Class 12 Biology Chapter 11 Organisms and Populations Solutions. Explore Class 12 Biology Chapter 11 Organisms and Populations Solutions with clear answers, important questions, and exam-focused explanations to understand ecological concepts, revise effectively, and prepare confidently for exams.

Class 12 Biology Chapter 11 Organisms and Populations Solutions

A. Very Short Answer Questions (1 mark each)

1. What is ecology?

Ans: Ecology is a branch of biology that studies the relationships of organisms with the abiotic and biotic components of their surroundings.

2. With which levels of biological organisation is ecology concerned?

Ans: Organisms, populations, biological communities and biomes.

3. What factors cause variations in the physical and chemical conditions of different habitats?

Ans: Soil, water, light and temperature.

4. Which organisms are called eurythermal organisms?

Ans: Organisms that can tolerate a wide range of temperature variations are called eurythermal organisms.

5. What are mesophytes?

Ans: Plants that grow in environments with a moderate water supply, neither excessively wet nor excessively dry, are called mesophytes.

6. What is homeostasis?

Ans: The ability of an organism to maintain a stable internal environment despite changes in its external surroundings is called homeostasis.

7. What is the normal body temperature of humans?

Ans: 37°C.

8. What is migration?

Ans: The movement of an organism from an unfavourable environment to a favourable environment is called migration.

9. What are the main factors that influence population density?

Ans: Births and deaths.

10. What is a population?

Ans: A population is a group of individuals of the same species living in a particular geographical area, sharing resources or competing for them.

11. Name two factors that influence a population.

Ans: Immigration and emigration.

12. Describe an ecotone in nature.

Ans: An ecotone is a transitional area between two biomes where the species of both biomes meet.

13. Each individual organism of a species is called __________.

Ans: An organism.

14. Aquatic plants prefer __________ habitats.

Ans: Aquatic habitats.

15. Winter sleep is called __________.

Ans: Hibernation.

16. A lichen is an example of __________.

Ans: An epiphyte.

17. __________ shows an association between algae and fungi.

Ans: Lichen.

18. Desert plants tolerate water scarcity through the development of __________.

Ans: Spines.

19. In the process of __________, the body temperature of an organism changes with the external temperature.

Ans: Conformation.

20. The scorpion is an example of a __________ organism.

Ans: Benthic.

B. Short-Answer Questions (Organisms and Populations Solutions)

21. Briefly describe the following: (a) Regulation, (b) Migration, and (c) Suspension.

Ans: (a) Regulation: Some organisms maintain homeostasis through physiological or behavioural mechanisms that regulate body temperature, water concentration and other internal conditions. Humans and other mammals use similar mechanisms to regulate body temperature. Humans can maintain their body temperature at approximately 37°C. When the external temperature is high, sweating helps cool the body. On cold days, when the body temperature falls considerably below 37°C, we begin to shiver. Shivering helps increase body heat.

(b) Migration: Migration is the temporary movement of organisms from an unfavourable habitat to a favourable one. When the unfavourable conditions disappear, they may return to their original habitat. Many animals, especially birds, migrate to distant, favourable regions during winter. For example, thousands of birds arrive at Keoladeo National Park in Rajasthan from Siberia and other extremely cold regions to spend the winter.

(c) Suspension: In bacteria, fungi and lower plants, various spores are covered by thick protective walls, which help them survive unfavourable conditions. In higher plants, seeds and sometimes vegetative reproductive structures help in dispersal and enable the plants to survive unfavourable periods. During this period, metabolic activities are reduced, and normal activities resume when favourable conditions return. Among animals, those unable to migrate may remain inactive for a certain period to avoid stressful conditions.

22. What are antibiosis and allelopathy?

Ans: Antibiosis is an interaction between two or more organisms in which one organism is harmed by another.

Allelopathy is a process in which chemicals produced by one plant species are used to inhibit the growth of another plant species.

23. Briefly describe the factors responsible for population growth. HS ’24

Ans: Population growth depends on the following factors:

(i) Natality: The number of individuals born in a population during a specified period. These individuals are added to the initial population density.

(ii) Mortality: The number of deaths in a population during a specified period.

(iii) Immigration: The number of individuals of the same species that enter a habitat during a specified period.

(iv) Emigration: The number of individuals that leave a population and move out of a habitat during a specified period.

