NCERT Class 12 Biology Chapter 13 Organisms and Populations

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NCERT Class 12 Biology Chapter 13 Organisms and Populations

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Organisms and Populations

Chapter: 13

BIOLOGY

UNIT – V ECOLOGY AND ENVIRONMENT

TEXT BOOK QUESTIONS ANSWERS

Q. 1. How is diapause different from hibernation?

Ans: Diapause is a stage of suspended development during unfavorable conditions in Zooplanktons and insects larvae.

Hibernation is winter sleep in animals like bears during winters.

Q. 2. If a marine fish is placed in a fresh water aquarium, will the fish be able to survive? Why or why not?

Ans: Marine fish placed in a fresh water aquarium will not survive due to osmotic changes in its environment. This results in the swelling up of the body leading to the death of marine fish.

Q. 3. Define phenotypic adaptation. Give one example.

Ans: Adaptation is any feature of the organism (morphological, physiological and behavioral) that enables the organism to survive and reproduce in its habitat e.g. Desert plants have leaves with thick cuticle and stomata arranged in deep pits to minimise water loss through transpiration.

Q. 4. Most living organisms cannot survive at temperature above 45°C°. How are some microbes able to live in habitats with temperatures exceeding 100°C?

Ans: Microbes are able to survive in temperatures exceeding 100°C because of adaptation of the organism (morphological, physiological and behavioral) that enables the organism to survive and reproduce in its habitat.

Q. 5. List the attributes that populations but not individuals possess. 

Ans: Population and individual organism has following different attributes:

(i) Individual has births and deaths but a population has birth rates and death rates.

(ii) Individual is either a male or a female but a population has a sex ratio.

Q. 6. If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population? 

Ans: N_t = N₀ e^rt

Q. 7. Name important defence mechanisms in plants against herbivory.

Ans: Defence mechanisms in plants against herbivory: 

(i) Calotropis plant produces poisonous cardiac glycosides that prevent cattle or goats browsing on this plant.

(ii) Chemical substances from plants like nicotine, caffeine, quinine, strychnine, opium, etc. are produced as defenses against grazers and browsers.

(iii) Cactus and Acacia have thorns or spines for defence mechanism.

Q. 8. 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: Interaction between a mango tree and an orchid is an example of commensalism.

Q. 9. What is the ecological principle behind the biological control method of managing with pest insects? 

Ans: Biological control of weeds and pests are based on the ability of the predator to regulate prey population e.g. Prickly pear cactus (in Australia) was brought under control by cactus-feeding moth [predator].

Q. 10. Distinguish between the following:

(a) Hibernation and Aestivation.

Ans: Hibernation and Aestivation: 

Hibernation: is winter sleep in animals like bears during winters.

Aestivation: is summer sleep in animals like snails and fish to avoid heat and dessication in summers.

(b) Ectotherms and Endotherms. 

Ans: Ectotherms and Endotherms:

Ectotherms cannot maintain a constant internal environment and their body temperature changes with surrounding temperature eg. invertebrates and chordates [except birds & mammals] and plants.

Endotherms maintain constant body temperature e.g. birds and mammals.

Q. 11. Write a short note on

(a) Adaptations of desert plants and animals.

Ans: Adaptations of desert plants and animals:

(i) Kangaroo rat in North American deserts meets the water requirements through water produced by internal fat oxidation and passing out concentrated urine to reduce water loss in excretion.

(ii) Desert lizards lie in sun to absorb heat on fall of body temperature below comfort zone and move into shade on increase in surrounding temperature.

(iii) Desert plants have leaves with thick cuticle and stomata arranged in deep pits to minimize water loss through transpiration.

(iv) Desert plants have CAM photosynthetic pathway that enables their stomata to remain closed during day time.

(v) Some desert plants like Opuntia have leaves reduced to spines and photosynthetic flattened stems. 

(b) Adaptations of plants to water scarcity.

Ans: Adaptations of plants to water scarcity:

(i) Desert plants have leaves with thick cuticle and stomata arranged in deep pits to minimize water loss through transpiration.

