NCERT Solutions Class 12 Biology Chapter 11 Organisms and Populations

In Class 12 Biology Chapter 11, Organisms and Populations, students learn about how living organisms interact with their environment and with each other. Factors like temperature, water, light, and soil that influence survival, as well as population attributes such as density, growth, and distribution, are also discussed in detail here. The chapter aims to help students understand how a balance is maintained in natural ecosystems.

To ensure a thorough grasp of these ecological concepts, ALLEN provides structured NCERT Solutions to help students master the theoretical and numerical side of ecology, such as calculating population density and interpreting age-structure diagrams. By practicing with ALLEN’s academic materials, students can build the conceptual clarity and speed necessary to handle both descriptive board questions and time-sensitive competitive exams like NEET.

1.0Download NCERT Solutions Class 12 Biology Chapter 11 Organisms and Populations : Free PDF

This chapter explains how organisms survive in different environments and how populations change over time. The NCERT Solutions for Class 12 Biology Chapter 11 Organisms and Populations help students revise key ecological ideas and practise textbook questions easily. Download the free PDF now to study anytime and prepare confidently for exams.

2.0Key Concepts of Class 12 Biology Chapter 11 Organisms and Populations

The chapter focuses on understanding environmental effects and population behaviour. Some of the key lessons covered in this chapter are given below:

• Abiotic Factors: Understanding how temperature, water, light, and soil affect living organisms.
• Adaptations: Learning how organisms adjust structurally and behaviourally to survive in different habitats.
• Population Attributes: Studying density, birth rate, death rate, age distribution, and sex ratio.
• Population Growth Models: Understanding exponential and logistic growth curves.
• Population Interactions: Learning about competition, predation, parasitism, commensalism, and mutualism.
• Human Population Growth: Understanding trends and their impact on resources and environment.

3.0NCERT Class 12 Biology Chapter 11 Organisms and Populations : Detailed Solutions

NCERT Detailed Solutions Class 12 Biology Chapter 11 Organisms and Populations

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

Ans: The attributes possessed by the population but not individuals are:

• Population density – It is the number of individuals of a species per unit area or volume.
• Birth rate (Natality) – The number of new individuals added to the population per unit time.
• Death rate (Mortality) – Number of individuals dying in a population per unit time.
• Age distribution – Proportion of individuals in different age groups (pre-reproductive, reproductive, post-reproductive).
• Sex ratio – Ratio of males to females in the population.

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

Ans: Initial population: N_0, After 3 years, population = 2N₀

For exponential growth:

$N=N_0{e}^{rt}$

So,

$2N_0=N_0{e}^{3r}$

Cancel N₀: $2=e^{3r}$

Take natural log: $\ln 2=3r$

$r=\frac{\ln 2}{3}$

$r\approx \frac{0.698}{3}\approx 0.231\text{ per year }$

3. Name important defence mechanisms in plants against herbivory. 

As: Morphological (Structural) Defenses

These are physical features that make it difficult or painful for an animal to eat the plant.

• Thorns: The most common morphological means of defense. For example, Acacia and Cactus possess sharp thorns to deter grazers.
• Spines and Hairs: Many plants have evolved sharp spines on their leaves or "stinging hairs" that inject irritants into the skin of the herbivore.
• Modified Leaves: Some plants have thick, waxy cuticles or leathery leaves that are difficult to chew and digest.

Chemical Defenses

Many plants produce and store chemicals that make the herbivore sick, inhibit its digestion, disrupt its reproduction, or even kill it.

• Cardiac Glycosides: The weed Calotropis (growing in abandoned fields) produces highly poisonous cardiac glycosides. This is why you never see cattle or goats browsing on this plant it affects the heart function of the herbivore.
• Alkaloids and Other Chemicals: Plants produce a wide variety of chemicals that we humans use commercially, but the plants actually produce them as defense mechanisms. These include:
• Nicotine (Tobacco)
• Caffeine (Coffee/Tea)
• Quinine (Cinchona tree)
• Strychnine
• Opium (Poppy)

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

Ans: The interaction is called commensalism. In this relationship, the orchid benefits by getting support and better exposure to sunlight, while the mango tree is neither harmed nor benefited. The orchid only uses the mango tree as a place to grow and does not take any nutrients from it.

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

Ans: The ecological principle behind biological control of pest insects is the predator–prey relationship.

In this method, natural enemies of the pest, such as predators, parasites, or pathogens, are used to control the pest population. This maintains a natural balance in the ecosystem without the use of chemical pesticides.

6. Define population and community. 

Ans:

• Population: A population is a group of individuals of the same species living in a specific area at a given time and capable of interbreeding.
• Community: A community is a group of different populations of different species living together in a particular area and interacting with one another.

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

(a) Commensalism 

(b) Parasitism 

(c) Camouflage 

(d) Mutualism 

(e) Interspecific competition 

Ans:

(a) Commensalism 

Definition: It is an interaction between two species in which one species benefits while the other is neither harmed nor benefited.

Example: An orchid growing on a mango tree.

(b) Parasitism 

Definition: It is an interaction in which one organism (parasite) benefits and the other (host) is harmed.

Example: Tapeworm in the human intestine.

(c) Camouflage 

Definition: It is the ability of an organism to blend with its surroundings to avoid being seen by predators or prey.

Example: A chameleon changing its colour.

(d) Mutualism 

Definition: It is an interaction in which both species benefit from each other.

Example: Lichen (an alga and a fungus living together).

(e) Interspecific competition 

Definition: It is the interaction between different species that compete for the same limited resources like food, space, or light.

Example: Weeds and crop plants compete for nutrients and water.

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

Ans: The phases involved in the population growth curve are:

• Lag phase – This is the initial phase where the population begins to establish itself. This is marked by a slow initial growth.
• Exponential (log) phase – As individuals adapt to the environment, there is a rapid increase in population.
• Deceleration phase – Due to the limited resources and increased competition, the growth rate slows down. 
• Stationary phase – The population levels off. Here, the number of births will roughly equal the number of deaths. The habitat would have reached maximum capacity.

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, other is affected. 

Ans: (d) One organism is benefited, other is affected. 

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

Ans: The three important characteristics of a population are:

• Population Density: Population density shows how many individuals of a species live in a given area or volume at a particular time. It helps ecologists understand whether a population is sparse or crowded and how resources like food, space, and shelter may affect its growth.
• Natality (Birth Rate): Natality refers to the rate at which new individuals are added to the population by reproduction. A higher natality increases population size, while a lower natality slows down population growth.
• Mortality (Death Rate): Mortality is the rate at which individuals die in a population. When mortality is higher than natality, the population size decreases, and when it is lower, the population continues to grow.

4.0Key Features and Benefits of Class 12 Biology Chapter 11 Organisms and Populations

• Graph-based explanations help students interpret population growth curves and ecological trends clearly.
• Real-world examples connect textbook ecology concepts with forests, oceans, and urban environments.
• Concept summaries support quick and quality revision before board examinations and class tests.
• Strong foundations of this chapter help in NEET preparation, especially for ecology and environment-based questions.
• Simple language keeps complex ecological relationships easy to understand and remember.