Organisms capable of using CO2 as principal carbon source are called :
- (a)Autotrophs
- (b)Heterotrophs
- (c)Parasites
- (d)Decomposers
Correct — A, Autotrophs. Autotrophs ('self-feeders') build their own organic food from simple inorganic molecules, using carbon dioxide as their principal source of carbon — green plants, algae and cyanobacteria do this by photosynthesis, and some bacteria by chemosynthesis. Because they fix CO2 into organic matter, autotrophs are the primary producers at the base of every food chain.
- (b)Heterotrophs — Heterotrophs ('other-feeders') cannot fix CO2; they get carbon by consuming ready-made organic food from other organisms.
- (c)Parasites — Parasites are a kind of heterotroph that draws nutrients from a living host, not from atmospheric carbon dioxide.
- (d)Decomposers — Decomposers obtain carbon by breaking down dead organic matter, so their carbon comes from organic remains, not from CO2 fixation.
Living things are grouped by how they get carbon and energy. Autotrophs fix inorganic carbon dioxide into organic compounds, whereas heterotrophs depend on organic carbon already made by autotrophs. The clue phrase 'CO2 as principal carbon source' points squarely to autotrophic nutrition.
The trap is that all four options are nutrition-related terms, but only autotrophs make their own food from CO2. Parasites and decomposers are both special heterotrophs, so they are ruled out the moment you notice the carbon source is inorganic CO2.
- Autotrophs fix inorganic CO2 into organic compounds; photoautotrophs use light energy while chemoautotrophs use chemical energy.
- Heterotrophs depend on organic carbon originally made by autotrophs.
- Green plants, algae and cyanobacteria are the main photoautotrophs.
- Autotrophs occupy the first trophic level, as producers.
- Confusing autotrophs with heterotrophs.
- Assuming all bacteria are heterotrophic — many are chemoautotrophs or photoautotrophs.
NDA/UPSC ask you to match a nutrition mode to its definition or to identify producers in a food chain — 'makes food from CO2' always means autotroph.
Consider the following kinds of organisms: 1. Copepods 2. Cyanobacteria 3. Diatoms 4. Foraminifera. Which of the above are primary producers in the food chains of oceans?
- (a) 1 and 2
- (b) 2 and 3
- (c) 3 and 4
- (d) 1 and 4
Answer(b) 2 and 3 — cyanobacteria and diatoms are photosynthetic autotrophs and the primary producers, while copepods and foraminifera are heterotrophic zooplankton.
UPSC prelims 2021 tested the same idea of autotrophic producers — cyanobacteria and diatoms fix carbon by photosynthesis (they are autotrophs), whereas the animal plankton are heterotrophs, reinforcing that producers are the CO2-fixing autotrophs.
- practice — not a real PYQ
Organisms that obtain their food by feeding on other organisms are called
- (a)autotrophs
- (b)heterotrophs
- (c)producers
- (d)photoautotrophs
Answer(b) heterotrophs — they cannot fix CO2 and depend on ready-made organic food.
- practice — not a real PYQ
Which of the following occupies the first trophic level in a food chain?
- (a)Herbivores
- (b)Carnivores
- (c)Green plants
- (d)Decomposers
Answer(c) Green plants — as autotrophic producers they form the base of the food chain.