NCERT Solutions for Class 12 Biology Chapter 13 Organisms and Populations

According to the latest CBSE Syllabus 2025-26, this chapter has been renumbered as Chapter 11.

NCERT Solutions for Class 12 Biology Chapter 11 Organisms and Populations describes how various organisms respond to their environment and how populations increase and evolve over time. It describes important factors such as ecological adaptations, interactions among organisms, and environmental factors influencing survival. Students also learn about population traits such as birth rate, death rate, and age structure.

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1. NCERT Solutions for Class 12 Biology Organisms and Populations PDF
2. NCERT Solutions for Class 12 Biology Chapter 11 Organisms and Populations
3. Previous year's MCQs on NCERT Chapter 11 Organisms and Populations
4. How to solve NCERT Chapter 11 questions effectively
5. Important Topics of Organisms and Populations
6. Adaptations of Organisms to Their Environment
7. Why is NCERT Chapter 11 is important for NEET preparation?
8. Additional topics from Organisms and Populations for NEET exam
9. NCERT Solutions for Class 12 Biology Chapter wise

The solutions help to explain how living organisms adapt to temperature change, water availability, and other factors in their environments. It also describes population growth patterns, competition, and other relationships between species. NCERT Solutions are useful when studying ecosystems and biodiversity conservation. NCERT Solutions for Class 12 Biology offer step-by-step solutions to textbook questions in a simple and easy to understand manner, making it easy for students to grasp important concepts such as adaptations, population dynamics, and ecological relationships in an organised and systematic way.

NCERT Solutions for Class 12 Biology Organisms and Populations PDF

Students can download the complete questions with detailed answers PDF of this chapter for better learning:

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NCERT Exemplar for Class 12 Biology Chapter 11 Organisms and Populations

NCERT Solutions for Class 12 Biology Chapter 11 Organisms and Populations

The detailed answers to all the questions in this chapter are given below:

Ques:1 List the attributes that populations possess but not individuals.

Ans: Traits that populations have but not individuals:

• Population size, density, birth rate, death rate, and age distribution are traits of a population.
• These traits characterise a group of organisms, not an individual.

Ques:2 If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?

Ans: Intrinsic rate of increase (r):

• The formula for exponential growth is $$N_t=N_0\times e^{rt}$$ .

• Given that the population doubles in 3 years, we use $$2N_0=N_0\times e^{3r}$$ .

• Solving for r, we get r ≈ 0.231 per year.

Ques:3 Name important defence mechanisms in plants against herbivory.

Ans: Plant defense mechanisms against herbivory:

• Thorns and spines (e.g., cactus) deter animals from consuming plants.
• Chemical defences such as bitter or toxic chemicals (e.g., alkaloids in tobacco).
• Mimicry and camouflage to evade detection (e.g., passionflower leaves resemble butterfly eggs).

Ques:4 An orchid plant is growing on the branch of the mango tree. How do you describe this interaction between the orchid and the mango tree?

Ans: Interaction between the mango tree and the orchid:

• This is a case of commensalism.
• The orchid gains from receiving support and sunlight, without affecting the mango tree in any way.

Ques:5 What is the ecological principle behind the biological control method of managing with pest insects?

Ans: Ecological principle supporting biological control of pest insects:

• It relies on predation and parasitism.
• Natural predators (e.g., ladybugs consuming aphids) or parasites (e.g., wasps laying eggs on caterpillars) regulate populations of pests using no chemicals.

Ques:6 Define population and community.

Ans:

Population: A collection of members of the same species inhabiting a particular location (e.g., all the tigers in a forest).

Community: Various species inhabiting the same location and interacting with each other (e.g., plants, animals, and microbes in a pond).

Ques:7 Define the following terms and give one example for each:

(a) Commensalism

Ans: One species gains, the other is not affected (e.g., barnacles on whales).

(b) Parasitism

Ans: One species gains, the other suffers (e.g., tapeworm in humans).

(c) Camouflage

Ans: An organism matches its environment to hide from predators (e.g., stick insects).

(d) Mutualism

Ans: Both species gain (e.g., bees pollinating flowers).

