Each year a large amount of plant material — cellulose — is deposited on Earth’s surface. What natural processes does this cellulose undergo before yielding carbon dioxide, water, and other end products?
Subtopic: Ecosystems · Decomposition & nutrient cycling
How to structure your answer
Introduction: Name cellulose as the main structural carbohydrate of plant litter and frame decomposition as the route back to CO₂ and water.
Body: Three sequential processes — leaching, fragmentation/comminution, and microbial catabolism (enzymatic hydrolysis + mineralisation, with humification of the residue).
Conclusion: Closes the carbon cycle and builds soil.
Written within the word limit
145 words · target 150 words · 9 min
Introduction: Cellulose, a long polymer of glucose units, is the chief structural material of fallen leaves, stems and wood. As litter on the ground it is broken down step by step by decomposers until it finally returns to carbon dioxide, water and stable humus.
Body:
- Leaching: Rain and percolating soil water first dissolve and wash out the soluble sugars and minerals from the litter.
- Fragmentation (comminution): Detritivores — earthworms, termites and mites — shred the litter into finer particles, exposing far more surface area for microbial attack.
- Catabolism: Fungi, bacteria and actinomycetes secrete cellulase enzymes that hydrolyse cellulose into glucose. Aerobic respiration then mineralises this glucose into carbon dioxide and water, while the more resistant residues are converted into dark, stable humus through humification.
Conclusion: These linked processes recycle carbon and nutrients back to the atmosphere and soil, closing the carbon cycle and sustaining long-term soil fertility.
What an examiner expects to see
- Cellulose = β-1,4 linked glucose polymer, main structural carbohydrate of plants
- Leaching: water dissolves and removes soluble sugars/minerals first
- Fragmentation/comminution by detritivores (earthworms, termites, mites) increases surface area
- Decomposers: fungi (acidic conditions), bacteria & actinomycetes (neutral/alkaline)
- Cellulase enzymes hydrolyse cellulose to glucose
- Mineralisation: aerobic respiration converts glucose to CO₂ and water
- Humification: resistant residues form stable dark humus
- Net result recycles carbon and nutrients, building soil fertility
Concrete cases, schemes and judgments
- Earthworms and termites as litter fragmenters
- Cellulolytic fungi (e.g. Trichoderma) and actinomycetes
- Forest-floor leaf litter decomposition
- Humus formation in topsoil