Class 12 Biology Chapter 13 Notes: Organisms and Populations

Class 12 Biology Chapter 13 Organisms and Populations Notes

13.1 Organism and Its Environment

Every organism lives in a specific environment and continuously interacts with both biotic (living) and abiotic (non-living) factors. Survival depends on the ability to adapt to environmental conditions.

Diapause vs Hibernation

Diapause

A period of suspended growth and development
Seen mainly in insects and invertebrates
Metabolic activity is greatly reduced
Helps organisms survive unfavourable seasons

Hibernation

A sleep-like inactive state
Seen in animals like fishes and amphibians
Helps survive cold and food scarcity during winter

Osmoregulation in Aquatic Animals

Marine Fish in Fresh Water

Marine fish cannot survive in fresh water
Reason:
- Marine fish are adapted to saline (hypertonic) environment
- Fresh water is hypotonic
- Excess water enters the body by osmosis
- Fish fails to maintain osmoregulation

Phenotypic Adaptation

Phenotypic adaptation refers to non-genetic changes shown by organisms in response to environmental stress.

Examples:

Acclimatization
Behavioural changes
Seasonal changes in body functions

Survival at Extreme Temperatures

Most organisms cannot survive above 45°C, but some microbes do.

Reasons:

Presence of branched chain lipids in cell membrane
Reduced membrane fluidity
Very little free water in the body
Occur in hot springs and hydrothermal vents

Abiotic Environmental Factors

Abiotic factors are non-living components that affect organisms.

Major Abiotic Factors:

Temperature – affects metabolism, distribution and survival
Water – essential for life; affects productivity
Light – required for photosynthesis and flowering
Humidity – affects water loss from organisms
Precipitation – rainfall, snow, dew
Soil (Edaphic factor) – supports plant life

Adaptations in Organisms

(a) Adaptations of Desert Plants and Animals

Plants:
- Small or no leaves
- Photosynthesis through stem
- Succulent stems for water storage
Animals:
- Smaller body size
- No drinking of water (e.g., kangaroo rat)

(b) Adaptations of Plants to Water Scarcity

Leaf curling
Closure of stomata
Leaf fall in deciduous plants
Accumulation of inorganic ions

(c) Behavioural Adaptations in Animals

Migration to favourable habitats
Desert lizards:
- Bask in sun when cold
- Hide in shade when hot

(d) Importance of Light to Plants

Required for photosynthesis
Influences flowering and germination
Affects phenology and pigmentation
Lack of light destroys chlorophyll

(e) Effect of Temperature / Water on Animals

Ectotherms (Poikilotherms):
- Body temperature depends on environment
- Example: fishes, reptiles
Endotherms (Homeotherms):
- Maintain constant body temperature
- Example: birds and mammals

13.2 Populations

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

Population Attributes (not found in individuals):

Population density
Natality (birth rate)
Mortality (death rate)
Age distribution
Sex ratio
Population dispersion
Growth patterns

Population Growth

Exponential Growth

Occurs when resources are unlimited
Intrinsic rate of increase (r) is high
Population doubles rapidly

Logistic Population Growth Curve

Also called S-shaped curve
Growth slows as population reaches carrying capacity (K)

Equation:

$\frac{d N}{d t} = r N (\frac{K - N}{K})$

Where:

N = population size
r = intrinsic rate of increase
K = carrying capacity

Defence Mechanisms in Plants Against Herbivory

Plants cannot escape predators, so they develop defences:

Chemical Defences:

Nicotine (tobacco)
Morphine (opium poppy)
Strychnine

Physical Defences:

Thorns and spines
Silica-rich leaves
Tough stems

Species Interactions

Orchid on Mango Tree

Type of interaction: Commensalism
Orchid benefits (support, light)
Mango tree is unaffected

Biological Control of Pests

Based on predator–prey relationship
Predators control pest population naturally
Eco-friendly and sustainable

Important Ecological Terms

Population

A group of individuals of the same species living in a defined area.

Community

A group of different species living together in a habitat.

Types of Interactions (with Examples)

Commensalism

One benefits, other unaffected
Example: E. coli in human intestine

Parasitism

One benefits, other harmed
Example: lice on humans

Mutualism

Both benefit
Example: insect pollination

Camouflage

Concealment from predators
Example: colour matching animals

Interspecific Competition

Competition between species
Example: Paramecium aurelia and P. caudatum

Examples

Endothermic animal – Human
Ectothermic animal – Frog
Benthic organism – Corals

Important Population Characteristics

1. Population Density

Number of individuals per unit area

2. Natality

Birth rate of population

3. Mortality

Death rate of population

MCQ Answer (Concept Based)

Parasitism is best explained by:
✔ One organism is benefited, other is affected.

13.3 Summary (Exam Ready ✨)

Organisms interact continuously with environment
Adaptations help survival in extreme conditions
Populations show unique characteristics
Logistic growth shows environmental limits
Species interactions shape ecosystems
Abiotic factors strongly influence distribution

✍️ Top 10 Short Question–Answers

Q1. What is meant by environment?

Answer: Environment includes all biotic and abiotic factors that affect an organism.

Q2. What is diapause?

Answer: A period of suspended growth and development to survive unfavourable conditions.

Q3. What is hibernation?

Answer: A sleep-like inactive state during cold and food scarcity.

Q4. Why cannot marine fish survive in fresh water?

Answer: Because fresh water is hypotonic, causing excess water entry and failure of osmoregulation.

Q5. What is phenotypic adaptation?

Answer: Non-genetic changes shown by organisms in response to environmental stress.

Q6. Name any one abiotic factor affecting organisms.

Answer: Temperature.

Q7. What are ectothermic animals?

Answer: Animals whose body temperature depends on environmental temperature.

Q8. What is a population?

Answer: A group of individuals of the same species living in a defined area.

Q9. What is population density?

Answer: Number of individuals per unit area.

Q10. Which interaction is shown by orchid growing on mango tree?

Answer: Commensalism.

📝Long Answer Questions

Q1. Explain abiotic factors affecting organisms.

Answer:
Abiotic factors are non-living components influencing organisms.

Major abiotic factors:

Temperature: Affects metabolism, growth and survival
Water: Essential for life and productivity
Light: Required for photosynthesis and flowering
Humidity: Affects water loss
Precipitation: Rainfall, snow and dew
Soil: Supports plant growth

These factors determine distribution of organisms.

Q2. Describe adaptations of desert plants and animals.

Answer:

Desert plants:

Small or absent leaves
Photosynthesis through stem
Succulent stems for water storage

Desert animals:

Small body size
Do not drink water directly
Example: Kangaroo rat

These adaptations help conserve water.

Q3. Explain population growth patterns.