Part 4 – Decomposition: Processes & Detritus Food Chain
Part 4 – Decomposition: Processes & Detritus Food Chain
(25 MCQs with answers and detailed explanations, NCERT Class 12 + NEET UG standard)
Q1.
Decomposition is the process of:
a) Conversion of inorganic matter into organic matter
b) Breakdown of complex organic matter into simpler inorganic substances
c) Conversion of light energy into chemical energy
d) Formation of humus from soil
Answer: b) Breakdown of complex organic matter into simpler inorganic substances
Explanation:
- (a) Incorrect → That’s synthesis.
- (b) Correct → Decomposers mineralize organic matter.
- (c) Photosynthesis.
- (d) Humification is a part of decomposition, not whole.
Q2.
Which of the following is NOT a step in decomposition?
a) Fragmentation
b) Humification
c) Mineralisation
d) Nitrogen fixation
Answer: d) Nitrogen fixation
Explanation:
- (a–c) → All are steps of decomposition.
- (d) → Nitrogen fixation is a nutrient cycle process, not decomposition.
Q3.
The first step of decomposition is:
a) Humification
b) Fragmentation
c) Catabolism
d) Mineralisation
Answer: b) Fragmentation
Explanation:
Fragmentation by detritivores (earthworm, etc.) initiates decomposition.
Q4.
Which organisms initiate decomposition by breaking litter into smaller particles?
a) Bacteria
b) Fungi
c) Detritivores
d) Algae
Answer: c) Detritivores
Explanation:
- Earthworms, termites, etc. break litter → fragmentation.
Q5.
Humus is:
a) Highly resistant, dark, amorphous organic matter in soil
b) Easily decomposable organic matter
c) Inorganic salts
d) Mineral nutrients
Answer: a) Highly resistant, dark, amorphous organic matter in soil
Explanation:
Humus is stable, resistant to microbial attack, and improves soil fertility.
Q6.
Mineralisation in decomposition refers to:
a) Release of inorganic nutrients from organic matter
b) Formation of humus
c) Conversion of inorganic into organic
d) Loss of energy as heat
Answer: a) Release of inorganic nutrients from organic matter
Explanation:
Mineralisation releases nutrients like N, P, S, etc.
Q7.
Which factor does NOT directly affect decomposition?
a) Temperature
b) Moisture
c) Oxygen availability
d) Light intensity
Answer: d) Light intensity
Explanation:
Light affects producers, not decomposers.
Q8.
Which ecosystem has faster decomposition?
a) Tropical rainforest
b) Desert
c) Tundra
d) Polar region
Answer: a) Tropical rainforest
Explanation:
Warm, moist conditions → rapid decomposition.
Q9.
Which ecosystem shows the slowest decomposition rate?
a) Grassland
b) Desert
c) Tundra
d) Forest
Answer: c) Tundra
Explanation:
Cold temperature and permafrost slow decomposition drastically.
Q10.
The detritus food chain begins with:
a) Green plants
b) Dead organic matter
c) Herbivores
d) Carnivores
Answer: b) Dead organic matter
Explanation:
DFC starts with detritus, unlike grazing food chain.
Q11.
Which organisms form the decomposer trophic level?
a) Bacteria and fungi
b) Herbivores
c) Carnivores
d) Algae
Answer: a) Bacteria and fungi
Explanation:
They break down organic matter → recycling.
Q12.
Fragmentation of detritus is done by:
a) Bacteria
b) Detritivores
c) Fungi
d) Protozoa
Answer: b) Detritivores
Explanation:
Earthworms, termites, etc. break into smaller pieces.
Q13.
Which step in decomposition produces water-soluble nutrients?
a) Fragmentation
b) Leaching
c) Catabolism
d) Mineralisation
Answer: b) Leaching
Explanation:
Leaching = removal of soluble nutrients into soil water.
Q14.
Which enzymes are secreted by microbial decomposers to degrade detritus?
a) Extracellular enzymes
b) Intracellular enzymes
c) Digestive enzymes of herbivores
d) None
Answer: a) Extracellular enzymes
Explanation:
Fungi and bacteria secrete extracellular enzymes → catabolism.
Q15.
Humification results in:
a) Formation of humus
b) Release of nutrients
c) Mineralisation directly
d) Respiration
Answer: a) Formation of humus
Explanation:
Stable dark material accumulates in soil.
Q16.
Which of the following decreases the rate of decomposition?
a) Warm temperature
b) Moist soil
c) Anaerobic conditions
d) Presence of decomposers
Answer: c) Anaerobic conditions
Explanation:
Decomposers are mostly aerobic; oxygen shortage slows decomposition.
Q17.
Which nutrient cycle is most dependent on decomposition?
a) Nitrogen cycle
b) Carbon cycle
c) Phosphorus cycle
d) All of the above
Answer: d) All of the above
Explanation:
Decomposition releases nutrients in all cycles.
Q18.
Decomposition releases:
a) CO₂
b) Water
c) Nutrients
d) All of the above
Answer: d) All of the above
Explanation:
Organic matter → CO₂, water, and mineral nutrients.
Q19.
Which factor favors rapid decomposition?
a) Low temperature
b) High moisture & oxygen
c) Low pH
d) Water scarcity
Answer: b) High moisture & oxygen
Explanation:
Warm, moist, oxygen-rich conditions speed decomposition.
Q20.
Decomposition of animal dung and plant litter is carried out by:
a) Autotrophs
b) Bacteria and fungi
c) Herbivores
d) Algae
Answer: b) Bacteria and fungi
Explanation:
Major decomposers of organic waste.
Q21.
Which is the correct sequence in decomposition?
a) Fragmentation → Leaching → Catabolism → Humification → Mineralisation
b) Leaching → Fragmentation → Mineralisation → Catabolism → Humification
c) Catabolism → Fragmentation → Leaching → Mineralisation → Humification
d) Mineralisation → Fragmentation → Humification → Catabolism → Leaching
Answer: a) Fragmentation → Leaching → Catabolism → Humification → Mineralisation
Explanation:
This is the correct stepwise sequence.
Q22.
Decomposers derive their nutrition from:
a) Living organisms
b) Dead organic matter
c) Photosynthesis
d) None
Answer: b) Dead organic matter
Explanation:
Saprotrophic nutrition → on dead remains.
Q23.
The rate of decomposition is slower if litter has:
a) High nitrogen content
b) Low lignin content
c) High lignin and chitin content
d) High moisture
Answer: c) High lignin and chitin content
Explanation:
Hard, complex polymers resist microbial attack.
Q24.
The “earthworm” is called:
a) Nature’s scavenger
b) Farmer’s friend
c) Primary producer
d) Secondary consumer
Answer: b) Farmer’s friend
Explanation:
Earthworms improve soil fertility by detritus feeding and nutrient recycling.
Q25.
Which of the following ecosystems has maximum humus accumulation?
a) Tropical rainforest
b) Grassland
c) Desert
d) Tundra
Answer: d) Tundra
Explanation:
Cold climate slows decomposition → humus builds up.
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