(e) Interspecific competition

Ans: Two species compete for the same resource (e.g., lions and hyenas competing for food).

Ques:8 With the help of a suitable diagram describe the logistic population growth curve.

Ans: The logistic growth curve shows how a population increases with finite resources.

• Initially, the population increases slowly (lag phase), followed by rapid increase (exponential phase).
• When resources run out, growth slows down to a stable stage (carrying capacity).

The equation of the logistic growth curve is as follows:
dN/dt = rN(K−N/K)
Where N = Population density at time t
R = Intrinsic rate of natural increase
K = Carrying capacity
Since resources for growth for populations are finite and become limiting, the logistic growth model is considered a more realistic one.

Ques:9 Select the statement which explains best parasitism.

(a) One organism is benefited.

(b) Both the organisms are benefited.

(c) One organism is benefited, other is not affected.

(d) One organism is benefited, and another is affected.

Ans: The best explanation of parasitism:

• (d) One organism is benefited, other is affected.

Ques:10 List any three important characteristics of a population and explain.

Ans:

• Population Density: The number of organisms per unit area. Influences resource utilisation and competition.
• Natality (Birth Rate): The number of new organisms added to a population via reproduction. Determines population growth.
• Death Rate (Mortality): The amount of people dying within a period. Affects population decrease.

Previous year's MCQs on NCERT Chapter 11 Organisms and Populations

Some PYQs are given below:

Q1. When does the growth rate of a population following the logistic model equal zero? The logistic model is given as dN/dt = rN(1-N/K):

1. when N/K is exactly one.

2. when N nears the carrying capacity of the habitat.

3. when N/K equals zero.

4. when the death rate is greater than the birth rate.

Answer:

The equation for logistic growth -

A plot of N about time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth and is described by the following equation:

- wherein

$$\frac{dN}{dt}=rN\left(\frac{K-N}{K}\right)$$

Where,

N = Population density at time t

r = Intrinsic rate of natural increase

K = Carrying capacity

So, the growth rate of the population will be equal to zero, when the value of N/K is equal to one.

Hence, the correct answer is (1) when N/K is exactly one.

Q2. Gause's principle of competitive exclusion states that:

1. More abundant species will exclude the less abundant species through competition.

2. Competition for the same resources excludes species having different food preferences.

3. No two species can occupy the same niche indefinitely for the same limiting resources.

4. Larger organisms exclude smaller ones through competition.

Answer:

As we learned in

Competitive Exclusion Principle -

Two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually. Gause's law of competitive exclusion or just Gause's law is a proposition that states that two species competing for the same resource cannot coexist at constant population values if other ecological factors remain constant. When two competing life forms attempt to occupy the same niche, only one outcome is possible: One life form will drive out the other.

Hence, the correct answer is (3) No two species can occupy the same niche indefinitely for the same limiting resources.

Q3. The principle of competitive exclusion was stated by

1. C. Darwin

2. G. F. Gause

3. MacArthur

4. Verhulst Pearl

Answer:

Competitive Exclusion Principle -

Two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually. It was stated by G.F. Gause.

Hence, the correct option is (2) G. F. Gause.

Q4. If '+' sign is assigned to beneficial interaction, '-' sign to detrimental and '0' sign to neutral interaction, then the population interaction represented by '+' '-' refers to

1. mutualism

2. amensalism

3. commensalism

4. parasitism

Answer:

Parasitism is a relationship between two living organisms of different species in which one organism i.e. parasite obtains its food directly from the host. In this relationship, the parasite is benefitted (+) and the host is harmed (-) so this type of population interaction is represented by (+) and (-).

Mutualism is a relationship between two living organisms from which both are benefitted.

Commensalism is a relationship between two living organisms, where one organism is benefitted while the other is not affected.

Ammensalism is a relationship between two living organisms, where one organism is harmed while the other is not affected.

Hence, the correct answer is Option (4) Parasitism.

Q5. Which of the following is correct for r-selected species?

