Do plants themselves turn cellulose into CO2?

Living plants respire their own sugars. Dead cellulose is mainly decomposed by microbes and fire, not by the dead plant.

Is methane an ‘other end product’?

Yes, in anaerobic soils and guts. The question’s CO2 and water are the aerobic terminal pair.

Each year a large amount of plant material, cellulose, is deposited on the

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Plants deposit a large annual mass of cellulose in litter, wood and roots. Most animals cannot digest the β-1,4 polymer without microbial cellulases. Fungi and bacteria hydrolyse cellulose to glucose; soil fauna shred and mix. Aerobic respiration yields carbon dioxide and water; anaerobic guts and wetlands also yield methane. Humus and soil organic carbon are slower residues, not instant end gases.

Model answer

Introduction

Land plants lock carbon in cellulose—the polymer that makes cell walls. Each year forests, grasslands and crops add a huge mass of that material as litter, roots and wood. It does not sit forever. Decomposers and, in part, herbivores and fire break cellulose into sugars and then into carbon dioxide, water and residues such as humus and, in wet soils, methane.

Body

What must be broken

Cellulose is a chain of glucose units joined by β-1,4 links. Most animals cannot hydrolyse it; they need microbes.
Lignin and hemicellulose travel with cellulose in wood; they slow decay and shape which organisms win.

Natural processes before the end products

Mechanical breakdown: soil fauna (earthworms, termites, millipedes) shred litter, raise surface area, and mix it with microbes.
Enzymatic hydrolysis: fungi (especially basidiomycetes and ascomycetes) and bacteria secrete cellulases (endoglucanase, exoglucanase, β-glucosidase). The polymer becomes cellobiose and then glucose.
Aerobic respiration of that glucose by microbes (and by animals that host cellulolytic symbionts, such as ruminants and termites) yields CO2 and H2O and energy.
Anaerobic pathways in waterlogged soils, sediments and guts: fermentation and methanogenesis yield organic acids, CO2 and CH4 rather than full oxidation.
Humification: not all carbon becomes gas in one season. Some becomes humus and soil organic carbon, a slower pool.
Fire and photodegradation in dry biomes oxidise litter partly to CO2 without passing through a gut; they are natural but not the forest-floor default.
Ruminants and termites are ecological processors of cellulose at landscape scale; their symbionts do the chemistry.

End products in order

Immediate: sugars, then microbial biomass.
Terminal under air: CO2 and water.
Terminal under anoxia: CO2, methane, and reduced organics.
Residual: humus, charcoal, and mineral-associated organic matter.

Why it matters for GS-III

This is the carbon cycle on land. Slowing decay (waterlogging, cold, lignin) stores carbon; accelerating it (tillage, drainage) releases CO2.

Flow diagram

flowchart TD
  C[Plant cellulose litter] --> M[Fauna shred]
  C --> F[Fungal bacterial cellulase]
  M --> G[Glucose]
  F --> G
  G --> A[Aerobic CO2 and H2O]
  G --> N[Anaerobic CO2 CH4]
  G --> H[Humus slow pool]

Conclusion

Yearly cellulose is dismantled first by shredders and then by microbial cellulases. Glucose is respired to carbon dioxide and water in air, or fermented to methane in wet systems, with humus as the slow remainder. The end products named in the question are the last step, not the first.

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Next question on this syllabus topic (2022 · Q17). View answer →
Do plants themselves turn cellulose into CO2?

Living plants respire their own sugars. Dead cellulose is mainly decomposed by microbes and fire, not by the dead plant.
Is methane an ‘other end product’?

Yes, in anaerobic soils and guts. The question’s CO2 and water are the aerobic terminal pair.

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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 ?