CBSE Class 12 Biology Organisms And Populations Question Bank

Very Short Answer Questions

Question. Pollinating species of wasps show mutualism with specific fig plants. Mention the benefits the female wasps derive from the fig trees from such an interaction.  
Answer. The wasp uses the fruit as oviposition, i.e., egg laying and the developing seeds for nourishing its larvae.

Question. Define population and community.
Answer. Population: Population is a group of individuals of same species, which can reproduce among themselves and occupy a particular area in a given time.
Community: It is an assemblage of several populations in a particular area and time and exhibit interaction and interdependence through trophic relationships.

Question. Name the type of association that the genus Glomus exhibits with higher plants.
Answer. Symbiosis/Mycorrhizae/Mutualism.

Question. Name the interaction between a whale and the barnacles growing on its back. 
Answer. Commensalism

Question. If a population growing exponentially doubles in size in 3 years, what is the intrinsic rate of increase (r) of the population?
Answer. t = log2N/r
or r = log2N/t =0.7931 /3 = 0.2643
Intrinsic rate of increase = 0.2643 × 100 = 26.43%

Question. With the help of suitable diagram describe the logistic population growth curve.
Answer. (ii) Logistic growth
• The resources become limited at certain point of time, so no population can grow exponentially.
• This growth model is more realistic.
• Every ecosystem or environment or habitat has limited resources to support a particular maximum number of individuals called its carrying capacity (K).
• When N is plotted in relation to time t, the logistic growth show sigmoid curve and is also called Verhulst–Pearl logistic growth. It is given by the following equation: and Fig.

Question. What is an interaction called when an orchid grows on a mango plant? 
Answer. Commensalism

Question. Why do people living in high altitude have more haemoglobin/high RBC count? 
Answer. To acclimatise at high altitude, the people have more haemoglobin/high RBC count to compensate for the low oxygen availability.

Question. What do phytophagous insects feed on? 
Answer. Phytophagous insects feed on plant sap and other parts of plant.

Question. What is Allen’s rule?
Answer. According to Allen’s rule, mammals in colder climate have shorter ears and shorter limbs to minimise heat loss.

Question. Give one function of aerenchyma in aquatic plants. 
Answer. Aerenchyma gives buoyancy to the aquatic plants due to presence of air chambers.

Short Answer Questions

Question. Why the plants that inhabit a desert are not found in a mangrove? Give reasons.
Answer. In mangroves the soil is oxygen deficit because of excess water present. Plants in mangroves develop special roots called breathing roots or pneumatophores for respiration. This adaptation is not present in desert plants because of which they cannot survive in mangroves.

Question. Explain brood parasitism with the help of an example.
Answer. Koel is a parasitic bird (which has lost the instinct to make its own nest to lay eggs), has evolved the technique of laying eggs in the nest of a crow. Its eggs bear resemblances to those of crow.

Question. What is mutualism? Mention any two examples where the organisms involved are commercially exploited in agriculture.
Answer. Interaction between two species in which both are benefited is called mutualism.
(i) Rhizobium in the roots (nodules) of legumes.
(ii) Mycorrhiza → Glomus living with the roots of higher plants.

Question. How do plants benefit from having mycorrhizal symbiotic association?
Answer. Mycorrhizal association is found between fungi and the roots of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while plant in turn provides energyyielding carbohydrates to fungi.

Question. (a) How is Cuscuta adapted to be a parasitic plant?
(b) Why do cattle avoid browsing on Calotropis plants? Explain.
Answer. (a) Cuscuta has lost its chlorophyll and leaves during evolution and thus it derives its nutrition from host plant, thus, it is a parasitic plant.
(b) Cattle avoid browsing on Calotropis plants because it produces poisonous cardiac glycosides.

Question. Besides acting as ‘conduits’ for energy transfer across trophic levels, predators play otherimportant roles. Justify. [CBSE Sample Paper 2016] 
Answer. Besides acting as ‘conduits’ of energy transfer across trophic levels, predators play other important roles like
(i) They keep prey population under control.
(ii) Predators also help in maintaining species diversity in a community by reducing the intensity of competition among competing prey species.