24. How does light help in the growth and development of plants?

Ans: Plants prepare food through photosynthesis, a process that uses solar energy. The food produced supports plant growth and development.

25. What is meant by the reproductive pattern of organisms? Explain.

Ans: Populations maximise their reproductive success in their habitats. Through natural selection, organisms evolve reproductive strategies that improve their reproductive success. Some organisms reproduce only once during their lifetime, whereas others reproduce several times. Some produce numerous small offspring, while others produce a smaller number of large offspring. The different characteristics of an organism’s life history have evolved in response to the biotic and abiotic factors operating in its habitat.

26. Write a short note on the adaptations of desert plants and animals.

Ans: Many desert plants develop a thick cuticle on the outer surface of their leaves or stems to reduce water loss through transpiration. Their roots may penetrate deeply into the soil to obtain water. Some plants close their stomata during the day and carry out a specialised form of photosynthesis. Certain desert plants, such as those resembling cacti, have no leaves or have leaves that fall off early. Their leaves are reduced to spines, while their flattened green stems carry out photosynthesis.

27. What is exponential growth? Briefly describe it.

Ans: In nature, no species has an unlimited supply of resources to support continuous exponential growth. Therefore, individuals of a species compete for limited resources. As a result, the organisms best adapted to the prevailing conditions survive and reproduce. The resources available in a habitat may support population growth up to a certain limit. Once this limit is exceeded, further growth becomes difficult.

When a population grows in a habitat with limited resources, its growth initially follows an exponential pattern. This is followed by a phase of decelerating growth. Finally, when the population density reaches the carrying capacity of the habitat, the population approaches a stable state.

C. Descriptive Questions (3–5 marks each)

28. Describe the major abiotic factors.

Ans: The major abiotic factors are discussed below.

Temperature: Temperature varies with the seasons and geographical location. It generally decreases from equatorial regions towards the poles and from the plains towards mountain peaks. Mango trees do not grow naturally in temperate countries such as Germany and Canada. Snow leopards are not found in the forests of Kerala, and tuna fish are not found in ocean waters outside the equatorial latitudes. Temperature is extremely important for living organisms because it affects enzyme activity, metabolism, respiration and other physiological processes.

Water: After temperature, water is one of the most important factors affecting organisms. Life first originated in water, and life cannot exist without it. Plant productivity and distribution depend greatly on water availability. The salinity of water is particularly important for aquatic organisms. Salt concentration varies considerably among freshwater habitats, seawater and hypersaline environments, where it may reach 30–35% or even higher in certain salt lakes. Some organisms tolerate a wide range of salinities, whereas others cannot. Some freshwater animals cannot survive in seawater.

Light: Light is particularly important for autotrophic organisms. Plants require light for photosynthesis. Some forest plants can photosynthesise under very low light intensity. Many plants also depend on light duration for flowering. Light is important to animals because variations in its intensity and duration, both daily and seasonally, influence feeding, reproduction and migration. Ultraviolet rays in sunlight can be harmful to many organisms. However, sunlight does not reach the deep ocean, where many marine organisms live.

Soil: Soil characteristics vary from place to place depending on climate and weathering processes, the movement or deposition of materials, and the way the soil develops. Important soil characteristics include soil structure, particle size and the arrangement of soil particles. These determine the water-holding capacity and drainage of the soil. Other factors, such as salinity and geographical location, also influence plant growth in a particular area. The types of animals that can live there are also affected. Similarly, the sediment characteristics of aquatic environments determine the distribution of bottom-dwelling animals.

29. What is adaptation? Describe the adaptations of organisms to extreme environmental conditions.

Ans: Adaptation refers to the structural, physiological or behavioural features that enable organisms to survive under extreme environmental conditions.

Some adaptations of organisms to extreme environmental conditions are as follows:

(i) Many desert plants have leaves modified into spines to reduce water loss through transpiration. Their flattened green stems carry out photosynthesis.

(ii) Mammals living in extremely cold regions have short ears, tails or limbs to reduce heat loss.

(iii) Some animals dig burrows and remain underground to escape the heat at the soil surface.

