Important Questions for CBSE Class 12 Biology Chapter 13 - Organism and Population 2024-25

Very Short Questions and Answers (1 Mark Questions)

1. Which are the factors responsible for the wide variety of habitats formed within each biome?

Ans: Regional and local variations

2. Freshwater animals are unable to survive for long in seawater. Give a reason.

Ans: Due to osmotic problems.

3. With which population growth model is the Verhulst Pearl equation associated?

Ans: Logistic Growth.

4. Define diapause. Which organisms exhibit it?

Ans: A stage of suspended development, zooplanktons.

5. Calculate the death rate if 6 individuals in a laboratory population of 60 fruit flies died during a particular week.

Ans: 6/60 =0.1 individuals per fruitfly per week.

6. In the biological control method, one living organism is used against another to check its uncontrolled growth. Which kind of population interaction is involved in this?

Ans: Predation.

7. An organism has to overcome stressful conditions for a limited period of time. Which strategies can it adopt to do so?

Ans: 

(i) Migration

(ii) Suspension of active life by hibernation/aestivation/spore formation.

8. Write what do phytophagous insects feed on?

Ans: Plant sap and other parts of the plant.

9. Why do leaves contain Sunken stomata?

Ans: Leaves contain sunken stomata i.e. Stomata are arranged in deep pits to minimize water loss by transpiration.

10. Name the type of interaction that is detrimental to both the species.

Ans: Competition.

11. What type of interaction is shown by sparrows eating the seeds?

Ans: Predation.

12. Define homeostasis?

Ans: Homeostasis refers to the maintenance of a steady internal environment by organisms.

13. Give an example of suspension? 

Ans: Hibernation is frogs, reptiles, or polar bears. 

14. What is Allen’s rule?

Ans: Mammals living in colder regions have short ears & limbs to minimize heat loss.

15. “Cuckoo bird lays eggs in the nest of crow” which type of interaction is shown in this relation?

Ans: Brood parasitism.

16. Give one function of aerenchyma in aquatic plants?

Ans: Aerenchyma in aquatic plants provides buoyancy & helps them in floating.

17. What does the J-shaped curve indicate?

Ans: The J-shaped curve indicates that the resources are unlimited in a habitat.

18. Name the type of interaction in which one species is harmed while the other is neither benefited or harmed?

Ans: Ammensalism.

19. Why are calotropis plants not browsed by herbivores?

Ans: Calotropis plants are not browsed by herbivores because calotropis plants produce a highly poisonous glycoside that is a cardiac poison & thus, directly kills the predator.

20. What are the two primary requirements of a parasite from the host? 

Ans: Food & shelter.

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

Ans: Predation, where predators prey upon pests & control their number.

22. Write the equation for Verhulst – pearl logistic growth of population. 

Ans:  $ \dfrac{dN}{dt} = rN \left( \dfrac{K - N}{K} \right) $

23. Name the mechanism employed by Ophrys to get its flowers pollinated? 

Ans: Mutualism.

24. List any two factors that determine the nature of the soil.

Ans: The two factors that determine the nature of the soil are Climate & weathering processes. 

Short Questions and Answers (2 Marks Questions)

1. What are the four levels of a biological organization with which ecology basically deals?

Ans: Organisms, populations, communities, and biomes.

2. Differentiate between stenohaline and euryhaline organisms.

Ans: Euryhaline: Organisms tolerant in a wide range of salinities. Stenohaline: Organisms tolerant to a narrow range of salinities.

3. List four features that enable the Xeric plants to survive in desert conditions.

Ans:

(i) Thick cuticle

(ii) Stomata in deep pits

(iii) Stomata closed during daytime

(iv) Leaves are reduced to spines (CAM photosynthetic pathway).

4. Mention the attributes which a population has but not an individual organism.

Ans: Birth rate, Death rate, Sex ratio, age groups.

5. Differentiate between stenothermal and eurythermal organisms.

Ans: Eurythermal: Organisms that can tolerate and thrive in a wide range of temperatures.

Stenothermal: Organisms restricted to a narrow range of temperature.

