CBSE Important Questions Class 12 Biology Chapter 11 – Organisms and Populations

Class 12 Biology Chapter 11 Organisms and Populations is the eleventh chapter of the NCERT Class 12 Biology textbook. It explains how organisms live in their habitat: the abiotic factors (temperature, water, light and soil), how organisms respond to them by regulating, conforming, migrating or suspending activity, the adaptations of desert plants and animals, population attributes such as birth rate, death rate and age pyramids, population growth models, and interactions such as predation, competition, parasitism, commensalism and mutualism.

These CBSE important questions for Class 12 Biology Chapter 11 cover thermophiles that live above 100°C, the effect of temperature on enzymes, adaptations of parasites, how rats and snakes escape desert heat, how long adaptation takes, the spines and swollen stems of the cactus, and the adaptations of desert animals, plants and camels. The set has multiple-choice, short-answer and long-answer questions with answers for board exam revision.

Q1. Most living organisms cannot survive at temperatures above 45°C. How are some microbes able to live in habitats with temperatures exceeding 100°C?

Answer: Most living organisms cannot survive at temperatures above 45°C because their proteins and enzymes lose their three-dimensional structure and stop working. However, some microbes, known as thermophiles, are able to live in habitats with temperatures exceeding 100°C due to the presence of enzymes which are stable to heat. These thermostable enzymes do not denature at high temperatures, so the metabolic activities of the bacteria can go on. Their cell membranes are also made of special lipids that stay stable in extreme heat.

Q2. The enzyme kinetics is affected by
(a) light.
(b) temperature.
(c) water.
(d) air.

Answer: (b) temperature.
The rate of an enzyme-catalysed reaction increases with temperature up to an optimum temperature for that enzyme. Above this temperature, the enzyme denatures (loses its shape) and the reaction rate falls. So temperature is the key factor affecting enzyme kinetics.

Q3. List any two adaptive features evolved in parasites enabling them to live successfully on their hosts.

Answer: 1. Presence of adhesive organs or suckers: Many parasites, such as tapeworms and flukes, have hooks, suckers or adhesive discs that help them attach firmly to the host's tissues or intestinal wall, so they are not dislodged.
2. High reproductive capacity: Parasites produce a very large number of eggs or offspring to increase the chance of reaching new hosts and to make up for the high mortality outside the host.
(Other adaptations include loss of sense organs and of the digestive system, and a complex life cycle with intermediate hosts.)

Q4. How do rats and snakes escape the intense heat of the desert?

Answer: Rats and snakes escape the intense heat of the desert by hiding deep inside burrows in the sand during the day, where the temperature is much lower than at the surface. They come out during the night when it is cool outside. This daytime burrowing and nocturnal activity (a behavioural adaptation) also helps them conserve water.

Q5. How much time does adaptation take to occur?

Answer: Adaptation does not take place in a short time. The abiotic factors of a region change slowly over thousands of years, and adaptations also develop over many generations through natural selection. The animals and plants which cannot change themselves and do not adapt to these changes die out, and those which adapt are able to survive.

Q6. Any structural or behavioural change in an organism that makes it best suited to survive and reproduce in its environment is termed as
(a) aestivation
(b) adaptation
(c) conformation
(d) hibernation

Answer: (b) adaptation
Adaptation is any heritable structural, physiological or behavioural change that improves the survival and reproduction of an organism in its environment (for example, cactus spines, bird migration or antifreeze proteins in fish). Aestivation is dormancy in summer to avoid heat, hibernation is dormancy in winter, and conformation means letting the body temperature change with the surroundings.

Q7. The leaves of the cactus are modified into spines, which help in
(a) adjustment
(b) penetration
(c) adaptation
(d) propagation

Answer: (c) adaptation
Cacti grow in dry regions where water is scarce. Their leaves are modified into spines, which greatly reduce the surface area exposed to the air and so reduce water loss by transpiration. The spines also protect the plant from herbivores. These structural changes help the plant survive in extreme water stress, so they are an adaptation.

Q8. Leaves in desert plants are either absent, very small or present in the shape of spines. This helps in
(a) the uptake of minerals
(b) reducing the loss of water
(c) increasing evaporation of water
(d) the condensation of water

Answer: (b) reducing the loss of water
In plants, most water is lost by transpiration from the leaf surface. In desert plants, the leaves are absent or reduced to spines, which greatly reduces the leaf surface area. Along with a thick cuticle and sunken stomata, this reduces the loss of water.

Q9. Any feature of an organism or its part which enables it to exist under the conditions of its habitat is known as
(a) adaptive variation
(b) adjustment
(c) acclimatization
(d) adaptation

Answer: (d) adaptation
Adaptation is a heritable structural, physiological or behavioural feature that enables an organism to live under the conditions of its habitat. Adjustment and acclimatisation are short-term, reversible responses of an individual to environmental change, and adaptive variation refers to minor differences within a population.

Q10. Write a short note on:
(a) Adaptations of desert animals
(b) Adaptations of plants to water scarcity
(c) Behavioural adaptations in animals

Answer: (a) Adaptations of desert animals: Desert animals meet all their water requirements through internal fat oxidation, in which water is a by-product, e.g. the kangaroo rat in the North American desert. They have the ability to excrete concentrated urine, so that a minimal volume of water is used to remove excretory products.
(b) Adaptations of plants to water scarcity: Plants have a thick cuticle on their leaf surfaces to lessen the effect of high temperature. The stomata on their leaves are located in deep pits to minimise water loss through transpiration. They also have a special photosynthetic pathway (CAM) that enables their stomata to remain closed during the daytime. In plants like Opuntia, the leaves are reduced to spines, and the stem is modified into a flat green structure which performs photosynthesis.
(c) Behavioural adaptations in animals: Behavioural adaptations help organisms cope with variations in their environment. For example, the desert lizard basks in the sun and absorbs heat when its body temperature drops below the comfort zone, but moves into the shade when the ambient temperature starts increasing. Some species are capable of burrowing into the soil to hide and escape the heat above the ground.

Q11. Camels have adapted to the ______ climate.
(a) arid
(b) Mediterranean
(c) polar
(d) tropical

Answer: (a) arid
An arid climate has very low rainfall, high temperatures and high evaporation. Camels are adapted to it: their hump stores fat that can be used for energy and water, they can tolerate losing a large amount of body water, their nasal passages trap moisture from exhaled air, thick fur on the back insulates them from the daytime heat, and broad flat feet stop them sinking into the sand.

Q12. Swollen stems of the cactus are important to
(a) absorb more oxygen
(b) absorb more carbon dioxide
(c) absorb more water
(d) absorb more sunlight

Answer: (c) absorb more water
Cacti are xerophytes adapted to dry regions. Their leaves are reduced to spines to cut water loss, so the stems take over the functions of photosynthesis and water storage. The swollen (succulent) stems provide a large volume for storing the water that the plant absorbs during the short periods of rain, which keeps the plant alive during drought.

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