NCERT Exemplar Problems Class 12 Biology Chapter 13 Organisms and Populations

Multiple Choice Questions
Single Correct Answer Type
1. Autecology is the
(a) Relation of a population to its environment
(b) Relation of an individual to its environment
(c) Relation of a community to its environment
(d) Relation of a biome to its environment
Answer. (b) Autecology is the relation of an individual to its environment.

2. Ecotone is
(a) A polluted area
(b) The bottom of a lake
(c) A zone of transition between two communities
(d) A zone of developing community
Answer. (c) Ecotone is a zone of transition between two communities.

3. Biosphere is
(a) A component in the ecosystem
(b) Composed of the plants present in the soil
(c) Life in the outer space
(d) Composed of all living organisms present on earth which interact with the physical environment.
Answer. (d) Biosphere is composed of all living organisms present on earth which interact with the physical environment.

4. Ecological niche is
(a) The surface area of the ocean
(b) An ecologically adapted zone
(c) The physical position and functional role of a species within the community
(d) Formed of all plants and animals living at the bottom of a lake.
Answer. (c) Ecological niche is the physical position and functional role of a species within the community.

5. According to Allen’s Rule, the mammals from colder climates have
(a) Shorter ears and longer limbs (b) Longer ears and shorter limbs
(c) Longer ears and longer limbs (d) Shorter ears and shorter limbs
Answer. (d) According to Allen’s Rule, the mammals from colder climates have shorter ears and shorter limbs.

6. Salt concentration (Salinity) of the sea measured in parts per thousand is
(a) 10-15 (b) 30-70 (c) 0-5 (d) 30-35
Answer. (d) The salt concentrations is measured as salinity in parts per thousand, is less than 5 in inland waters, 30-35 in the sea and greater than 100 percent in some hypersaline lagoons.

7. Formation of tropical forests needs mean annual temperature and mean annual precipitation as
(a) 18-25 °C and 150-400 cm (b) 5-15 °C and 50-100 cm
(c) 30-50 °C and 100-150 cm (d) 5-15 °C and 100-200 cm
Answer. (a)

8. Which of the following forest plants controls the light conditions at the ground?
(a) Lianas and climbers  (b) Shrubs
(c) Tall trees (d) Herbs
Answer. (c) Tall trees control the light conditions at the ground.

9. What will happen to a well growing herbaceous plant in the forest if it is transplanted outside the forest in a park?
(a) It will grow normally.
(b) It will grow well because it is planted in the same locality.
(c) It may not survive because of change in its micro climate.
(d) It grows very well because the plant gets more sunlight.
Answer. (c) A well growing herbaceous plant in the forest if it is transplanted outside the forest in a park. It may not survive because of change in its micro climate.

10. If a population of 50 paramoecium present in a pool increases to 150 after an hour, what would be the growth rate of population?
(a) 50 per hour (b) 200 per hour
(c) 5 per hour (d) 100 per hour
Answer. (d)
Initial population of paramecium Pi = 50
After one hour population of paramecium Pf = 150
Growth rate after one hour = Pf – Pi
= 150-50= 100

11. What would be the per cent growth or birth rate per individual per hour for the same population mentioned in the previous question (Question 10)?
(a) 100 (b) 200 (c) 50 (d) 150
Answer. (b)

12. A population has more young individuals compared to the older individuals. What would be the status of the population after some years?
(a) It will decline
(b) It will stabilise
(c) It will increase
(d) It will first decline and then stabilise
Answer. (c) A population has more young individuals compared to the older individuals. It will increase the status of the population after some years.

13. What parameters are used for tiger census in our country’s national parks and sanctuaries?
(a) Pug marks only (b) Pug marks and faecal pellets .
(c) Faecal pellets only (d) Actual head counts
Answer. (b) Sometimes population size is indirectly estimated without actually counting them or seeing them. E.g. ; The tiger census in our National Parks and Tiger Reserves is often based on pug marks and faceal pellets.