6. What are the four ways through which living organisms respond to abiotic factors?

Ans: (i) Regulate (ii) Conform (iii) migrate (iv) Suspend 

7. Why do clownfish and sea anemones pair up? What is this relationship called?

Ans: Clownfish live in tentacles of sea Anemone and get protection from predators. Interaction – commensalism.

8. Distinguish between ectotherms & Endotherms?

Ans: Ectotherms are those animals whose body temperature changes & matches with that of the environment in which they are living whereas Endotherms are those animals whose body temperature is maintained relatively constant by physiological regulation.

9. “Lichens are considered good examples of obligate mutualisms”. Comment?

Ans: Lichens show an intimate mutualistic relationship between a fungus & algae or cyanobacterium where the fungus helps in the absorption of nutrients & provides it to bacteria while the algae or cyanobacterium prepares the food.

10. Give any two examples of defense mechanisms in plants against herbivory?

Ans:

i) plants develop certain morphological means of defense e.g. thorns in bougainvillea & spines in cactus.

ii) plants produce & store certain chemicals which function by directly killing them or by inhibiting them from feeding.

11. What is Brood parasitism? Give an example. What adaptation has evolved in this phenomenon?

Ans: Brood parasitism is a phenomenon in which one bird species lays its eggs in the nests of another bird species.

• Evolution has occurred in such a way that the eggs of the parasitic bird resemble those of the host bird in size, color, etc., to avoid the host bird detecting the foreign eggs and ejecting them from the nest.

• For example, the cuckoo bird lays its eggs in the nests of crows.

• Brood parasitism is considered a parasitic type of interspecific interaction because the parasite, i.e., eggs of the cuckoo bird, depend on the crow's nest for food and shelter.

• However, the crow is harmed because there is competition for limited food and shelter between the crow's eggs and the cuckoo's eggs.

• In parasitic interspecific interaction, the parasite is benefited while the host is harmed.

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

Ans: Orchids grow as epiphytes on mango trees. This is an example of commensalism in which orchids are benefited by getting shelter while the tree is neither benefited nor harmed.

13. State Gauss’s competitive exclusion principle?

Ans: Gause’s competitive exclusion principle states that two closely related species competing for the same resources cannot exist together as the competitively inferior one will be eliminated but this is true only when resources are limited & not otherwise.

14. What is migration? Why do animals show this phenomenon?

Ans: Migration is a phenomenon in which organisms can move away temporarily from the stressful conditions in the habitat with hospitable conditions e.g. birds undertake long-distance migration during winter.

15.  How do desert lizards maintain a fairly constant body temperature?

Ans: Desert lizards manage to deal with high temperatures by keeping their body temperature fairly constant by behavioral means. They bask in the sun & absorb heat when their body temperature is below the comfort level & move into the shade when it is higher.

16.  Differentiate between Hibernation & aestivation?

Ans: Hibernation is the phenomenon of spending a cold period in an inactive stage by an animal whereas aestivation is the phenomenon of spending dry & hot conditions in an inactive stage by an animal.

17.  Name the bind of interaction present between the following:-

i. Indian Nightingale & crow

Ans: Indian Nightingale & crow:- Brood parasitism

ii. Nodulated roots & rhizobium

Ans: Nodulated roots & rhizobium:- Mutualism

iii. Plasmodium man

Ans: Plasmodium man:- Parasitism

iv. Orchids & Mango tree

Ans: Orchids & Mango tree:- Commensalism.

18. Define carrying capacity?

Ans: The maximum number of individuals of a population that can be sustained by a given habitat is called its carrying capacity.

19.  If a marine fish is placed in a freshwater aquarium, will the fish be able to survive. Why or why not?

Ans: No, marine fish are unable to survive in a freshwater aquarium because they are adapted to live in saline seawater. They are unable to cope with an outside hypotonic environment because of the Osmoregulation problem.

20. Out of the two population growth models, which one is more realistic & Why?

Ans: A logistic or S-shaped growth curve is more realistic because no population can continue to grow exponentially, as the resource availability becomes limited at a certain point in time.

21. What role do predators play an important role in the ecosystem? 

Ans: Predators play an important role in the ecosystem:-

• They act as conduits for energy transfer to higher trophic levels.

• They Keep the prey population under control which otherwise can reach very high population density.   

• They help in maintaining species diversity in a community.

