Energy Flow in Ecosystems and Trophic Levels

Attempt the quiz above first, then review every question with its correct answer and a short explanation below.

1. Q1.Out of the total incident solar radiation, what percentage constitutes Photosynthetically Active Radiation (PAR)?

   • A.More than 80%
   • B.Exactly 50%
   • C.Less than 50%✓ Correct Answer
   • D.Between 10% and 20%

   Explanation: According to NCERT Class 12 Biology Chapter 14: 'Of the incident solar radiation, less than 50 per cent of it is photosynthetically active radiation (PAR).'

2. Q2.What percentage of the Photosynthetically Active Radiation (PAR) is actually captured by plants for photosynthesis?

   • A.1% to 2%
   • B.2% to 10%✓ Correct Answer
   • C.15% to 20%
   • D.50% to 60%

   Explanation: NCERT Class 12 Biology states that plants capture only 2-10 per cent of the PAR. This small amount of energy sustains the entire living world.

3. Q3.The flow of energy in an ecosystem is always:

   • A.Bidirectional
   • B.Unidirectional✓ Correct Answer
   • C.Multidirectional
   • D.Cyclic

   Explanation: NCERT Chapter 14 emphasizes that the flow of energy is unidirectional: from the sun to producers, and then to consumers. Energy captured by autotrophs does not revert back to solar input, and energy passed to herbivores does not go back to autotrophs.

4. Q4.Which law of thermodynamics states that ecosystems require a constant input of energy to counteract entropy and maintain order?

   • A.First Law
   • B.Second Law✓ Correct Answer
   • C.Third Law
   • D.Zeroth Law

   Explanation: NCERT Class 12 Biology explains that ecosystems are not exempt from the Second Law of Thermodynamics. They need a constant supply of energy to synthesize molecules and counteract the universal tendency toward increasing disorderliness (entropy).

5. Q5.Which of the following represents the correct order of energy transfer across trophic levels?

   • A.Primary Consumer -> Secondary Consumer -> Producer -> Tertiary Consumer
   • B.Producer -> Primary Consumer -> Secondary Consumer -> Tertiary Consumer✓ Correct Answer
   • C.Producer -> Secondary Consumer -> Primary Consumer -> Tertiary Consumer
   • D.Tertiary Consumer -> Secondary Consumer -> Primary Consumer -> Producer

   Explanation: Energy flows from producers (first trophic level) to primary consumers/herbivores (second trophic level), then to secondary consumers/primary carnivores (third trophic level), and finally to tertiary consumers/secondary carnivores (fourth trophic level).

6. Q6.In aquatic ecosystems, the major channel for energy flow is the:

   • A.Detritus Food Chain (DFC)
   • B.Grazing Food Chain (GFC)✓ Correct Answer
   • C.Auxiliary Food Chain
   • D.Parasitic Food Chain

   Explanation: NCERT Class 12 Biology states: 'In an aquatic ecosystem, GFC (Grazing Food Chain) is the major conduit for energy flow.' In contrast, in terrestrial ecosystems, a much larger fraction of energy flows through the Detritus Food Chain.

7. Q7.The Detritus Food Chain (DFC) begins with:

   • A.Living green plants
   • B.Dead organic matter✓ Correct Answer
   • C.Zooplankton
   • D.Primary consumers

   Explanation: According to NCERT Chapter 14, 'The detritus food chain (DFC) begins with dead organic matter. It is made up of decomposers which are heterotrophic organisms, mainly fungi and bacteria.'

8. Q8.Primary carnivores occupy which trophic level in a grazing food chain?

   • A.Second trophic level
   • B.Third trophic level✓ Correct Answer
   • C.Fourth trophic level
   • D.First trophic level

   Explanation: Producers are T1. Herbivores (primary consumers) are T2. Carnivores that eat herbivores (primary carnivores / secondary consumers) occupy the third trophic level (T3) as per NCERT definitions.

9. Q9.If 20,000 J of energy is available at the producer level in an ecosystem, how much energy will be transferred to the tertiary consumer level?

   • A.2,000 J
   • B.200 J
   • C.20 J✓ Correct Answer
   • D.2 J

   Explanation: Based on Lindeman's 10% law: Producer (20,000 J) -> Primary Consumer (2,000 J) -> Secondary Consumer (200 J) -> Tertiary Consumer (20 J).

10. Q10.Decomposers meet their energy and nutrient requirements by degrading:

    • A.Living plants
    • B.Living animals
    • C.Dead organic matter or detritus✓ Correct Answer
    • D.Inorganic chemicals in soil

    Explanation: NCERT Class 12 Biology states that decomposers (saprotrophs) meet their energy and nutrient requirements by degrading dead organic matter or detritus. They secrete digestive enzymes outside their body to break it down.

