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Organisms and Populations – Class 12 Biology notes, important questions and MCQs

NCERT Class 12 Biology, chapter 11. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Organisms adapt to abiotic factors like temperature, water, light and soil. Populations have attributes such as density, birth and death rates and growth patterns, and interact with other species.

Organism and environment

Regulators maintain homeostasis (mammals); conformers do not (most fish); some migrate or suspend (hibernation, aestivation, diapause).
Allen's rule: mammals in colder areas have shorter ears and limbs.
Adaptations: CAM in desert plants, kangaroo rat makes metabolic water.

Populations and interactions

Exponential growth dN/dt = rN (J-shaped); logistic growth dN/dt = rN(K − N)/K (S-shaped), K = carrying capacity.
Interactions: mutualism (+/+), competition (−/−), predation and parasitism (+/−), commensalism (+/0), amensalism (−/0).
Gause's competitive exclusion principle; brood parasitism (cuckoo lays eggs in crow's nest).

Key terms

Carrying capacity: maximum population an environment can support
Commensalism: one species benefits, the other is unaffected
Predation: interaction in which one species (predator) kills and eats another (prey); +/− interaction

Formulas

dN/dt = rN(K − N)/K

Important questions with answers

Why do mammals in cold regions have shorter ears and limbs?2 marks

Shorter ears and limbs reduce the surface area through which body heat is lost (Allen's rule).

Explain three adaptations of desert plants.3 marks

Thick cuticle and sunken stomata reduce water loss; leaves reduced to spines with photosynthesis by flattened green stems (Opuntia); CAM pathway keeps stomata closed during the day and open at night.

Compare exponential and logistic growth with their equations and curves, and explain which is more realistic.5 marks

Exponential growth: when resources are unlimited, dN/dt = rN; the population grows faster and faster, giving a J-shaped curve (Nₜ = N₀e^(rt)). Logistic growth: when resources are limited, dN/dt = rN(K − N)/K; growth is slow at first, fastest at K/2 and then slows down as N approaches the carrying capacity K, giving an S-shaped (sigmoid) curve. Logistic growth is more realistic because no environment has unlimited resources; competition for food and space eventually limits every population.

Quick revision

Logistic growth is S-shaped.
Lichen = alga + fungus (mutualism).
Orchid on mango tree is commensalism.

Exam tips

Age pyramid: broad base = growing, bell = stable, urn = declining.
For population growth questions, remember Nₜ = N₀e^(rt) for exponential growth and that r = birth rate − death rate.

Practice MCQs

Lichen is an example of
1. mutualism
2. parasitism
3. predation
4. competition
Show answer
Answer: (a) mutualism. Alga + fungus.
S-shaped growth curve is called
1. exponential
2. declining
3. linear
4. logistic
Show answer
Answer: (d) logistic. Logistic.
An orchid growing on a mango branch shows
1. commensalism
2. parasitism
3. mutualism
4. amensalism
Show answer
Answer: (a) commensalism. Orchid benefits, tree unaffected.
Cuckoo laying eggs in a crow's nest is
1. brood parasitism
2. mutualism
3. competition
4. predation
Show answer
Answer: (a) brood parasitism. Brood parasitism.
The interaction between a bee and a flower is
1. mutualism
2. parasitism
3. competition
4. amensalism
Show answer
Answer: (a) mutualism. Both benefit.
In dN/dt = rN(K − N)/K, growth is maximum when N is
1. K
2. K/2
3. 0
4. 2K
Show answer
Answer: (b) K/2. Peak of rN(1 − N/K).
Competitive exclusion principle was given by
1. Darwin
2. MacArthur
3. Gause
4. Odum
Show answer
Answer: (c) Gause. Gause.
In a pond, there were 20 lotus plants last year; 8 new plants were added through reproduction. The birth rate is
1. 0.2
2. 0.4
3. 0.8
4. 8
Show answer
Answer: (b) 0.4. 8/20 offspring per plant per year.

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Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.