(ii) Desert plants have CAM photosynthetic pathway that enables their stomata to remain closed during day time.

(iii) Some desert plants like Opuntia have leaves reduced to spines and photosynthetic flattened stems.

(c) Behavioural adaptations in animals.

Ans: Behavioural adaptations in animals:

(i) Organisms show behavioral responses to cope with variations in their environment.

(ii) It is seen in desert lizards. 

(iii) Desert lizards keep their body temperature fairly constant by behavioral means.

(iv) Desert lizards lie in sun to absorb heat on fall of body temperature below comfort zone and move into shade on increase in surrounding temperature.

(d) Importance of light to plants.

Ans: Importance of light to plants:

Plants are autotrophic which need sunlight for photosynthesis. Light affects photoperiodic responses occurring in plants. It plays role in seed germination.

(e) Effect of temperature or water scarcity and the adaptations of animals.

Ans: Effects of temperature or water scarcity and the adaptations of animals.

(i) Adaptations to temperature: 

(a) Allen’s’ Rule: Mammals from colder climates generally have smaller extremities like ears, limbs to minimize heat loss.

(b) Polar seas aquatic mammals like seals have a thick fat layer or blubber underneath their skin that acts as an insulator and reduces loss of body heat. 

(c) Desert lizards lie in sun to absorb heat on fall of body temperature below comfort zone and move into shade on increase in surrounding temperature.

(ii) Adaptation of water scarcity: 

(a) Kangaroo rat in North American deserts meets the water requirements through if water produced by internal fat oxidation and passing out concentrated urine to reduce water loss in excretion. 

Q. 12. List the various abiotic environmental factors. 

Ans: Major Abiotic Factors are temperature, water, light and soil.

Q. 13. Give an example for:

(a) An endothermic animal.

Ans: Endothermic animal:e.g. birds and mammals.

(b) An ectothermic animal.

Ans: Ectothermic animal:e.g. Fishes and amphibians.

(c) An organism of benthic zone. 

Ans: Organism of benthic zone: Benthic zone is the ecological region at the lowest level of a water body. Organisms of benthic zone includes crustaceans and polychaetes.

Q. 14. Define population and community.

Ans: Population is group of interbreeding individuals of a species sharing or competing for similar resources in a defined geographical area.

Community is group of individuals of different inter related populations of different species living in a defined geographical area.

Q. 15. Define the following terms and give one example for each:

(a) Commensalism.

Ans: Commensalism: This is the interaction in which one species benefits and the other is neither harmed nor benefited e.g. Association of cattle egret and grazing cattle.

(b) Parasitism.

Ans: Parasitism: It is relationship between two organisms of different species in which parasite organism gets food directly from host organism e.g. human liver fluke.

(c) Camouflage.

Ans: Camouflage: It is defence mechanism of Preys. Insects and frogs are cryptically colored or camouflaged to avoid being seen by predator.

(d) Mutualism.

Ans: Mutualism: Mutualism is interaction between two organisms of different species which give benefits to both e.g. Lichen is mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.

(e) Interspecific competition.

Ans: Interspecific competition: Interspecific interactions are the interactions of populations of two different species. They could be beneficial, detrimental (harmful) or neutral (neither harm nor benefit) to one of the species or both. It can be Predation, Commensalism, Parasitism, Predation, Amensalism, Competition and Mutualism.

Q. 16. With the help of suitable diagram describe the logistic population growth curve.

Ans: Logistic Growth is given by equation:

dN/dt = r N K -N/K

Where N = Population density at time t 

r = Intrinsic rate of natural increase

K = Carrying capacity

Logistic growth is more realistic model since resources for growth are finite and limiting. 

Q. 17. 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.

Q. 18. List any three important characteristics of a population and explain.

Ans: Population has birth rates and death rates. Birth rate and death rate in population refer to per capita births and deaths respectively.

Population has a sex ratio. Sex ratio is ratio of male and female in a population.