11. Q11.Which of the following represents an interconnection between the Grazing Food Chain (GFC) and the Detritus Food Chain (DFC)?

    • A.They never share any common species.
    • B.Some organisms of DFC are prey to GFC animals.✓ Correct Answer
    • C.Energy flows only from DFC to GFC, never vice versa.
    • D.They are completely isolated systems.

    Explanation: NCERT notes that the two food chains are connected. For instance, an earthworm (a decomposer/detritivore in DFC) is preyed upon by a sparrow or crow (members of GFC). This creates a food web.

12. Q12.Organisms that feed on detritus and break it into smaller particles are called:

    • A.Carnivores
    • B.Herbivores
    • C.Detritivores✓ Correct Answer
    • D.Producers

    Explanation: According to NCERT Chapter 14, detritivores (like earthworms) break down detritus into smaller particles. This process is called fragmentation.

13. Q13.Why does the amount of usable energy decrease at each successive trophic level?

    • A.Because top predators use up all the solar energy.
    • B.Because energy is lost as respiratory heat during metabolic activities.✓ Correct Answer
    • C.Because decomposers absorb all the energy.
    • D.Because plants stop photosynthesizing at night.

    Explanation: NCERT Class 12 Biology states that energy decreases at each trophic level because a significant portion is lost as heat during respiration and other metabolic processes at each level, in compliance with the second law of thermodynamics.

14. Q14.In a terrestrial ecosystem, what is the fate of the major portion of energy produced by autotrophs?

    • A.It is consumed by herbivores.
    • B.It goes directly to secondary carnivores.
    • C.It dies and enters the Detritus Food Chain (DFC).✓ Correct Answer
    • D.It evaporates into space as light.

    Explanation: In terrestrial ecosystems, only a small fraction of net primary productivity is eaten by herbivores directly. A much larger fraction of energy flows through the dead organic matter route (detritus food chain).

15. Q15.Which of the following groups of organisms acts as the primary decomposers in terrestrial ecosystems?

    • A.Fungi and Bacteria✓ Correct Answer
    • B.Earthworms and Nematodes
    • C.Algae and Lichens
    • D.Insects and Birds

    Explanation: As per NCERT Class 12 Biology, the primary decomposers are heterotrophic micro-organisms, mainly fungi and bacteria, which secrete enzymes to break down organic debris.

16. Q16.How does energy enter a Detritus Food Chain (DFC)?

    • A.Direct capture of sunlight by decomposers.
    • B.Through dead remains and waste products of other organisms.✓ Correct Answer
    • C.By consumption of living herbivores.
    • D.Through volcanic vents.

    Explanation: Energy enters the DFC through organic waste, exudates, and dead bodies of plants and animals (detritus), which contain stored chemical energy derived originally from photosynthesis.

17. Q17.A food web is different from a food chain because in a food web:

    • A.Energy flow is cyclic.
    • B.Organisms have multiple alternative feeding relationships.✓ Correct Answer
    • C.Producers are not required.
    • D.Decomposers are absent.

    Explanation: NCERT Class 12 Biology explains that in nature, simple linear food chains rarely exist. Organisms are interconnected via multiple feeding preferences, forming a network of food chains called a food web.

18. Q18.Which of the following statements matches the first law of thermodynamics in an ecosystem context?

    • A.Energy can be created by green plants.
    • B.Energy is lost as heat and cannot be transformed.
    • C.Solar energy is transformed into chemical energy by producers.✓ Correct Answer
    • D.Decomposers destroy energy completely.

    Explanation: The first law of thermodynamics states that energy can neither be created nor destroyed, only transformed. In an ecosystem, producers transform solar radiant energy into chemical energy stored in organic matter.

19. Q19.Which of the following parameters decreases from base to apex in all types of ecosystems?

    • A.Number of individuals
    • B.Biomass
    • C.Energy flow rate✓ Correct Answer
    • D.Bioaccumulation of toxins

    Explanation: While numbers and biomass can sometimes have inverted pyramids (e.g., in oceans or single-tree ecosystems), the pyramid of energy (energy flow rate) is strictly upright in all ecosystems. Energy always decreases as you move up.

20. Q20.The percentage of energy that a trophic level passes on to the next is known as:

    • A.Photosynthetic efficiency
    • B.Net productivity
    • C.Ecological efficiency✓ Correct Answer
    • D.Assimilation efficiency

    Explanation: Ecological efficiency is the percentage of energy transferred from one trophic level to the next. In most ecosystems, it is approximately 10% (the 10% law).