14. Which of the following would necessarily decrease the density of a population in a given habitat?
(a) Natality > mortality (b) Immigration > emigration
(c) Mortality and emigration (d) Natality and immigration
Answer. (c) Mortality and emigration would necessarily decrease the density of a population in a given habitat.

15. A protozoan reproduces by binary fission. What will be the number of protozoans in its population after six generations?
(a) 128 (b) 24 (c) 64 (d) 32
Answer. (c)
Population after nth generations = 2n
Population after 6th generations = 26 =64

16. In 2005, for each of the 14 million people present in a country, 0.028 were born and 0.008 died during the year. Using exponential equation, the number of people present in 2015 is predicted as
(a) 25 millions (b) 17 millions
(c) 20 millions (d) 18 millions
Answer. (b)

17. Amensalism is an association between two species where
(a) One species is harmed and other is benefitted
(b) One species is harmed and other is unaffected
(c) One species is benefitted and other is unaffected
(d) Both the species are harmed.
Answer. (b)

18. Lichens are the associations of
(a) Bacteria and fungus (b) Algae and bacterium
(c) Fungus and algae (d) Fungus and virus
Answer. (c) Lichens are the associations of fungus and algae.

19. Which of the following is a partial root parasite? .
(a) Sandal wood (b) Mistletoe
(c) Orobanche (d) Ganoderma
Answer. (a) Sandal Wood is a partial root parasite.

20. Which one of the following organisms reproduces sexually only once in its life time?
(a) Banana plant (b) Mango
(c) Tomato (d) Eucalyptus
Answer. (a) Banana plant organisms reproduce sexually only once in its life time.

Very Short Answer Type Questions
1. Species that can tolerate narrow range of temperature are called
Answer. Stenothermic

2. What are Eurythermic species?
Answer. Species that tolerate wide range of temperature are called Eurythermic . species.

3. Species that can tolerate wide range of salinity are called .
Answer. Euryhaline

4. Define stenohaline species.
Answer. Species that tolerate narrow range of salinity are called stenohaline species.

5. What is the interaction between two species called?
Answer. Interspecific interaction

6. What is commensalism?
Answer. Commensalism is the interaction in which one species benefits and the other is neither harmed nor benefited.

7. Name the association in which one species produces poisonous substance or a change in environmental conditions that is harmful to another species.
Answer. Amensalism

8. What is Mycorrhiza?
Answer. Mycorrhiza is a symbiotic association between a fungus and the roots of higher plants.

9. Emergent land plants that can tolerate the salinities of the sea are called  
Answer. Mangroves

10. Why do high altitude areas have brighter sunlight and lower temperatures as compared to the plains?
Answer. High altitude areas have brighter sunlight because at high altitude there is a very low concentration of dust particles and atmospheric gases which absorbs the sunlight. There is a low atmospheric pressure at high altitudes. Lower atmospheric pressure results in lower temperatures at high altitudes.

11. What is homeostasis?
Answer. To maintain the constancy of internal environment despite varying external environmental conditions is called homeostasis.

12. Define aestivation.
Answer. Aestivation is a state of dormancy characterized by inactivity and a lowered metabolic rate in response to high temperatures and arid conditions.

13. What is diapause and its significance?
Answer. Under unfavourable conditions many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development.

14. What would be the growth rate pattern, when the resources are unlimited?
Answer. Exponential.

15. What are the organisms that feed on plant sap and other plant parts called?
Answer. Phytophagous

16. What is high altitude sickness? Write its symptoms.
Answer. If one had ever been to any high altitude place (>3,500m like Rohtang Pass near Manali and Mansarovar .in Tibetan Autonomous Region), thepathological effect caused by acute exposure to low partial pressure of oxygen at high altitude is called altitude sickness. Its symptoms include nausea, fatigue and heart palpitations.