22. Most living organisms cannot survive at temperatures above 450c. How are some microbes able to live in habitats with temperatures exceeding 1000c.

Ans: Some microbes are able to live in habitats with temperatures exceeding 1000 c because they possess a minimum amount of free water in their body. Removal of water provides resistance to high temperatures.

23. Below is a graph depicting the organismic response to changing external conditions. Name the type of organisms which will show:-

i) pattern A

Ans: Conformers show pattern A where body temperature changes with the ambient temperature.

ii) pattern B

Ans: Regulators show pattern B where body temperature remains constant.

24. Mention any two ways in which organisms tide over unfavorable conditions by suspending their activities.

Ans: 

i) Hibernation – the phenomenon of spending a cold period in inactive stage by an animal

e.g. frogs, reptiles, polar bears.

ii) Aestivation – the phenomenon of spending dry & hot conditions in an inactive stage by an animal e.g. snail, fish.

25. Why are predators “prudent in nature?

Ans: Predators are said to be prudent in nature because if a predator is too efficient & overexploits its prey, then the prey might become extinct & following it the predator will also become extinct for lacking food.

Long Questions and Answers (3 Marks Questions)

1. How does the shape of the age pyramid reflect the growth status of a population?

Ans: The shape of pyramids reflects the growth status of the population (a) growing (b) Stable (c) declining.

2. Darwin showed that even a slow-growing animal like an elephant could reach enormous numbers in absence of checks. With the help of your understanding of growth models, explain when is this possible? Why is this notion unrealistic?

Ans: It is only possible if the growth of the model is Exponential, i.e., having unlimited resources. This notion seems to be an unrealistic situation because resources are limited. Hence, it follows a logistic growth model.

3. How will you measure population density in the following cases?

(i) fish in a lake

Ans: fish caught per trap.

(ii) tiger census in a national park

Ans: number per unit area

(iii) single huge banyan tree with a large canopy.

Ans: percentage cover in biomass.

4. Species facing competition might evolve a mechanism that promotes coexistence rather than exclusion. Justify this statement in light of Gause’s competitive exclusion principle, citing suitable examples.

Ans: State Gause’s competitive exclusion principle. Mechanisms are resource partitioning. E.g., the experiment performed by MacArthur on Warblers.

5. Describe the specific adaptation of xerophytes with respect to the root system, stem   & leaves.

Ans: 

(i) Adaptations in Roots - Well developed and extensively branched long root system is seen in Xerophytes. Though some perennial xerophytes of succulent nature possess extensive but shallow root systems. They seem to absorb water from dewdrops & small rain droplets.

(ii) Adaptations in Stem - Stems of woody xerophytes are comparatively stunted hard & rigid. In the case of acacia, the main stem & branches may occur as a thick, fleshy, flattened & green modified structure called phylloclade.

(iii) Adaptations in Leaves - Leaves are basically short sized which decreases the chances of getting overheated when exposed to solar radiation & thus by reducing the rate of transpiration. Leaves of xerophytes are known to store water, they are usually thick, fleshy green & leathery.

6. List the important characteristics of a population Explain? 

Ans: A population has the following four major characteristics:

(i) Population Density - The size of a population defined with respect to a definite unit of space is known as population density. The maximum limit of density depends upon energy flow in an ecosystem, nutritional status of trophic level & metabolic equilibrium. Population density can

be mathematically calculated as:

(ii) Birth Rate / Natality - The birth rate or natality denotes the produced new number of individuals produced by a natural method in per unit time. 

(iii) Death Rate / Mortality - It denotes the death rate of individuals in the population. It is expressed as the number of individuals dying in a given period. 

(iv) Carrying Capacity - A finite number of organisms can accommodate in a certain space of a particular habitat or ecosystem depending on its size & productivity. This is called carrying capacity.

7. Describe the specific adaptations of hydrophytes with respect to roots, stem & leaves?

Ans: 

(i) Adaptations in Roots - Root systems are haphazardly developed & unbranched, some floating plants or submerged plants lack roots. There is an absence of root hairs except in rooted floating hydrophytes.

(ii) Adaptations in Stem - Stems are long, slender & flexible in submerged hydrophytes whereas in the case of free-floating hydrophytes stems are modified as thick, stout, and stoloniferous occur horizontally on the water surface.

