BPSC 70th CCE Prelims General Studies · Question 142 of 143

Correct — D, Decomposition. Decomposition is defined as the breakdown of complex dead organic matter — the detritus of fallen leaves, dead roots, bark, animal remains and excreta — into simpler inorganic substances by decomposers, which are chiefly bacteria and fungi and are collectively called saprotrophs. That is word for word what the stem describes: organic matter, broken down, by micro-organisms. Ecology treats it as a sequence of five steps rather than a single event. Fragmentation, in which detritivores such as earthworms break the detritus into smaller pieces and enormously increase its surface area; leaching, in which water-soluble nutrients percolate into the soil and are lost from the litter; catabolism, in which bacterial and fungal enzymes degrade the fragments into simpler compounds; humification, which accumulates the dark, colloidal, highly resistant substance called humus that acts as the soil's nutrient reserve; and mineralisation, in which humus is finally broken down to release inorganic nutrients — nitrogen, phosphorus, potassium — back into the soil for plants to take up again. The whole process is oxygen-requiring and its rate is set by temperature, soil moisture and the chemical quality of the litter: warm and moist conditions speed it up, while lignin- and chitin-rich detritus slows it down and nitrogen-rich, sugar-rich material accelerates it. Without this step the carbon and nitrogen cycles would simply stop, because every nutrient locked in dead tissue would stay locked there.

• (a)Photosynthesis — The exact opposite of the process described. Green plants use light energy to build organic matter out of carbon dioxide and water, releasing oxygen — it is a synthesis, not a breakdown, and it is carried out by autotrophs, not by micro-organisms acting on dead material.
• (b)Respiration — Also a breakdown, which is why it tempts, but it happens inside a living organism's own cells, oxidising its own food to release energy as ATP. Decomposition acts on dead matter outside the decomposer's body, through enzymes it secretes. Respiration is a step within decomposition, not a synonym for it.
• (c)Fermentation — A specific anaerobic microbial pathway — yeast turning sugar into ethanol and carbon dioxide, or lactic acid bacteria souring milk into curd. It is one narrow route by which micro-organisms break down one class of substrate in the absence of oxygen, not the general term for breaking down organic matter.

Every ecosystem runs on a one-way flow of energy and a closed cycle of nutrients, and decomposition is what closes the nutrient cycle. Producers fix energy, consumers pass it along the food chain, and decomposers return the matter. The organisms that do it are saprotrophs — fungi, which dominate in acidic, woody and lignin-rich litter, and bacteria, which dominate in nitrogen-rich and wetter substrates — and both work by secreting enzymes onto dead material and absorbing the soluble products, a mode of nutrition called extracellular digestion. Alongside them work detritivores such as earthworms, termites and millipedes, which do not decompose chemically but fragment the litter and so accelerate the work of the microbes; the earthworm's role here is why Darwin called it nature's plough. Rate is everything in practice: in a warm, moist tropical forest litter can disappear within a year, while in cold or waterlogged, oxygen-poor conditions decomposition slows to the point where organic matter accumulates — which is how peat bogs form and how coal seams began.

This is a definition question with three near-relatives as distractors, and the safe method is to test each option against two features of the stem at once: it must be a breakdown, and it must be performed by micro-organisms on organic matter that is not their own body. Photosynthesis fails the first test — it builds rather than breaks. Respiration passes the breakdown test but fails the second, because an organism respires its own food inside its own cells. Fermentation passes both but is too narrow: it names one anaerobic pathway rather than the general process, and the examiner's use of the broad phrase 'organic matter' signals that the broad term is wanted. Only decomposition satisfies everything the stem says. That habit — reading the stem for its scope, not just its keywords — is what separates a candidate who can distinguish a general term from a special case, which is the single commonest way biology definition questions are set.

• Decomposition is the breakdown of dead organic matter, called detritus, into simpler inorganic substances by decomposers — bacteria and fungi, collectively saprotrophs
• Its five steps are fragmentation, leaching, catabolism, humification and mineralisation
• Humus is the dark, colloidal, highly resistant product of humification and acts as the soil's reserve of nutrients; mineralisation finally releases them as inorganic ions
• Decomposition is an oxygen-requiring process; it is accelerated by warmth and moisture and slowed by lignin- and chitin-rich litter, while nitrogen-rich detritus decomposes faster
• Detritivores such as earthworms, termites and millipedes fragment litter and speed up microbial action, but the chemical breakdown itself is done by bacteria and fungi
• Respiration is the intracellular oxidation of an organism's own food; fermentation is a specific anaerobic pathway such as yeast converting sugar to ethanol and carbon dioxide

• Choosing respiration because it is also a breakdown — it happens inside the living cell on the organism's own food
• Choosing fermentation, which is one anaerobic pathway rather than the general process the stem describes
• Treating detritivores such as earthworms as decomposers; they fragment litter, while bacteria and fungi do the chemical work

BPSC asks school-level biology as a one-line definition with three closely related processes as distractors, so the discipline that pays is being able to state each term in a single precise sentence rather than recognising it vaguely. UPSC takes the same content further into ecology — which kinds of organism count as decomposers, what happens to organic matter in a bio-toilet, why upwelling raises marine productivity — so the concept has to be usable in an applied setting rather than merely named.

• practice — not a real PYQ

  Which of the following is the correct sequence of steps in decomposition ?

  • (a)Catabolism → Fragmentation → Leaching → Humification → Mineralisation
  • (b)Fragmentation → Leaching → Catabolism → Humification → Mineralisation
  • (c)Leaching → Fragmentation → Humification → Catabolism → Mineralisation
  • (d)Fragmentation → Catabolism → Mineralisation → Leaching → Humification

  Answer(b) Fragmentation → Leaching → Catabolism → Humification → Mineralisation — detritus is first broken into small pieces, soluble nutrients wash down, enzymes degrade the rest, humus accumulates and is finally mineralised into inorganic nutrients.

• practice — not a real PYQ

  Decomposition in an ecosystem is slowest under which of the following conditions ?

  • (a)Warm and moist
  • (b)Cold and waterlogged
  • (c)Warm and well-aerated
  • (d)Litter rich in nitrogen and sugars

  Answer(b) Cold and waterlogged — low temperature and a lack of oxygen both suppress microbial activity, which is why organic matter accumulates as peat in such places.