CBSE Class 12 Biology Study Module – Organisms and Populations (NCERT Based)

Unit V: Ecology and Environment | Chapter 13: Organisms and Populations
For CBSE Class 12 Board Examinations (Latest NCERT Syllabus)

CBSE Class 12 Biology Chapter 13 – Organisms and Populations | NCERT Ecology Revision Notes

Course & Examination Details

• Course: CBSE Class 12 Biology
• Unit: Unit V – Ecology and Environment
• Chapter: Chapter 13 – Organisms and Populations
• Board: Central Board of Secondary Education (CBSE)
• Syllabus Base: Strictly as per NCERT Textbook
• Exam Relevance: CBSE Class 12 Board Examinations, NEET (Foundation Level), School Assessments

Study Module with Revision Notes

(Designed for conceptual clarity, answer-writing practice, and exam-oriented preparation)

Introduction to Ecology: Organisms and Populations

Ecology is the branch of biology that deals with the interactions among organisms and between organisms and their environment. Chapter 13, Organisms and Populations, focuses on how living beings survive, grow, reproduce, and interact with both abiotic (non-living) and biotic (living) components of the ecosystem. This chapter forms the ecological foundation for understanding higher-level topics such as communities, ecosystems, biodiversity, and environmental issues.

The study of organisms cannot be separated from their environment because environmental factors strongly influence survival and distribution. Similarly, populations exhibit specific characteristics that help ecologists understand population dynamics, growth patterns, and interactions.

1. Organism and Environment

Every organism lives in a specific environment that provides the necessary conditions for survival. The environment includes all external factors—physical, chemical, and biological—that influence organisms.

Key Concepts

• Organisms are not independent entities; they are continuously interacting with their surroundings.
• Environmental factors determine the distribution, abundance, and survival of organisms.
• The relationship between organisms and the environment is dynamic and reciprocal.

Levels of Ecological Organization

• Organism → Population → Community → Ecosystem → Biome → Biosphere

Each level represents increasing complexity of interactions.

2. Major Abiotic Factors

Abiotic factors are non-living components of the environment that affect organisms directly or indirectly.

2.1 Temperature

• Temperature is the most ecologically relevant factor.
• It affects enzyme activity, metabolism, growth, and reproduction.
• Most organisms have an optimum temperature range.

Categories based on temperature tolerance:

• Eurythermal: Organisms that tolerate a wide range of temperatures
• Stenothermal: Organisms that tolerate a narrow range of temperatures

2.2 Water

• Life originated in water, making it a critical abiotic factor.
• Availability of water determines the distribution of organisms.

Based on water requirements:

• Hydrophytes: Plants living in water
• Mesophytes: Plants requiring moderate water
• Xerophytes: Plants adapted to dry conditions

Salinity also influences organisms, especially aquatic species.

2.3 Light

• Light is essential for photosynthesis.
• Duration, intensity, and quality of light influence plant growth and flowering.
• Aquatic ecosystems show light stratification with depth.

2.4 Soil

• Soil composition, pH, mineral content, and texture affect vegetation.
• Soil indirectly affects animals by influencing plant distribution.

3. Responses to Abiotic Factors

Organisms respond differently to environmental stress caused by abiotic factors.

3.1 Regulation

Some organisms maintain a constant internal environment (homeostasis) despite external changes.

• Seen in birds and mammals.
• Energetically expensive process.

3.2 Conformation

• Internal conditions change with external environment.
• Common in plants, fishes, and reptiles.

3.3 Migration

• Temporary movement of organisms to favorable conditions.
• Common in birds, fishes, and some mammals.

3.4 Suspension

• Organisms survive unfavorable conditions by reducing metabolic activity.
• Includes:
  • Hibernation (winter sleep)
  • Aestivation (summer sleep)
  • Diapause in insects

4. Adaptations

Adaptations are morphological, physiological, or behavioral traits that enable organisms to survive and reproduce in specific environments.

4.1 Adaptations in Desert Organisms

• Thick cuticle and reduced leaves in plants
• CAM pathway in desert plants
• Concentrated urine in animals to conserve water

4.2 Adaptations in Aquatic Organisms

• Streamlined bodies in fishes
• Presence of gills
• Floating leaves in aquatic plants

4.3 Adaptations at High Altitudes

• Increased red blood cell count
• Higher hemoglobin affinity for oxygen
• Compact body size in animals

5. Population Attributes

A population is a group of individuals of the same species living in a defined area at a given time.

5.1 Population Size (Density)

Population density can be measured by:

• Number
• Biomass
• Percent cover (in plants)

5.2 Birth Rate (Natality)

• Number of individuals added per unit population per unit time.

5.3 Death Rate (Mortality)

• Number of individuals dying per unit population per unit time.

5.4 Age Distribution

• Pre-reproductive
• Reproductive
• Post-reproductive

Age pyramids indicate population growth status:

• Expanding
• Stable
• Declining

5.5 Sex Ratio

• Ratio of males to females in a population.
• Influences reproductive potential.

6. Population Growth

Population growth depends on the balance between natality, mortality, immigration, and emigration.

6.1 Exponential Growth

Occurs when resources are unlimited.

Equation:
dN/dt = rN

• Produces J-shaped curve
• Rare in nature

6.2 Logistic Growth

Occurs when resources are limited.

Equation:
dN/dt = rN (K − N)/K

• Produces S-shaped (sigmoid) curve
• K = carrying capacity

6.3 Carrying Capacity

• Maximum population size an environment can sustain.
• Depends on resource availability.

7. Population Interactions

Populations interact with one another in various ways that affect survival and reproduction.

7.1 Mutualism

• Both species benefit.
• Example: Pollination by insects

7.2 Competition

• Both species are harmed.
• Occurs when resources are limited.

7.3 Predation

• One species (predator) benefits, the other (prey) is harmed.
• Helps maintain population balance.

7.4 Parasitism

• Parasite benefits, host is harmed.
• Parasites show high reproductive potential.

7.5 Commensalism

• One species benefits, the other is unaffected.
• Example: Epiphytes on trees

7.6 Amensalism

• One species is harmed, the other remains unaffected.

Revision Highlights (Quick Notes for Exams)

• Ecology studies interactions between organisms and environment.
• Temperature and water are the most important abiotic factors.
• Organisms show regulation, conformation, migration, or suspension.
• Adaptations are environment-specific.
• Population attributes apply only to populations, not individuals.
• Logistic growth is most common in nature.
• Population interactions maintain ecosystem stability.

CBSE Exam Focus Tips

• Practice diagrams of population growth curves.
• Learn definitions and examples of population interactions.
• Prepare short notes on adaptations and abiotic factors.
• Use NCERT terminology strictly in answers.

Conclusion

Chapter 13 Organisms and Populations forms the backbone of ecological understanding in Class 12 Biology. Mastery of this chapter helps students grasp how life adapts, survives, and interacts within environmental constraints. These NCERT-aligned revision notes provide a structured, exam-ready approach for scoring well in CBSE board examinations.