The process of breaking down organic matter by micro organisms is called

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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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2024 Q142 — Environment & Ecology PYQ with Answer

State PCS 2024 Q142 — Environment & Ecology PYQ with Answer