Question. Most living organisms cannot survive at temperature above 45°C. How are some microbes able to live in habitats with temperatures exceeding 100°C?
Answer. Most living organisms cannot survive above 45°C because
(a) Above 45°C enzymes get denatured.
(b) Protoplasm precipitates at high temperature.
However, some microbes (Archaebacteria) are found at 100°C because of  (a) reduced fluidity of cell membrane due to presence of branched chain lipids in their cell membrane.
(b) presence of heat-tolerant enzymes.

Question. Explain parasitism and co-evolution with the help of one example of each. 
Answer. Mode of interaction between two species in which one species (parasite) depends on the other species (host) for food and shelter is called parasitism. In this one organism is benefitted and the other is harmed.
For example, Human liver fluke or Malarial parasite or Cuscuta.
Co-evolution is the relationship between two interacting organisms where an organisms fails to survive in the absence of the other.
For example, Fig and fig wasp or Ophrys and bumble bee.

Question. Plants that inhabit a rain-forest are not found in a wetland. Explain. 
Answer. Plants in wetland are adapted differently. The soil in wetland lacks oxygen. So, for respiration the roots have to grow above the soil. These roots are called breathing roots or pneumatophores. This feature is not present in roots of plants growing in rainforest.

Question. Apart from being part of the food chain, predators play other important roles. Mention any two such roles supported by examples. 
Answer. (i) Keeps prey population under control. For example, the invasive prickly. Pear cactus in Australia was brought under control only after a cactus feeding predator (a moth) was introduced in the country.
(ii) Maintains species diversity by reducing intensity of competition among prey species. For example, when the starfish Pisaster was removed from its community of American Pacific Coast, more than 10 species of invertebrates became extinct.

Long Answer Questions

Question. (a) State how the constant internal/environment is beneficial to organisms.
(b) Explain any two alternatives by which organisms can overcome stressful external conditions. 
Answer. (a) Constant internal environment permits all biochemical reaction and physiological functions to proceed with maximal efficiency and thus enhance the overall fitne ss of the species.
(b) Organisms can overcome stressful external conditions with the following ways:
(i) Regulation: Maintaining internal environment by maintaining constant body temperature or osmotic concentration.
(ii) Suspend (conform): By suspending metabolic activities through hibernation or aestivation or diapause.
(iii) Migration: Organisms migrate temporarily to more hospitable areas.

Question. (a) List the biotic components an organism interacts with in its natural habitat.
(b) Mention how have organisms optimised their survival and reproduction in a habitat.
Answer. (a) Plants, animals and microorganisms.
(b) (i) Some organisms regulate to maintain homeostasis by physiological and behavioural means.
(ii) In some animals and plants the osmotic concentration of the body fluids change with that of the ambient water osmotic concentration (Conform).
(iii) Some animals migrate to avoid unfavourable conditions.
(iv) Some bacteria, fungi and lower plants, under unfavourable conditions slow down metabolic rate and form thick-walled spores to overcome stressful conditions (Suspend).

Question. How do organisms which cannot migrate, tend to overcome adverse environmental conditions?
Explain taking one example each from vertebrates and angiosperms respectively.
Answer. Organisms which cannot migrate tend to overcome adverse environmental conditions by developing several methods/features. For example, some vertebrates escape the stress caused by unfavourable environmental conditions by escaping in time like bears go into hibernation during the winter months. In angiosperms, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress. They reduce their metabolic activity and go into an inactive, i.e., ‘dormant’ state. They germinate to form new plant when the favourable conditions return.

Question. Explain with the help of suitable examples the three different ways by which organisms overcome their stressful conditions lasting for short duration. 
Answer. If the stressful conditions remain for short duration, the organism has following alternatives, i.e., either conform, migration, suspension.
(i) Conform: In some animals called conformers, osmotic concentration of body fluids change with that of the ambient water osmotic concentration. For example, small animals have larger surface area relative to their volume. They lose body heat very fast in low temperature. So, they expand energy to generate body heat through metabolism.
(ii) Migration: The temporary movement of organism from the stressful habitat to a more hospitable area and return when stressful period is over is called migration. For example, migratory birds from Siberia come to Keoladeo National Park (Bharatpur) in every winter.
(iii) Suspend: Those animals who fail to migrate, might avoid the stress by escaping in time. Hibernation of bears during winter or aestivation of snails and fish to avoid summer are examples of this phenomenon.