(iii) Adaptations in Leaves - Leaves are thin, long, ribbon shapes and submerged forms. In free-floating plants, the petioles of leaves show indefinite power of growth.

8. Name& explain the bind of interaction in the following.

(i) Algae & fungi in Lichens

Ans: Algae & fungi in lichens- Lichens show an intimate mutualistic interaction with both fungi helps in absorption of nutrients & provides protection, while algae or cyanobacterium prepares the food.

(ii) Head louse & humans

Ans: Head louse & humans- Head louse shows ectoparasites on humans in which head louse is getting nutrition from the human body & is thus benefited while human beings are harmed.

(iii) Hermit crab & sea anemone 

Ans: Hermit crab & sea anemone shows commensalism as hermit crab is benefited because it gets protection from predators which stays away from stinging tentacles of the sea anemone.

9. Mention the different defense mechanisms to reduce the impact of predation? 

Ans: Plant species evolved various defense mechanisms to reduce impact of predation:-

i) Certain insect species & frogs have camouflage or cryptic coloration to avoid detection by their predators.

ii) Some animals like monarch butterflies are highly distasteful to their predators because they accumulate a certain chemical by feeding on poisonous weeds during its caterpillar stage.

iii) Some prey are poisonous & hence are avoided by predators.

iv.) Plants have evolved certain morphological, or chemical defense mechanisms against herbivores e.g. thorns in bougainvillea.

v) plants also produce certain chemicals which function as:-

a. They make animals feel sick. 

b. They may inhibit them from feeding. 

c. They may interfere with digestion. 

d. They may directly kill them.

10. Mutualism often involves the co-evolution of mutualists. Describe taking the example of an animal-plant (wasp-fig) relationship.

Ans: 

• Plants require the help of animals for pollination of their flowers and dispersal of their seeds.

• Animals that help plants in this process must be compensated for the services they provide.

• Plants offer rewards or fees to the animals in the form of seed dispersers.

• Plant-animal interactions often involve co-evolution of the mutualists, which means that the evolution of the flower and its pollinator species occurs together.

• For example, a fig can be pollinated only by its partner wasp species and no other species.

• The female wasp not only uses the fruit as an oviposition site but also uses a growing seed within the fruit to feed her larvae.

• While looking for suitable egg-laying locations, the wasp pollinates the fig inflorescence.

• The fig feeds the wasp some of its growing seeds as nourishment for the developing wasp larvae in exchange for pollination services.

11. How do kangaroo rats live in the absence of water in North American deserts?

Ans: The kangaroo rat in North American deserts is capable of meeting all its water requirements through its internal oxidation of fat, where water is a by-product, it can also concentrate its urine to a minimal volume.

12. How is diapause different from Hibernation?

Ans: Diapauses are the phenomenon of spending unfavorable climatic conditions during their development. Hibernation is a phenomenon of spending the winter in resting or dormant conditions by cold-blooded animals to escape the cold by hiding them in hollow tree trunks or burrows or caves, etc, revealing minimum physiological activity.

13. Study the three representative figures of the age of the pyramid relating to the human population & answer the following question.

(i) Mention the given to the three binds of age profile (a), (b), and (c)

Ans: 

(a) is called young population

(b) is called stable population

(c) is called declining population

(ii) Which one of them is ideal for a population & why?

Ans: Among the three, a stable population is ideal because it has an identical birth death rate.

(iii) How does such an age – profile helps policy-making concerned about our growing population & prepare for future generations.

Ans: Age profile helps policymakers get concerned about our wing population & to make an idea for future population growth so that they make future plans.

14. In the adjacent population growth curve:-

(i) What is the name given to curves (a) & (b)?

Ans: Curve (a) is known as exponential growth curve & curve (b) is known as a logistic growth curve.

(ii) What is the status of food & space in the curve (a) & (b)?

Ans: Food & space is less in curve ‘a’ whereas plenty of food & space is available in curve ‘b’.

(iii) In absence of predators, which curve “a” or “b” would appropriately depict the prey population?

Ans: Curve “a”.

(iv) When does curve ‘b’ change into curve ‘a’?

