UPSC Mains 2022 GS Paper 3 Q6: model answer
Each year a large amount of plant material, cellulose, is deposited on the surface of Planet Earth. What are the natural processes this cellulose undergoes before yielding carbon dioxide, water, and other end products?
What the question wants
Explain the natural processes cellulose goes through before turning into carbon dioxide, water and other end products.
How to structure it
Process step by step.
- What cellulose is
- Breakdown by microbes
- Aerobic route
- Anaerobic route
- Other routes and significance
How to think it through
Cellulose: polymer of beta-glucose, most abundant organic compound. Decomposition: fragmentation by detritivores (earthworms, termites); enzymatic hydrolysis by cellulases from fungi (Trichoderma) and bacteria into cellobiose, glucose; aerobic respiration: glucose → CO2 + H2O + energy; anaerobic: fermentation → organic acids, alcohols, methane (methanogens) in wetlands, rumen; humification into soil organic matter; fire combustion. Significance: carbon cycle, soil fertility; biofuels.
Model answer
Every autumn, forests drop billions of tonnes of leaves. Most of it is cellulose, the most abundant organic compound on Earth. Within a few years, nearly all of it has vanished back into the air and soil.
What cellulose is
- A long chain of glucose units in plant cell walls; very tough to break.
The natural processes
- Fragmentation: earthworms, termites and insects chew litter into small pieces.
- Enzyme action: fungi such as *Trichoderma* and bacteria release cellulases that cut cellulose into cellobiose and then glucose.
- Aerobic decomposition: microbes respire glucose with oxygen into carbon dioxide, water and energy.
- Anaerobic decomposition in wetlands, paddy fields and animal guts: fermentation gives organic acids and alcohols, then methane from methanogens.
- Humification: part becomes stable humus, storing carbon in soil.
- Lignin around cellulose slows decay, so wood rots more slowly than leaves.
- Fire burns it directly to CO2.
Why it matters
- Returns carbon to the carbon cycle and nutrients to soil.
- Basis of 2G ethanol and biogas.
- Methane from paddy fields and cattle is a strong greenhouse gas.
Rotting leaves are one of nature's largest recycling plants.
Draw: A flow: leaf litter → fragments → glucose → CO2 + H2O (or CH4), with humus branching off.
Keywords, cases and data to drop in
cellulose, beta-glucose, detritivores, cellulase, Trichoderma, aerobic respiration, fermentation, methanogens, humus, carbon cycle, 2G ethanol.
Where most answers lose marks
- Writing about plastics or deforestation; this is a biology question.
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Question text as printed in the official UPSC Civil Services (Main) question paper. The model answer is original Battle IAS work; use it to learn structure, not to memorise.
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