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CBSE · Class 12 · Biology

Organisms and Populations

Standard difficulty · 20 questions · 40 marks

Name: Date: Score: ____ / 40

Show your working for Sections B and C. Answers are on the last page — try every question first!

Section A — Choose the correct answer8 × 1 = 8 marks
1.
The interaction in which one species benefits and the other is unaffected is:
  1. amensalism
  2. parasitism
  3. mutualism
  4. commensalism
2.
The relationship in the sea anemone and the clown fish is:
  1. commensalism
  2. amensalism
  3. competition
  4. predation
3.
Which adaptation is shown by plants with CAM photosynthesis?
  1. Diapause, a stage of suspended development in unfavourable conditions
  2. A thick cuticle and stomata sunk in deep pits, which reduce water loss
  3. Getting water by oxidising its internal fat, and excreting very concentrated urine
  4. Stomata kept closed during the day
4.
A pond had 160 lotus plants last year, and 2 new plants were added by reproduction this year. The birth rate is:
  1. 0.0125 per lotus per year
  2. 80 per lotus per year
  3. 0.0123 per lotus per year
  4. 0.2 per lotus per year
5.
A field of 20 m2 has 160 Parthenium plants. The population density is:
  1. 8 per m2
  2. 0.125 per m2
  3. 80 per m2
  4. 3,200 per m2
6.
In logistic growth, dN/dt = rN(K − N)/K. If r = 0.5 per year, K = 4,000 and N = 3,000, dN/dt is:
  1. 2,000 per year
  2. 1,500 per year
  3. 500 per year
  4. 375 per year
7.
A population of 210 has 40 births, 17 immigrants, 23 deaths and 9 emigrants in a year. Its size at the end of the year is:
  1. 299
  2. 227
  3. 235
  4. 219
8.
Assertion (A): The relationship in a lichen (a fungus and an alga) is an example of mutualism.
Reason (R): In every interaction between two species, both species are harmed.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is not the correct explanation of A.
  3. A is true, but R is false.
  4. A is false, but R is true.
Section B — Short answer8 × 2 = 16 marks
9.
Distinguish between regulators and conformers.
10.
(a) In a laboratory population of 150 fruit flies, 30 die in a week. Find the death rate. (b) 75 lotus plants gave 6 new plants in a year. Find the birth rate.
11.
A deer population of 710 has 80 births, 60 deaths, 31 immigrants and 7 emigrants in a year. What is its size after one year? Is it growing or shrinking?
12.
A population grows logistically with r = 0.4 per year and carrying capacity K = 2,000. Find dN/dt when N = 1,600, and when N = K. What would dN/dt be at N = 1,600 with unlimited resources?
13.
Name the interaction, and its signs (+, − or 0) for the two species, in (i) the cactus-feeding moth that brought Opuntia under control in Australia and (ii) flamingoes and resident fishes competing for zooplankton in South American lakes.
14.
How are these adapted: (i) bears; (ii) mammals of cold climates (Allen’s rule)?
15.
In a population, 34% are pre-reproductive, 48% reproductive and 18% post-reproductive. What shape will its age pyramid have, and is the population expanding, stable or declining?
16.
Five 1 m2 quadrats in a 500 m2 meadow contain 10, 10, 2, 11, 5 plants of one species. Estimate the density and the total population.
Section C — Word problems4 × 4 = 16 marks
17.
Describe the ways organisms respond to stressful abiotic conditions.
18.
Compare exponential and logistic growth, with their equations and curves. For a population with r = 0.5 per year and K = 1,000, find the growth rate at N = 200 under each model. Which is more realistic, and why?
19.
A population of 760 rabbits has 23 births, 32 deaths, 39 immigrants and 30 emigrants in a year. Find (a) the birth rate, (b) the death rate, (c) the population after a year. Name two other attributes of a population that an individual does not have.
20.
Explain commensalism, predation, mutualism with their signs for the two species, using these examples: an orchid growing on a mango branch; the cactus-feeding moth that brought Opuntia under control in Australia; the Mediterranean orchid Ophrys and the bee that pollinates it.
AmbifulWorksheets
Practice that never repeats
CBSE · Class 12 · Biology

Answer key

Organisms and Populations · AW-12B-OAP-2026-V1 · For parents — fold back or tear off before handing the sheet to your child.

QAnswerWorking
1(d) commensalism—
2(a) commensalismsigns: + and 0
3(d) Stomata kept closed during the day—
4(a) 0.0125 per lotus per year2 ÷ 160
5(a) 8 per m2160 ÷ 20
6(d) 375 per year0.5 × 3,000 × 1,000/4,000
7(c) 235Nₜ+1 = Nₜ + (B + I) − (D + E)
8(c) A is true, but R is false.A is true: it is mutualism (+ and +). R is false.
9Regulators maintain homeostasis by physiological and behavioural means, as birds and mammals keep a constant body temperature. Conformers cannot; their body temperature or osmotic concentration changes with the surroundings.—
10(a) 0.2 per fly per week; (b) 0.08 per lotus per year30 ÷ 150; 6 ÷ 75
11754; growingNₜ+1 = Nₜ + (B + I) − (D + E)
12128 per year; 0 per year; 640 per year (exponential, dN/dt = rN)dN/dt = rN(K − N)/K
13(i) predation, + and −; (ii) competition, − and −—
14(i) Hibernation, to escape the winter cold; (ii) shorter ears and limbs, which reduce heat loss—
15Bell-shaped; stablethe young about equal the reproductive group
167.6 per m2; about 3,800 plantsmean = 38 ÷ 5; total = mean × 500
17Regulate: maintain homeostasis physiologically, as mammals keep body temperature constant by sweating or shivering. Conform: let the body change with the surroundings, as most animals and plants do. Migrate: move temporarily to a better area, as Siberian birds come to Bharatpur in winter. Suspend: bacteria, fungi and lower plants form thick-walled spores; seeds become dormant; bears hibernate; snails and fish aestivate; zooplankton enter diapause.—
18Exponential: dN/dt = rN, with unlimited resources; the J-shaped curve. Logistic: dN/dt = rN(K − N)/K; growth slows as N approaches the carrying capacity K; the sigmoid (S-shaped) curve. At N = 200: exponential 100 per year, logistic 80 per year. Logistic is more realistic because resources are always limited—
19(a) 0.0303 per rabbit per year; (b) 0.0421 per rabbit per year; (c) 760. Other attributes: sex ratio and age distribution (age pyramid), also population density23 ÷ 760; 32 ÷ 760; 760 + (23 + 39) − (32 + 30)
20Commensalism (+ and 0): an orchid growing on a mango branch. Predation (+ and −): the cactus-feeding moth that brought Opuntia under control in Australia. Mutualism (+ and +): the Mediterranean orchid Ophrys and the bee that pollinates it. In mutualism both gain; in commensalism one gains and the other is unaffected; in parasitism and predation one gains at the other’s cost; in competition both lose—