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 ?
Approach · directive: “what”
What it asks · Trace how plant cellulose breaks down in nature (fragmentation, microbial enzymes, aerobic and anaerobic decay, humus formation) to give carbon dioxide, water and other products.
The question has 2 parts — answer each
- What: the natural processes that break cellulose down — fragmentation, microbial enzymatic hydrolysis and aerobic respiration — before it yields carbon dioxide and water
- What: the other pathways and end products — anaerobic decay, herbivore digestion, humus, fire and fossil carbon
Open with · Cellulose, the most abundant plant polymer, returns to the atmosphere through the carbon cycle, driven mostly by microbes.
Cover
- Fragmentation: earthworms, termites and other detritivores shred litter, exposing more surface to microbes.
- Enzymatic hydrolysis: fungi and bacteria secrete cellulase enzymes that split cellulose chains into cellobiose and then glucose.
- Aerobic decomposition: microbes respire the glucose, releasing carbon dioxide, water and energy.
- Anaerobic decay in waterlogged soils, paddy fields, wetlands and animal guts: fermentation and methanogenesis give organic acids, methane and carbon dioxide.
- Herbivores: ruminants and termites depend on gut microbes to digest cellulose, returning carbon as carbon dioxide and methane.
- Other end products: lignin-rich residues form humus, fire rapidly oxidises litter to carbon dioxide and water, and buried plant matter can turn into peat and coal over time.
Close with · These processes recycle carbon and nutrients and sustain soil fertility, though methane release links them to climate change.
Question: UPSC's CS (Main) 2022, GS Paper III — paper ↗. Approach: Minimalist IAS, checked 30 Sept 2026 (how we verify) — UPSC publishes no model answers. ·
Model answer · 222 words (UPSC limit 150) · Minimalist IAS
Cellulose, a long chain of glucose units and the most abundant plant polymer, returns to the atmosphere through the carbon cycle, and microbes do most of the work.
Breaking down the polymer
- Fragmentation: earthworms, termites and other detritivores shred fallen leaves and wood; the smaller pieces expose far more surface to microbial attack.
- Enzymatic hydrolysis: fungi and bacteria secrete cellulase enzymes that cut the cellulose chains into cellobiose and then into glucose.
From glucose to carbon dioxide and water
- Aerobic respiration: in well-aerated soil and litter, microbes oxidise the glucose, releasing carbon dioxide, water and energy; this is the main route by which plant carbon returns to the air.
Other pathways and end products
- Anaerobic decay: in waterlogged soils, wetlands, paddy fields and animal guts, fermentation and methanogenesis yield organic acids, methane and carbon dioxide.
- Herbivores: ruminants and termites cannot digest cellulose themselves; gut microbes ferment it and the carbon leaves as carbon dioxide and methane.
- Humus: lignin-rich residues resist decay and form dark humus that holds water and nutrients in the soil.
- Fire and burial: fire oxidises litter rapidly to carbon dioxide and water, while plant matter buried away from oxygen can become peat and, over geological time, coal.
These processes close the carbon cycle and sustain soil fertility, though the methane released in wet conditions ties decomposition to climate change.
Written by Minimalist IAS from facts checked at source (how we verify) — a little fuller than exam length, so every part of the question is covered; in the hall, keep the structure and trim the detail. UPSC publishes no model answers: compare your structure and coverage with this, then write your own.