1. A large number of progeny with small size

2. A large number of progeny with large size

3. The small number of progeny with small size

4. The small number of progeny with large size

Answer:

r-selected species are the ones that are found in the sparsely crowded ecological niche to exploit it and aim at the production of a large number of small-sized progeny with lower survival rates due to the short gestation period and early attainment of maturity.

Hence, the correct answer is (1) A large number of progeny with small size

How to solve NCERT Chapter 11 questions effectively

In order to easily answer Organisms and Populations questions, keep these simple points in mind:

• First Concepts: Read NCERT chapter slowly to know the concepts like interactions of population, adaptations, growth models.
• Take Help of NCERT Solutions: Take help from NCERT Solutions for proper concepts of textbook problems.
• Practice Previous Year Questions (PYQs): Practice PYQs in order to know the type of questions that NEET asks.
• Practice Diagrams and Graphs: Take special care while drawing diagrams like the curve of logistic growth, as they are often asked in the exam.
• Updated Periodically: Update periodic definitions, examples, and concepts of ecology as and when often.
• Highlight Important Lines: Highlight important lines in NCERT which may come as direct questions in NEET.

NCERT Exemplar Class 12 Solutions

Important Topics of Organisms and Populations

The key topics covered in this chapter are:

Population

• Population refers to a collection of organisms belonging to the same species in a definite area.
• They are affected by the availability of food, climatic conditions, and environmental parameters.
• Population analysis helps in species survival as well as in conservation.

Population Features

• Size: Number of entities present in the population.
• Density: Number of individuals per area unit.
• Birth Rate & Death Rate: No. of births and deaths per population over an interval of time.
• Age Structure: Percentage composition of people in varying age groups.

Population Growth

Populations increase or decrease depending on births, deaths, immigration, and emigration.

Two common types of growth patterns:

• Exponential Growth: Fast population growth without any limiting factors.
• Logistic Growth: Slowing growth due to the limitation of resources.

Variation in Life History

• Organisms possess various survival and reproductive strategies.
• Some species produce a lot of offspring and reproduce rapidly, whereas others produce few offspring but provide care for them.
• Environmental pressures affect life history characteristics such as lifespan and breeding cycles.

Population Interactions

• Competition: Two species competing for the same resources.
• Predation: One animal (predator) preys on and consumes another (prey).
• Mutualism: Both species gain from the association (e.g., flowers and bees).
• Parasitism: One animal (parasite) resides on or within another (host) and harms it.

These are the key topics covered in the organisms and populations NCERT solutions for class 12, giving students a detailed knowledge of health and disease.

Also Check NCERT Books and NCERT Syllabus here:

• NCERT Books Class 12 Biology

• NCERT Syllabus Class 12 Biology

• NCERT Books Class 12

• NCERT Syllabus Class 12

Adaptations of Organisms to Their Environment

Organisms evolve structural, physiological, and behavioural adaptations to thrive in various environments.

Examples:

• Polar bears have thick fur and layers of fat to live in cold climates.
• Desert plants such as cacti have thick cuticles and small leaves to reduce water loss.
• Camouflage helps animals such as chameleons to blend into their environment.

Why is NCERT Chapter 11 is important for NEET preparation?

Some points explaining why Organisms and Populations are necessary:

1. Matches NEET Syllabus: Covers topics that are often asked in NEET.
2. Helps Understand Ecology: Gives details of how living things respond to the environment.
3. Important for Biology Section: Ecology is a big part of the Biology questions in NEET.
4. Easy to Understand: Explains complex ideas in a simple way.
5. Good for Quick Revision: Helps you quickly review key points before the exam.

Additional topics from Organisms and Populations for NEET exam

Here is a table for all the important topics from the chapter:

NCERT Solutions for Class 12 Biology Chapter wise

Below mentioned are the solutions:

This chapter makes students aware of how organisms adapt to their surroundings and how populations increase and evolve. It defines key terms such as adaptations, competition, and population dynamics in an easy-to-understand manner. NCERT Solutions for Class 12 Biology offer easy explanations and step-by-step solutions to make learning easy for students. Through this chapter, students learn more about ecosystems and the balance of nature.

NCERT Solutions for Class 12- Subject-wise