Explore the reasoning
Low temperature and little oxygen slow aerobic bacteria and fungi, so decomposition and CO₂ release stay slow and carbon remains in the peat. Draining and warming would let those microorganisms work faster, so more of that carbon would be respired as CO₂. The assumption is that the change is in conditions, not in a new type of plant.
Recall this lesson
Lesson recall questions
Question 1 of 5: How do decomposers recycle carbon?
Microbes, decay and carbon
Practice question
Retrieval and explanation · suggested answer
How do decomposers recycle carbon?
Lesson: 6C 1: Microorganisms and Decomposition
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
Compost is managed decomposition
6C · Microorganisms and DecompositionThe US EPA describes composting as microorganisms breaking down organic materials. That is 6.16 in a bin. The page does not mention carbon dioxide, forensic timelines, or a…Specification 6.16Read & use the scienceClose article
Why this matters
The US EPA describes composting as microorganisms breaking down organic materials. That is 6.16 in a bin. The page does not mention carbon dioxide, forensic timelines, or a nitrogen cycle. Use it only as a familiar place where the same microbes work. The carbon-dioxide step is the specification link to respiration and photosynthesis. Oxygen availability changes the products and rate of decomposition. A landfill often develops anaerobic conditions and can release methane, while a managed compost pile is turned to maintain oxygen.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.16 know the role of microorganisms in the decomposition of organic matter and the recycling of carbon
Use the science
Why is “the EPA says compost gives off a measured mass of CO₂” an over-reading?
Explore a possible response
The page says microorganisms break the material down. It does not give a carbon budget. The release of CO₂ is your biology from respiration of those microbes, not a figure copied from that page.
Explore the source
- US EPA: composting at homeU.S. Environmental Protection Agency
Compost versus landfill: the oxygen test
6C · Microorganisms and DecompositionThe US Environmental Protection Agency describes composting as managed aerobic decomposition. Microorganisms break organic matter down and return nutrients to soil; their respiration returns carbon dioxide to the…Specification 6.16Read & use the scienceClose article
Why this matters
The US Environmental Protection Agency describes composting as managed aerobic decomposition. Microorganisms break organic matter down and return nutrients to soil; their respiration returns carbon dioxide to the atmosphere. In a landfill, buried food waste may become anaerobic and produce methane instead. This is why an advert saying “biodegradable” does not tell you what will happen to a product in every disposal setting. Decomposition depends on organisms and conditions, not a magic property of the label. The Unit 4 carbon-cycle link is the microbial conversion of organic carbon into gases and other compounds, with oxygen availability shaping the pathway.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.16 know the role of microorganisms in the decomposition of organic matter and the recycling of carbon
Use the science
Why could the same food scrap produce a different greenhouse-gas mixture in a turned compost heap and a sealed landfill?
Explore a possible response
Oxygen supports aerobic decomposition in compost; anaerobic microbes can generate methane in oxygen-poor landfill material.
Explore the source
- CompostingUS Environmental Protection Agency