NDA (II) 2023 General Ability Test · Question 60 of 146

Correct — A, 1%. NCERT Class 10 Science puts the figure in almost the same words as the stem — the green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and convert it into food energy. The number is small because most of the sunlight landing on a leaf never becomes chemical energy at all. Part of it is reflected off the leaf surface, part passes straight through, a large part lies in wavelengths that chlorophyll cannot absorb, and much of what is absorbed ends up as heat or drives the evaporation of water rather than the building of carbohydrate. What survives all of that is the roughly one per cent that becomes food energy and enters the ecosystem.

• (b)5% — Five times the textbook figure. It has no standing in the ecosystem chapter, and the value the syllabus fixes for the entry of solar energy into a terrestrial food chain is about one per cent, not five.
• (c)8% — Another inflated value with no textbook basis. It sits close enough to ten per cent to catch a candidate who half-remembers the trophic transfer number and rounds it downwards.
• (d)10% — This is the deliberate trap. Ten per cent is the average share of the food eaten at one trophic level that becomes the consumer's own body and is passed on to the next level — the ten per cent law. It describes transfer between levels, not the capture of sunlight by producers, and NCERT states both numbers on the same page.

Energy enters an ecosystem in one place only, through green plants capturing sunlight in photosynthesis, and then flows one way along the food chain, shrinking at every step. Two numbers describe the flow. About 1% of the sunlight falling on leaves is captured by producers, and about 10% of what is eaten at any level passes on to the next. Nutrients cycle, but energy does not — it leaves as heat.

Almost every wrong answer here comes from swapping the entry figure with the transfer figure. Keep them apart by asking what the percentage is being taken of — a share of incoming sunlight is the one per cent, a share of food already eaten is the ten per cent. One honest caveat worth carrying forward — senior-school ecology measures photosynthetic efficiency against only the photosynthetically active part of the radiation and so quotes a higher percentage, but the NDA key follows the Class 10 statement, which is about the sunlight falling on the leaves.

• NCERT Class 10 Science, Our Environment — green plants in a terrestrial ecosystem capture about 1% of the energy of sunlight that falls on their leaves and convert it into food energy.
• An average of 10% of the food eaten at one trophic level is turned into the consumer's own body and made available to the next level.
• Because so little energy is available at each successive level, food chains generally consist of only three or four steps.
• There are generally more individuals at the lower trophic levels of an ecosystem, and the greatest number is of the producers.

• Reading the ten per cent transfer between trophic levels as the share of sunlight captured.
• Assuming a leaf uses most of the light that lands on it.
• Mixing up energy flow, which is one-way and lossy, with nutrient cycling, which is circular.

NDA asks for one of the two figures, or for the reason food chains are short — fix about one per cent for the sunlight captured by producers and about ten per cent for the transfer at each trophic step.

• practice — not a real PYQ

  In a food chain, the average share of the energy available at one trophic level that is passed on to the next level is about

  • (a)1%
  • (b)10%
  • (c)50%
  • (d)90%

  Answer(b) 10% — an average of ten per cent of the food eaten becomes the consumer's own body and is available to the level above it.

• practice — not a real PYQ

  Food chains in an ecosystem generally consist of only three or four steps because

  • (a)there are very few species in most ecosystems
  • (b)so little usable energy remains after each transfer
  • (c)decomposers bring every food chain to an end
  • (d)producers are always fewer in number than consumers

  Answer(b) so little usable energy remains after each transfer — the loss at each step is so great that very little is left beyond four trophic levels.