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NEET Organisms and Populations NCERT Notes | NRI & US Prep

NEET Biology: Organisms and Populations NCERT Notes (Ecology Prep Guide for NRI Students)

AP Biology / APES vs. NCERT Ecology Terminology

Organisms and Populations Notes, Class 12 Ecology Notes, Organisms and Populations NCERT Notes, Population Growth NEET, Ecology Notes NEET, Population Interactions Notes, Organisms and Populations PYQ, NEET 2026 Biology Notes.



Organisms and Populations is an important Ecology chapter for NEET. Questions are frequently asked from adaptations, population growth, population interactions, and ecological principles. Most NEET questions are directly based on NCERT examples.
Organism and Its Environment
Every organism interacts continuously with its environment.
Environment Includes
Biotic factors
Abiotic factors
Major Abiotic Factors
1. Temperature
Most important ecological factor.
Organisms Based on Temperature
Eurythermal
Can tolerate wide temperature range.
Examples:
Humans
Tiger
Stenothermal
Examples:
Coral reefs
2. Water
Essential for all living organisms.
Organisms Based on Water Availability
Xerophytes
Adapted to dry conditions.
Examples:
Cactus
Opuntia
Hydrophytes
Adapted to aquatic habitat.
Examples:
Hydrilla
Lotus
3. Light
Affects:
Photosynthesis
Flowering
Growth
NCERT Point
Many plants depend on photoperiod for flowering.
4. Soil
Important characteristics:
pH
Mineral composition
Topography
Responses to Abiotic Factors
1. Regulate
Maintain constant body temperature.
Examples:
Birds
Mammals
2. Conform
Body temperature changes with surroundings.
Examples:
Amphibians
Reptiles
3. Migrate
Move temporarily to favorable habitats.
Examples:
Birds
4. Suspend
Dormancy during unfavorable conditions.
Examples:
Spores
Seeds
Adaptations
Adaptations help organisms survive in their habitat.
Desert Adaptations
Kangaroo Rat
Meets water requirement through metabolism
Rarely drinks water
Camel
Conserves water efficiently
Opuntia
Leaves modified into spines
Stem performs photosynthesis
Population
Population refers to a group of individuals of the same species living in a particular area.
Example:
All humans living in India.
Population Attributes
1. Natality
Birth rate.
Formula
Births per population per unit time.
2. Mortality
Death rate.
3. Sex Ratio
Number of males and females.
4. Age Distribution
Population categories:
Pre-reproductive
Reproductive
Post-reproductive
Population Growth
Exponential Growth
Occurs when resources are unlimited.
Equation
Nt = N₀eสณแต—
Graph
J-shaped curve
Logistic Growth
Occurs when resources become limited.
Graph
S-shaped curve
Carrying Capacity (K)
Maximum population size supported by environment.
NCERT Point
Logistic growth is more realistic.
Population Interactions
One of the most important topics for NEET.
1. Mutualism (+,+)
Both species benefit.
Examples
Lichen (Algae + Fungus)
Mycorrhiza
Fig and Wasp
2. Commensalism (+,0)
One benefits, other unaffected.
Examples
Orchid on Mango Tree
Cattle Egret and Grazing Cattle
3. Parasitism (+,-)
Parasite benefits, host harmed.
Examples
Cuscuta
Tapeworm
Lice
NCERT Point
Many parasites show host specificity.
4. Predation (+,-)
Predator kills prey.
Examples
Tiger and Deer
Bird and Insect
Importance
5. Competition (-,-)
Both species harmed.
Example
Two species competing for same resource.
NCERT Point
Competition may occur even if resources are abundant.
6. Amensalism (-,0)
One harmed, other unaffected.
Example
Penicillium inhibiting bacterial growth.
Important NCERT Examples
Concept
Example
Mutualism
Lichen
Commensalism
Orchid on Mango Tree
Parasitism
Cuscuta
Predation
Tiger and Deer
Amensalism
Penicillium
Desert Adaptation
Kangaroo Rat
Most Important NCERT Points
✔ Organisms respond to environmental changes through regulation, conformity, migration, or suspension.
✔ Kangaroo rat rarely drinks water.
✔ Logistic growth is more realistic than exponential growth.
✔ Carrying capacity determines maximum population size.
✔ Lichen is a mutualistic association.
✔ Cuscuta is a parasitic plant.
✔ Competition may occur even when resources are abundant.
NEET PYQ Practice Questions
Q1. Growth curve under unlimited resources is:
A. S-shaped
B. J-shaped
C. Circular
D. Linear
✅ Answer: B. J-shaped
Q2. Carrying capacity is associated with:
A. Exponential Growth
B. Logistic Growth
C. Natality
D. Mortality
✅ Answer: B. Logistic Growth
Q3. Example of Mutualism:
A. Cuscuta
B. Tapeworm
C. Lichen
D. Tiger
✅ Answer: C. Lichen
A. Orchid on Mango Tree
B. Cuscuta
C. Lichen
D. Tapeworm
✅ Answer: A. Orchid on Mango Tree
Q5. Kangaroo rat is adapted to:
A. Aquatic habitat
B. Desert habitat
C. Forest habitat
D. Polar habitat
✅ Answer: B. Desert habitat
Last-Minute NEET Revision
Must Revise
Abiotic factors
Adaptations
Population attributes
Exponential vs Logistic growth
Carrying Capacity
Population interactions
NCERT examples
Conclusion
Organisms and Populations is a scoring Ecology chapter with direct NCERT-based questions. Master the population interactions, growth curves, and adaptation examples to score easy marks in NEET Biology.
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