Question. Explain with the help of an example each, any two ways by which the animals cope with the stressful conditions lasting for a short period in their habitat.
Answer. Regulate
QU Some organisms maintain homeostasis by physiological and behavioural means, such organisms are called regulators. All birds and mammals and few lower vertebrate and invertebrate species maintain homeostasis by thermoregulation and osmoregulation.
QU The success of mammals is largely due to their ability to maintain a constant body temperature.
QU In summers, when outside temperature is more than our body temperature, we sweat profusely and the resulting evaporative cooling brings down the body temperature.
QU In winters, when temperature is lower we shiver, a kind of exercise that produces heat and raises the
body temperature.
QU Plants do not have such mechanism to maintain internal temperatures.

Question. How do organisms cope with stressful external environmental conditions which are localised or of short duration? 
Answer. The following methods are employed by organisms to cope with stressful conditions:
(i) Migrate temporarily from the stressful habitat to a hospitable area,
(ii) suspend activities,
(iii) form thick walled spores,
(iv) form dormant seeds,
(v) hibernate during winter,
(vi) aestivate during summer,
(vii) planktons undergo diapause. (Any six)

Question. Highlight the differences between the population interactions given below. Given an example of each.
(a) Parasitism (b) Amensalism (c) Mutualism 
Answer. Table 13.10 Differences between parasitism, amensalism and mutualism

Question. How does the Mediterranean orchid Ophrys ensure its pollination by bees?
Answer. The petals of the Ophrys resembles the female of a bee species in size, colour and odour, etc. Male bee mistakes the Ophrys for female bee and tries to copulate. Few pollen grains adhered with the body of the male bee fall over stigma of the flower thereby leading to pollination.

Question. List any three important characteristics of a population and explain them.
Answer. The three important characteristics of a population are as follows:
(i) Population density: Population density of a species is the number of individuals of a species per unit area or volume.
PD = N/S
where, PD = Population density
N = Number of individuals in a region
S = Number of unit area in a region.
(ii) Birth rate: It is expressed as the number of births per 1,000 individuals of a population per year.
(iii) Death rate: It is expressed as the number of deaths per 1,000 individuals of a population per year.

Question. Different animals respond to changes in their surroundings in different ways. Taking one example each, explain “some animals undergo aestivation while some others hibernate”. How do fungi respond to adverse climatic conditions? 
Answer. Some animals go into aestivation to avoid summer problems like, heat and dessication. For example, snails and fish.
Some animals go into hibernation to avoid winter related problem like, extreme cold. For example, bear. Fungi form thick walled spores and suspend their activities to respond to adverse climatic conditions.

Question. Define the following terms and give one example for each:
(a) Commensalism (b) Parasitism (c) Camouflage
(d) Mutualism (e) Interspecific competition.
Answer. (a) Commensalism: It is an interaction between two different species where one is benefited and other remains unaffected, e.g., clown fish and sea anemone. Here, the clown fish gets protection from predators which stay away from stinging tentacles of anemone but anemone does not derive any benefit from fish.
(b) Parasitism: It is an interaction between two organisms in which one is benefited and the other is harmed, i.e., one organism lives at the cost of other organism. e.g., Cuscuta, a parasitic plant that is found growing on hedge plants, do not have chlorophyll and thus derives its nutrition from the host.
(c) Camouflage: It is a phenomenon of blending of an organism with the surrounding due to similar colour, marking and shape so as to avoid the predators, e.g., leaf-like insect such as grasshopper.
(d) Mutualism: It is the interaction between two species in which both organisms are benefited to maintain the life process, e.g., lichen (association between algae and fungi). Here, fungi helps in absorption of nutrients and water while the algal partner manufactures food.
(e) Interspecific competition: It is the competition among the members of different species for limited natural resources. The Abingdon tortoise in Galapagos Islands became extinct within a decade after goats were introduced on the Island, apparently due to the greater browsing efficiency of the goats.

Please click on below link to download CBSE Class 12 Biology Organisms And Populations Question Bank