17. Give a suitable example for commensalism.
Answer. Cattle egret and grazing cattle.

18. Define ectoparasite and endoparasite, and give suitable examples.

  • Parasites that feed on the external surface of the host organism are called ectoparasites. The most familiar examples of this group are the lice on humans and ticks on dogs.
  • Endoparasitesare those that live inside the host body at different sites (liver, kidney, lungs, red blood cells, etc.).’”The human liver fluke (a trematode parasite) is an endoparasite.

19. What is brood parasitism? Explain with the help of an example.
Answer. Brood parasitism in birds is a fascinating example of parasitism in which the parasitic bird lays its eggs in the nest of its host and let the host incubate them. During the course of evolution, the eggs of the parasitic bird have evolved to resemble the host’s egg in size and colour to reduce the chances of the host bird detecting the foreign eggs and ejecting them from the nest.

Short Answer Type Questions
1. Why are coral reefs not found in the regions from. West Bengal to Andhra Pradesh but are found in Tamil Nadu and on the east coast of India?
Answer. High salinity, optimal temperature and less siltation are essential to colonise corals. If siltation and fresh water inflow are very high, the corals don’t colonise. In contrast when the siltation and fresh water inflow by the rivers are very less, the coral do colonise.

2. If a fresh water fish is placed in an aquarium containing sea water, will the fish be able to survive? Explain giving reasons.
Answer. If a fresh water fish is placed in an aquarium containing sea water, it will not be able to survive because of the osmotic problems, they would face. Sea water is hypertonic as compared to fish, so it lost water through exosmosis and die due to dehydration.

3. Why do all the fresh water organisms have contractile vacuoles whereas , majority of marine organisms lack them?
Answer. In majority of fresh water organisms, contractile vacuoles is present which help in osmoregulation (remove excess water from body). In marine organism there is no need of removal of water from body (due to hyptonic condition), hence contractile vacuoles are absent.

4. Define heliophytes and sciophytes. Name a plant from your locality that is either heliophyte or sciophyte.

  • Heliophytes also called sun-loving plants, are those that require for their optimum growth full exposure to the sun. E.g., Mango
  • Sciophytes also called shade-loving plants, are those plants that require reduced light intensity. E.g., Lycopodium

5. Why do submerged plants receive weaker illumination than exposed floating plants in a lake?
Answer. Submerged plants receive weaker illumination than exposed floating plants in a lake because on passing of light through water much more amount of light is lost.

6. In a sea shore, the benthic animals live in sandy, muddy and rocky substrata
and accordingly developed the following adaptations.
a. Burrowing  
b. Building cubes
c. Holdfasts/peduncle
Find the suitable substratum against each adaptation.
Answer. a. Sandy, b. Muddy, c. Rocky

7. Categorise the following plants into hydrophytes, halophytes, mesophytes and xerophytes. Give reasons for your answers.
a. Salvinia b. Opuntia
c. Rhizophora d. Mangifera
Answer. a. Hydrophyte, b. Xerophyte, c. Halophyte, d. Mesophyte

8. In a pond, we see plants which are free-floating; rooted-submerged; footed emergent; rooted with floating leaves. Write the type of plants against each of them.
Answer. a. Submerged, b. Rooted emergent, c. Rooted with floating leaves, d. Free- floating, e. Rooted submerged

9. The density of a population in a habitat per unit area is measured in different units. Write the unit of measurement against the following:
a. Bacteria …………..
b. Banyan …………..
c. Deer …………..
d. Fish …………..
Answer. a. Nos. / Vol; b. Coverage / area; c. Biomass / area; d. Nos. / area; e. Wt. / area

a. Label the three tiers 1, 2, 3 given in the above age pyramid.
b. What type of population growth is represented by the above age pyramid?
Answer. (a)
1. Pre-reproductive pogulation
2. Reproductive population
3. Post-reproductive population
(b) Expanding or growing population

11. In an association of two animal species, one is a termite which feeds on wood and the other is a protozoan Trichonympha present in the gut of the termite. What type of association they establish?
Answer. They shows mutualism.

12. Lianas are vascular plants rooted in the ground and maintain erectness of their stem by making use of other trees for support. They do not maintain direct relation with those trees. Discuss the type of association the lianas have with the trees.
Answer. This association is called commensalism.

