Energy and nutrients flow from one trophic level to another in an ecosystem. Which one of the following statements correctly describes the direction of flow of energy and nutrients in an ecosystem?
- (a)Energy flow is cyclic and nutrient flow is unidirectional.
- (b)Energy flow is unidirectional and nutrient flow is cyclic.
- (c)Both energy and nutrient flow are unidirectional.
- (d)Both energy and nutrient flow are cyclic.
Correct — B, Energy flow is unidirectional and nutrient flow is cyclic. Energy and matter behave differently in an ecosystem, and the question turns on that difference. Energy enters an ecosystem once, as sunlight, is fixed by producers, and then passes to herbivores and on to carnivores. At every transfer most of it is lost as heat in respiration, and heat cannot be recaptured by any organism. NCERT states both halves: 'you find unidirectional flow of energy from the sun to producers and then to consumers', and 'there is unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment'. That is why a food chain has only a few links and why the pyramid of energy is always upright. Nutrients behave the other way. Carbon, nitrogen, phosphorus and the rest are taken up by producers, passed along the chain, and returned to the soil, water and air by excretion and by the decomposers that break down dead matter — from where they are absorbed again. NCERT calls this nutrient cycling, the storage and movement of nutrient elements through the various components of the ecosystem, and notes that nutrients are repeatedly used through this process. So the earth is closed to matter and open to energy, and the answer follows.
- (a)Energy flow is cyclic and nutrient flow is unidirectional. — The two claims are swapped. Energy cannot cycle, because each transfer degrades a large part of it into heat that leaves the ecosystem; nutrients can, and do, because decomposers return the elements in dead matter to the abiotic pool.
- (c)Both energy and nutrient flow are unidirectional. — Right about energy and wrong about nutrients. If nutrients moved only one way an ecosystem would exhaust its supply of nitrogen and phosphorus within a few generations; the biogeochemical cycles are what prevent that.
- (d)Both energy and nutrient flow are cyclic. — Right about nutrients and wrong about energy — and wrong in a way that would breach the laws of thermodynamics. Energy dissipated as heat at each trophic level cannot be gathered up and fed back to the producers, which is why sunlight has to keep arriving.
An ecosystem is powered by an external energy source and stocked from an internal store of matter. Producers capture a small fraction of the incoming solar radiation and turn it into chemical energy; NCERT's figure is that plants capture only two to ten per cent of the photosynthetically active radiation reaching them. That fixed energy then moves up the trophic levels, shrinking sharply at each step as respiration converts much of it to heat. Matter follows the same path but does not stop at the top: decomposers return the elements locked in dead bodies and waste to the soil, water and atmosphere, where producers take them up again. The routes those elements follow are the biogeochemical cycles — carbon, nitrogen, phosphorus, sulphur and water.
The quickest way to settle a question of this shape is to ask what happens to each thing at the end of the chain. Energy ends up as heat radiated away, and no organism eats heat, so the flow can only be one-way and the ecosystem needs a fresh supply of sunlight every day. Nutrient atoms end up in a dead body or in waste, and decomposers put them back into circulation, so the flow closes into a loop. Once those two endings are clear, the option matching 'unidirectional' with energy and 'cyclic' with nutrients is the only one that can stand. This is also the reason food chains are short — so little energy survives each transfer that there is not enough left to support a fifth or sixth level — which is a point CDS has tested separately.
- NCERT: 'you find unidirectional flow of energy from the sun to producers and then to consumers'.
- NCERT: 'There is unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment.'
- NCERT: 'The storage and movement of nutrient elements through the various components of the ecosystem is called nutrient cycling; nutrients are repeatedly used through this process.'
- NCERT: 'Plants capture only 2-10 per cent of the PAR' — the photosynthetically active radiation available to them.
- Because energy is lost as heat at every transfer, the pyramid of energy is always upright and food chains carry only a few trophic levels.
An ecosystem is open to energy and closed to matter, which is exactly what option (b) says.
- Assuming that because matter cycles, energy must cycle too; the second law of thermodynamics forbids it.
- Thinking nutrients run out at the top of the chain. Decomposers return them, which is why the pyramid does not apply to matter the way it applies to energy.
- Confusing the flow of energy with the flow of biomass; biomass can accumulate, but the energy in it is still on a one-way journey.
As a statement item pairing energy and nutrients with 'cyclic' and 'unidirectional', or as a question on the number of trophic levels, the pyramid of energy, or the role of decomposers.
Which one of the following group of organisms forms a food chain ?
- (a) Grass, human and fish
- (b) Grass, goat and human
- (c) Tree, tree cutter and tiger
- (d) Goat, cow and human
Answer(b) Grass, goat and human
Builds the chain that the energy flow travels along. A valid chain needs a producer followed by consumers who actually eat the level below, which is the structure through which energy moves one way and nutrients move round.
CDS_GK_2021_II_Q132021Which of the following limits the number of trophic levels in a food chain?
- (a) Deficient food supply
- (b) Polluted air
- (c) Decrease in the available energy at higher trophic levels
- (d) Parasitic organisms
Answer(c) Decrease in the available energy at higher trophic levels
The consequence of unidirectional energy flow, asked in its own right. Energy that leaves as heat at each transfer is gone for good, so the amount available shrinks up the chain until no further level can be supported.
- practice — not a real PYQ
The number of trophic levels in a food chain is limited mainly by
- (a)the number of decomposers present
- (b)the loss of energy as heat at each transfer
- (c)the recycling of nutrients by decomposers
- (d)competition among producers
Answer(b) the loss of energy as heat at each transfer — so little of the energy taken in at one level is passed on to the next that after a few links there is not enough left to support another.
- practice — not a real PYQ
Which one of the following organisms returns nutrients locked in dead organic matter to the abiotic environment?
- (a)Producers
- (b)Primary consumers
- (c)Secondary consumers
- (d)Decomposers
Answer(d) Decomposers — by breaking down dead bodies and waste they release the elements back into the soil, water and air, closing the nutrient cycle.