CBSE Class 12 Biology – Organisms and Populations
NCERT-aligned chapter resource | Biology Code 044
1. Organism and Its Environment
Organisms interact continuously with abiotic factors such as temperature, water, light and soil, and with biotic factors such as competition, predation and mutualism. Survival depends on the ability to tolerate or regulate environmental variation.
2. Major Abiotic Factors
| Factor | Biological significance |
|---|---|
| Temperature | Affects enzyme activity, metabolism and distribution |
| Water | Essential for biochemical reactions and determines habitat availability |
| Light | Drives photosynthesis and influences behaviour/seasonality |
| Soil | Influences plant distribution through texture, pH, minerals and water availability |
3. Responses to Environmental Conditions
Organisms may regulate internal conditions, conform to the environment, migrate temporarily, or enter dormancy. Examples include thermoregulation in mammals and aestivation in some animals.
4. Population Attributes
Population-level properties include density, birth rate, death rate, age distribution and sex ratio. These cannot be assigned in the same way to an individual organism.
5. Population Growth
Under unlimited resources, exponential growth follows dN/dt=rN. When resources become limiting, logistic growth includes carrying capacity K and follows dN/dt=rN(K−N)/K.
6. Population Interactions
| Interaction | Effect | Example |
|---|---|---|
| Predation | +/− | Predator and prey |
| Competition | −/− | Species sharing a limiting resource |
| Mutualism | +/+ | Mycorrhiza |
| Parasitism | +/− | Parasite and host |
Worked Numerical
If a population increases from 500 to 550 in one time interval, the absolute increase is 50 individuals. If the initial population is used as the reference, the percentage increase is (50/500)×100=10%.
Chapter-Specific MCQs
- Carrying capacity is represented by: (A) K (B) r only (C) N only (D) t. Answer: A.
- Mutualism has the interaction effect: (A) +/+ (B) +/− (C) −/− (D) 0/−. Answer: A.
- Which is a population attribute? (A) Birth rate (B) Individual height only (C) Individual heartbeat only (D) Cell size. Answer: A.
Competency Questions
- Explain why logistic growth is more realistic than unlimited exponential growth for natural populations.
- Interpret a graph showing population growth approaching K.
- Predict how removal of a major predator could affect prey population and resource use.