13. Give the scientific names of any two micro organisms inhabiting the human intestine.
Answer. 1. Escherichia coli
2. Enterococcusfaecalis

14. What is a tree line?
Answer. When we go up the altitude, beyond a particular height no trees are found and the vegetation comprises only of shrubs and herbs. The altitude beyond which no tree is seen is known as tree line.

15. Define zero population growth rate’. Draw an age pyramid for the same.
Answer. Yes. An inverted bell shaped age pyramid is obtained. The young of pre- reproductive age group individuals are less in number and both pre- reproductive and reproductive stages are in the same level.

16. List any four characters that are employed in human population census.
Answer. 1. Birthrates
2. Death rates
3. Sex ratio
4. Age distribution

17. Give one example for each of the following types.
(a) Migratory animal (b) Camouflaged animal
(c) Predator animal (d) Biological control agent
(e) Phytophagous animal (f) Chemical defense agent
Answer. (a) Migratory animal—Siberian crane, Salmon
(b) Camouflaged animal—Frog, insects
(c) Predator animal—Tiger, sparrow
(d) Biological control agent—Moth (against prickly pear cactus)
(e) Phytophagous animal—Insects like Locusta
(f) Chemical defense agent—Cardiac glycosides produced by Calotropis

18. Fill in the blanks:

19. Observe the set of 4 figures A, B, C and D, and answer the following questions: •
(i) Which one of the figures shows mutualism?
(ii) What kind of association is shown in D?
(iii) Name the organisms and the association in C.
(iv) What role is the insect performing in B?
Answer. (i) Figure ‘A’ shows’ mutualism (plant-animal relationship).
(ii) Figure ‘D’ shows predation (leopard killing deer and eating it)
(iii) Figure ‘C’ shows commensalism (cattle egret and grazing cattle)
(iv) In figure ‘B’ insect is phytophagous that feed on sap of the flower.

Long Answer Type Questions
1. Comment on the following figures 1, 2 and 3:
A, B, C, D, G, P, Q, R, S are species
Answer. Fig. 1: It is a single population.and all individuals are of the same species, i.e. A—Individual interact among themselves and their environment.
Fig. 2: It is a community and it contains three populations of species A, B and C. They interact with each other and their environment.
Fig. 3: It is a biome. It contains three communities of which one is in climax and other two are in different stages of development. All three communities are in the same environment and they interact with each other and their environment.

2. An individual and a population has certain characteristics. Name these attributes with definitions.
Answer. A population has certain attributes that an individual organism does not. An individual may have births and deaths, but a population has birth rates and death rates. In a population these rates refer to per capita births and deaths, respectively. The rates, hence, expressed as change in numbers (increase or decrease) with respect to members of the population.
• Another attribute characteristic of a population is sex ratio. An individual is either a male or a female but a population has a sex ratio (e.g., 60 per cent of the population are females and 40 per cent males).
• A population at any given time is composed of individuals of different ages. If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid. For human population, the age pyramids generally show age distribution of males and females in a combined diagram. The shape of the pyramids reflects the growth status of the population (a) whether it is growing, (b) stable or (c) declining.

3. The following diagrams are the age pyramids of different populations. Comment on the status of these populations.
Answer. Fig. A: It is a pyramid shaped age pyramid. In this figure, the base, i.e., pre-reproductive stage is very large when compared with the reproductive
and past reproductive stages of the population. This type of age structure indicates that the population would increase rapidly.
Fig. B: It is an inverted bell shaped pyramid. In this figure, the pre- reproductive and reproductive stages are same. This type of age structure indicates that the population is stable.
Fig. C: It is ‘Urn’ shaped pyramid. In this figure, the pre-reproductive and reproductive stages are less than the post reproductive stage of this population. In this population, more older people are present. This type of age structure indicates that the population definitely is declining,

4. Comment on the growth curve given below.
Answer. A population growing in a habitat with limited resources show initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity. A’ plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth and is described by the following equation:

5. A population of Paramoecium caudatum was grown in a culture medium. After 5 days the culture medium became pvercrowded with Paramoecium and had depleted nutrients. What will happen to the population and what type of growth curve will the population attain? Draw the growth curve.
Answer. It shows logistic growth. (See Ans no. 4)

6. Discuss the various types of positive interactions between species.
Answer. Both the species benefit in mutualism. The interaction where one species is benefitted and the other is neither benefitted nor harmed is called commensalism.

