CBSE Class 12 Biology sample paper 2025-26: question 33(B)
From the official CBSE sample question paper for 2025-26, section E, question 33(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.
Q33(B) · 5 marks · Long answerCompetency-basedEcosystem
A. How does a simple food chain exemplify the First Law of Thermodynamics?
B. The table below shows the number of species in different parts of the world.
| Name of Place | Number of Bird species |
|---|---|
| Columbia | 1400 |
| India | 1200 |
| Northern South America | 1300 |
| New York | 105 |
| Denmark | 504 |
Identify the common factor in regions with a higher number of bird species and suggest at least two reasons for this greater diversity.
Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.
Hints
Open one hint at a time and try again before the next.
Hint 1
Recall what the First Law says about creating or destroying energy, and trace the energy from the sun through the trophic levels.
Hint 2
Look at where Colombia, India and Northern South America lie compared with New York and Denmark.
Hint 3
Think about glaciation history, how seasonal the climate is, and how much solar energy each region receives.
Step-by-step solution
Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.
- 1 mark
A. The First Law of Thermodynamics states that energy can be neither created nor destroyed, only changed from one form to another. In a food chain, producers convert solar energy into chemical energy stored in organic matter through photosynthesis.
- 1 mark
When herbivores and then carnivores eat organisms of the previous level, this chemical energy passes along the chain. Part of it is released as heat during respiration, and on death the remaining organic matter passes to decomposers (detritivores). At every stage energy is only transformed or transferred, never created or destroyed, which illustrates the First Law.
- 1 mark
B. Common factor: Colombia, India and Northern South America, which have the most bird species, all lie close to the equator in the tropics (about 23.5° N to 23.5° S). New York and Denmark are temperate regions at higher latitudes and have far fewer species. Diversity falls as we move from the equator towards the poles (latitudinal gradient).
- 1 mark
Reason 1: Tropical regions have stayed relatively undisturbed for millions of years, while temperate regions suffered frequent glaciations. Since speciation is a function of time, the tropics have had a longer evolutionary time for species to diversify.
- 1 mark
Reason 2: Tropical environments are less seasonal, more constant and predictable, which promotes niche specialisation and greater species diversity. Also acceptable: the tropics receive more solar energy, which raises productivity and may indirectly support greater diversity.
Answer: A. Energy is transformed from solar to chemical energy and to heat along the food chain, never created or destroyed. B. High-diversity regions lie in the tropics, near the equator. Reasons: long undisturbed evolutionary time (no glaciation), less seasonal and more constant environment, and more solar energy leading to higher productivity.
Where the marks are
- States the First Law and the conversion of solar to chemical energy by producers (1 mark)
- Explains transfer through consumers, loss as heat and passage to detritivores, with energy only transformed (1 mark)
- Identifies tropical or near-equatorial location as the common factor (1 mark)
- Any two correct reasons for greater tropical diversity (2 marks)
Revise the chapter
- Chapter: Ecosystem (notes, formulas, where marks are lost and every past question)
- Topic: Energy Flow and Food Chains
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