Organisms and Populations – Complete NCERT Notes (Class 12)

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Introduction
Ecology is the study of interactions between organisms and their environment. This chapter focuses on two levels:
Organism level → How individuals respond to environment
Population level → How groups of organisms behave and grow
🌱 1. Organism and Its Environment
Every organism interacts with two types of environment:
🔹 (A) Abiotic Factors
Temperature
Water
Light
Soil
🔹 (B) Biotic Factors
Other organisms (predators, competitors, parasites, etc.)
🌡️ Temperature Effects
Most organisms tolerate a specific temperature range.
Stenothermal → Narrow range (e.g., coral reefs)
Eurythermal → Wide range (e.g., humans)
👉 Temperature affects:
Enzyme activity
Metabolism
Distribution of species
💧 Water Importance
Life originated in water.
Organisms are adapted to:
Aquatic habitats
Terrestrial habitats
👉 Example:
Fish → gills
Plants in desert → thick cuticle, reduced leaves
☀️ Light
Essential for photosynthesis
Affects:
Flowering (photoperiodism)
Animal behavior
👉 Example:
Plants differ in light requirement (shade vs sunlight)
🌍 Soil
Composition varies → affects vegetation
Parameters:
pH
Mineral content
Texture
🧬 2. Responses to Abiotic Factors
Organisms respond in different ways:
🔹 (A) Regulate
Maintain constant body conditions
👉 Example: Humans regulate body temperature
🔹 (B) Conform
Body conditions change with environment
👉 Example: Most animals
🔹 (C) Migrate
Move to favorable environment
👉 Example: Birds
🔹 (D) Suspend (Dormancy)
Reduce metabolic activity
👉 Examples:
Hibernation
Aestivation
🐾 Adaptations
Adaptations are traits that help organisms survive.
Types:
Morphological → Body structure
Physiological → Internal functioning
Behavioral → Actions
👉 Examples:
Polar bear → thick fur
Desert plants → spines instead of leaves
👥 3. Population Attributes
A population has characteristics not seen in individuals.
🔹 Key Attributes:
Birth Rate (Natality)
Death Rate (Mortality)
Sex Ratio
Age Distribution
📊 Age Pyramid
Types:
Expanding → More young individuals
Stable → Equal distribution
Declining → More old individuals
📈 4. Population Growth
🔹 (A) Exponential Growth
Occurs when resources are unlimited
Produces J-shaped curve
🔹 (B) Logistic Growth
Resources become limited
Growth slows near carrying capacity (K)
Produces S-shaped curve
🤝 5. Population Interactions
Species interact in different ways:
Interaction
Effect
Example
Mutualism
+/+
Lichen
Commensalism
+/0
Orchid on tree
Parasitism
+/-
Tapeworm
Predation
+/-
Tiger & deer
Competition
-/-
Plants for nutrients
🐝 Special Interactions
🔹 Mutualism
Both benefit
👉 Example: Algae + Fungus (lichen)
🔹 Parasitism
One benefits, other harmed
👉 Example: Cuscuta plant
🔹 Predation
Maintains ecological balance
Controls population
🧠 Exam Quick Revision
⭐ Important Points:
Organisms adapt to abiotic factors
Population has unique attributes
Growth curves → J and S shaped
Interactions define ecosystem balance
🎯 NEET / Board Focus:
Definitions (very important)
Diagrams (age pyramid, growth curves)
Examples (NCERT-based)
Differences (mutualism vs parasitism)

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