Each year a large amount of plant material, cellulose, is deposited on the surface of Planet Earth. What…

Question

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 ?

Ghissu Editorial · Accepted Answer

Cellulose is fragmented by detritivores, hydrolysed into glucose by fungal and bacterial cellulases and respired into carbon dioxide and water, with anaerobic digestion, gut symbiosis and fire as other routes. In wetlands and paddy fields, methanogenic archaea release methane alongside carbon dioxide.

**Introduction**

Cellulose, a chain of glucose units joined by beta-1,4 bonds, is Earth's most abundant organic compound, bound in plant cell walls with hemicellulose and lignin. Since most animals cannot break this bond, microbes return its carbon to the cycle.

**Aerobic Decomposition of Cellulose on Land**

SKETCH: flow | From Leaf Litter to CO2
- Leaf litter
- Fragmentation
- Cellulase hydrolysis
- Glucose
- Aerobic respiration
- CO2, water, energy

1. **Fragmentation**: Rain leaches soluble sugars, and earthworms, termites and millipedes shred litter, enlarging the surface open to microbes.
2. **Enzymatic hydrolysis**: Fungi such as Trichoderma and bacteria such as Cellulomonas secrete cellulases that cut the chains into cellobiose and then glucose.
3. **Mineralisation**: Microbes respire the glucose through glycolysis and the Krebs cycle, releasing carbon dioxide, water and heat, which is why a compost heap warms up.
4. **Humification**: Lignin-shielded remains, slowly attacked by white-rot fungi, become humus that releases carbon over years.

**Other Natural Pathways and Their End Products**

1. **Anaerobic digestion**: In wetlands and paddy fields, Clostridium-type bacteria ferment cellulose into acids and hydrogen, which methanogenic archaea convert into methane and carbon dioxide.
2. **Gut symbiosis**: Rumen microbes in cattle and Trichonympha in termite guts yield fatty acids, carbon dioxide and methane.
3. **Natural fire**: Lightning-set fires oxidise cellulose within minutes into carbon dioxide and water vapour.
4. **Climate control**: Decay is fastest in warm, moist tropics and slowest in cold or waterlogged bogs, where plant matter piles up as peat.

**Conclusion**

These processes close the carbon loop and enrich soils with humus and nutrients. Indian Oil's Panipat 2G ethanol plant (2022) now copies them to turn rice straw into fuel.

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 ?

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