7. In an aquarium two herbivorous species of fish are living together and feeding on phytoplanktons. As per the Gause’s Principle, one of the species is to be eliminated in due course of time, but both are surviving well in the aquarium. Give possible reasons.
Answer. Each species has a specific position or functional role within the community, called niche. According to the Gausse’s principle, no two species can live in the same niche. In this case, two herbivorous species are living in the same niche and feeding on phytoplanktons. It may be because of the availability of sufficient phytoplanktons and or less number of individuals of the fish species. Of the two species might have occurred and though neither of the species have been eliminated, niche overlapping may effect the growth and development of individuals of the species.

8. While living in and on the host species, the animal parasite has evolved certain adaptations. Describe these adaptations with examples.
Answer. In accordance with their life styles, parasites evolved special adaptations such as the loss of unnecessary sense organs, presence of adhesive organs or suckers to cling on to the host, loss of digestive system and high reproductive capacity. The life cycles of parasites are often complex, involving one or two intermediate hosts or vectors to facilitate parasitisation of its primary host.

9. Do you agree that regional and local variations exist within each.biome? Substantiate your answer with suitable example.
Answer. Yes, regional and local variations exist within each biome. Regional and local variations within each biome lead to the formation of a wide variety of habitats. On planet Earth, life exists not just in a few favourable habitats but even in extreme and harsh habitats-^scorching Rajasthan desert, perpetually rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, permafrost polar regions, high mountain tops, boiling thermal springs, and stinking compost pits, to name a few. Even our intestine is a unique habitat for hundreds of species of microbes..

10. Which element is responsible for causing soil salinity? At what concentration does the soil become saline?
Answer. Soil salinity is the salt content in the soil. Salts are a natural component in soils and water. The ions responsible for salination are: Na+, K+, Ca2+, Mg2+ and Cl.

11. Does light factor affect the distribution of organisms? Write a brief note giving suitable examples of either plants or animals.
Answer. Since plants produce food through photosynthesis, a process which is only possible when sunlight is available as a source of energy, we can quickly understand the importance of light for living organisms, particularly autotrophs. Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesise optimally under very low light conditions because they are constantly overshadowed by tall, canopied trees. Many plants are also dependent on sunlight to meet their photoperiodic requirement for flowering. For many animals too, light is important in that they use the diurnal and seasonal variations in light intensity and duration (photoperiod) as cues for timing their foraging, reproductive and migratory activities.

12. Give one example for each of the following:
(i) Eurythermal plant species ………………
(ii) A hot water spring organism ………………
(iii) An organism seen in deep ocean trenches ………………
(iv) An organism seen in compost pit ………………
(v) A parasitic angiosperm ………………
(vi) A stenothermal plant species ………………
(vii) Soil organism ………………
(viii) A benthic animal ………………
(ix) Antifreeze compound seen in antarcticfish ………………
(x) An organism which can conform ………………
Answer. (i) Eurythermal plant species—Red algae
(ii) A hot water spring organism—Thermus aquaticus
(iii) An organism seen in deep ocean trenches—Sea cucumbers
(iv) An organism seen in compost pit—Earthworm
(v) A parasitic angiosperm—Cuscuta reflexa
(vi) A stenothermal plant species—Conifers
(vii) Soil organism—Earthworm
(viii) A benthic animal—Crabs, Sponges
(ix) Antifreeze compound seen in Antarctic fish—Antifreeze glycoproteins orAFGPs
(x) An organism which can conform—Frog

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