NCERT Solutions for Class 12 Biology Chapter 11 Organisms and Populations
NCERT Solutions for Class 12 Biology Chapter 11 Organisms and Populations covers Abiotic Factors, Responses to Abiotic Factors, Adaptations, Population Attributes, Age Pyramids, Population Growth Models (Exponential and Logistic), and Population Interactions.
Overview of Topics & Subtopics (Click to Jump)
1. Organisms and Their Environment & Adaptations
Short Note:
• Abiotic Factors: Key environmental parameters include Temperature, Water, Light, and Soil.
• Responses to Abiotic Stress:
1. Regulate: Maintaining internal homeostasis (e.g., mammals, birds).
2. Conform: Internal environment changes according to external ambient conditions (e.g., 99% animals, plants).
3. Migrate: Moving temporarily away to a hospitable area (e.g., migratory birds in Keoladeo National Park, Bharatpur).
4. Suspend: Development/metabolism is suspended (e.g., Hibernation in bears, Aestivation in snails, Diapause in zooplankton).
• Allen's Rule: Mammals from colder climates generally have shorter ears and limbs to minimize heat loss.
2. Populations & Growth Models
Short Note:
• Population Attributes: Birth rate (Natality), Death rate (Mortality), Sex ratio, and Age distribution (represented by Age Pyramids: Expanding, Stable, Declining).
• Exponential Growth (J-shaped Curve): Occurs when resources (food and space) are unlimited. Formula: dN/dt = rN (where r is intrinsic rate of natural increase).
• Logistic Growth (S-shaped / Sigmoid Curve): Occurs when resources are limited, reaching Carrying Capacity (K). Formula: dN/dt = rN [(K - N) / K] (Verhulst-Pearl Logistic Growth).
3. Population Interactions
Short Note:
• Mutualism (+/+): Both species benefit (e.g., Lichens [Algae & Fungi], Mycorrhizae, Fig tree & Wasp).
• Competition (-/-): Both species suffer (e.g., Gause's Competitive Exclusion Principle).
• Predation (+/-): One benefits (predator) while the other is harmed (prey).
• Parasitism (+/-): Parasite derives benefit at the expense of host (e.g., Liver fluke, Brood parasitism in Cuckoo/Koel).
• Commensalism (+/0): One benefits while the other is unaffected (e.g., Orchid growing on Mango branch, Barnacles on Whale).
• Amensalism (-/0): One is harmed while the other remains unaffected (e.g., Penicillium producing penicillin inhibiting bacterial growth).
Intext & Conceptual Questions
| Feature | Regulators | Conformers |
|---|---|---|
| Homeostasis | Maintain a constant internal body environment (thermoregulation & osmoregulation) regardless of external conditions. | Cannot maintain constant internal conditions; internal state changes with environmental changes. |
| Energy Requirement | High energy expenditure required to maintain constant internal state. | Low energy expenditure; body temperature fluctuates with ambient temperature. |
| Examples | Mammals, Birds, and very few lower vertebrates. | Nearly 99% of animals, aquatic species, and almost all plants. |
| Interaction Type | Species A | Species B | Common Example |
|---|---|---|---|
| Mutualism | + | + | Lichen (Algae + Fungi), Mycorrhizae |
| Competition | - | - | Abingdon tortoise & goats in Galápagos Islands |
| Predation | + | - | Tiger and Deer, Prickly pear cactus & moth |
| Parasitism | + | - | Cuscuta on hedge plants, Brood parasitism |
| Commensalism | + | 0 | Epiphytic Orchid on Mango branch, Cattle Egret & Cattle |
| Amensalism | - | 0 | Penicillium fungi and Bacteria |
Textbook Chapter End Exercise Questions
Allen's Rule: Mammals residing in colder climates generally possess shorter ears, snouts, and extremities (limbs) to minimize body heat loss.
Adaptations in Cold Climate Animals:
1. Thick Blubber Layer: Aquatic mammals such as seals have a thick layer of fat (blubber) below their skin that acts as an insulator to reduce body heat dissipation.
2. Thick Fur / Hair: Polar bears and snow leopards possess dense fur coat to trap an insulating layer of air.
3. Behavioral Huddling: Animals aggregate together in groups to conserve energy and body heat.
Verhulst-Pearl Logistic Growth: Describes a realistic population growth model where natural resources (food and space) are finite. As population density increases, competition intensifies until the population reaches the maximum carrying capacity (K) of the environment, giving a Sigmoid (S-shaped) curve.
Mathematical Equation:
dN / dt = rN [ (K - N) / K ]Where:
• N: Population density at time t
• r: Intrinsic rate of natural increase
• K: Carrying capacity of the habitat
Brood Parasitism: A specialized form of parasitism in birds where the parasitic bird lays its eggs in the nest of a host bird, leaving the host to incubate and rear the parasitic chicks.
Example: The Cuckoo (Koel) lays its eggs in the nest of a Crow. Through evolution, the cuckoo's eggs have evolved to closely resemble the host crow's eggs in size, color, and pattern, preventing the crow from detecting and ejecting the foreign eggs.
Read More & Chapter Resources
These short notes are designed for quick revision. Read the main points once, then use the related material below to practise the same chapter. Revising a concept and checking it with questions can make your preparation more focused.