30. Discuss the characteristics of a population.

Ans: Individuals of a species rarely live in isolation in nature. Instead, they usually live in groups within a defined geographical area, share resources or compete for them, and interbreed to form a population. Examples of populations include all the cormorants in a wetland, rats in a particular habitat, teak trees in a forest, bacteria in a cultivated field, and lotus plants growing in a pond.

A population has certain characteristics that an individual organism does not possess. An individual is born and dies, whereas a population has a birth rate and a death rate. Another characteristic of a population is its sex ratio. An individual organism is either male or female, but a population may consist of, for example, 60% males and 40% females.

At a particular time, a population contains individuals of different age groups. The size of a population indicates its status in a habitat. Competition between species, predation and the effects of pesticides are among the ecological factors whose effects can be assessed by examining changes in population size.

For example, if approximately 200 Parthenium plants grow around a single banyan tree in an area, the density of the banyan population may be considered relatively low. Consequently, the banyan tree may have a relatively small influence within that community.

31. What is the effect of competition on interactions among populations? Explain.

Ans: Darwin considered interspecific competition an important driving force of biological evolution. Competition begins when closely related species strive to obtain the same resources. However, unrelated species may also compete for the same resources. For example, fish and flamingos in shallow South American lakes may compete for similar resources, such as zooplankton.

In interference competition, the feeding ability of one species decreases in the presence of another species that interferes with or prevents its access to food, even when sufficient resources are available. Thus, competition is an interaction in which the fitness of one species may decrease because another species is present.

Gause’s competitive exclusion principle states that two closely related competitors cannot coexist indefinitely when they compete for the same limited resources. The inferior competitor may eventually be eliminated. This outcome occurs when resources are limited; when resources are sufficiently abundant, it may not occur.

Species facing competition may avoid exclusion by adopting different resource-use strategies. For example, two competing species may feed at different times or use different feeding methods.

32. How does light influence the distribution of organisms? Briefly describe with suitable examples.

Ans: The ultimate source of energy for almost all life on Earth is sunlight. Green plants capture solar energy and make it available to other organisms through food. Light stimulates various biological processes. For example, sunlight causes leaves to open and stimulates the opening and closing of the petals of certain flowers. Pollen formation may not occur without adequate light. Light also influences seed germination, growth, reproduction, and movement in many organisms.

In animals, light helps with vision, orientation within their habitats, and feeding. Prolonged absence of light may cause the light-sensitive organs of animals, particularly their eyes, to become reduced or non-functional. For example, cave-dwelling bats and certain rodent-like animals may exhibit reduced vision.

The availability of sunlight decreases with increasing depth in the ocean. Animals living in the deep sea, where sunlight does not penetrate, obtain energy by oxidising fat stored in their bodies.

D. Additional Questions and Answers

1. How do we regulate our body temperature during winter?

Ans: During winter, our bodies shiver more frequently to regulate body temperature. Shivering produces heat in the body, gradually increasing its temperature.

2. Mention three important characteristics of a population.

Ans: Population growth, population density and age distribution.

3. What is meant by a conformer?

Ans: A conformer is an organism whose internal body conditions, such as the concentration of body fluids, change in response to changes in the external environment.

4. Name the organism that can tolerate the highest salinity.

Ans: Stenohaline organisms.

5. Why are small animals generally not found in polar regions?

Ans: Temperatures in polar regions often fall below 0°C. Therefore, considerable energy is required for temperature regulation. Small animals have a relatively large surface area compared with their body volume, causing them to lose heat rapidly. Consequently, they must expend considerable energy to maintain their body temperature.

6. Why do plants produce nicotine, caffeine and opium?

Ans: Plants produce these substances to discourage herbivores from feeding on them.

7. What was Gause’s experiment?

Ans: Gause demonstrated that when food resources are limited, the more competitive species may gradually eliminate the less competitive species.

8. Name two types of organisms adapted to light conditions.

Ans: Sciophytes and heliophytes.

9. What does the term neutralism mean in a biological community?

Ans: Neutralism is an interaction in which the presence of one species has no effect on another species in a biological community.

10. How do animals temporarily escape unfavourable conditions?

Ans: Animals temporarily escape unfavourable conditions in the following ways:

(i) Some animals undergo hibernation to survive extremely cold conditions.

(ii) Some animals undergo aestivation to escape extremely high temperatures.

(iii) Diapause occurs in certain zooplankton species found in ponds and lakes.

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