Ans: When the food resources in a given place become unlimited the curve (b) assumes a J – shape & changes into a curve (a).

Very Long Questions and Answers (5 Marks Questions)

1. What is altitude sickness? What are its causes and symptoms? How does the human body try to overcome altitude sickness?

Ans: Breathlessness at high altitudes. Cause: Low atmospheric pressure at high altitudes due to which the body does not get enough oxygen. Symptoms: Nausea, fatigue, and heart palpitations.

The body adapts by :

(a) increasing red blood cell production

(b) decreasing binding affinity of hemoglobin

(c) by increasing breathing

2. Orchid flower, Ophrys co-evolves to maintain a resemblance of its petal to female bees. Explain how and why does it do so?

Ans: They employ ‘Sexual deceit’. one petal bears an uncanny resemblance to the female of the bee. The male bee is attracted to what it perceives as a female ‘pseudocopulation,’ during which pollen dusted on the male bee is a body. A male bee transfers pollen to another flower when the same bee pseudocopulation with another flower. Ophrys does this in order not to make pollination success reduce unless it co-evolves with the female bee.

3. Describe the exponential growth model of a diagram along with a curve?

Ans: This kind of curve is observed in the case of underpopulation of reindeer growing in a predator-free natural environment having plenty of food. In this case, the curve formed is J- curve the small population first takes time to adjust to the new environment so there is no increase in the population. Once they get adopted they multiply exponentially. This growth & multiplication continues so far the food is available. After some time the food supply becomes less as compared to the population increases. This causes mass starvation & mortality & results in the formation of J Shaped curve.

The J-shaped growth form is described by equation

4. Describe the logistic growth model of the population along with a suitable curve. Why is this curve more realistic?

Ans: The logistic growth curve shows a sigmoid or an S-shaped curve. It has three phases:-

i) Lag-phase:- It is considered to be an early phase showing little or no growth. The lag phase is one in which the under the population of cells adapt to or stabilize with the growth conditions before embarking on their multiplication.

ii) Log phase or Exponential phase:- It is the middle phase of rapid or geometric rise. Once stabilized cells start to multiply rapidly when the small population is stabilized, the multiply becomes faster because of the plenty of food & other requirements of life.

iii) Stationary phase or steady phase:- Soon after the number of food decreases in proportion to the number of cells & this results in the onset of the stationary phase. So in this phase,  the number of new cells formed becomes approximately equal to the number of cells dead & thus there is no net increase in the number of cells.

5. Give an example to show that completely unrelated species can also compete for the same resources?

Ans: Completely unrelated species can also compete for the same resources. For e.g. The visiting flamingoes & the native fishes compete for the same food, i.e., zooplanktons living in shallow lakes of South America.

6. What is the Age pyramid? What are the different types of age pyramids?

Ans: In a population of an organism, the geometrical diagrammatic representation of different age groups is referred to as the Age of pyramids. These are of three types:-

i) Expanding pyramid:- It is a broad-based, triangular pyramid that represents a population containing a large number of young people. It is a rapidly expanding population with a high birth rate.

ii) Stable pyramid:-  A moderate proportion of young to old is represented by this pyramid. As the rate of growth becomes slow & stable i.e.- pre-reproductive & reproductive age groups become more or less equal in size.

iii) Declining Pyramid:- The type of pyramid of population decreasing in size is characterized by a narrow base because there are fewer pre-reproductive individuals than in the other two age categories.

7. Differentiate between regulators & conformers? Why do small animals not show regulations?

Ans: Differentiate between regulators & conformers are:

• Regulators are organisms that maintain homeostasis by physiological or behavioral means and ensure a constant internal environment.

• Regulators include birds, mammals, some lower vertebrates, and invertebrates.

• In summer, when the outside temperature is higher than the body temperature, regulators sweat profusely to cool down the body temperature through evaporative cooling.

• Non-regulators are organisms that cannot maintain a constant internal environment, and their body temperature changes with the ambient temperature.

• The majority of animals and nearly all plants are non-regulators.

• Thermoregulation is an energy-expensive process and small organisms do not show regulation.

• Small animals tend to lose body heat very fast when it is cold outside because of a large surface area relative to volume.

• Small animals have to expend a lot of energy to generate body heat through metabolism.

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