CBSE Class 12 Biology Organisms And Population Worksheet Set B

Question. The maintenance of internal favourable conditions, by a self regulated mechanisms inspite of the fact that there are changes in environment, is known as 
(a) entropy
(b) enthalpy
(c) homoeostasis
(d) steady state
Answer : C

Question. The rate of formation of new organic matter by rabbit in a grassland, is called 
(a) Net productivity
(b) Secondary productivity
(c) Net primary productivity
(d) Gross primary productivity
Answer : B

Question. If 4 individuals in a laboratory population of 40 fruitflies died during a specified time interval (i.e., a week), the death rate in the population during that period is 
(a) 1
(b) 0.1
(c) 0.01
(d) 0.4
Answer : B

Question. Territoriality occurs as a result of
(a) competition
(b) parasitism
(c) predation
(d) co-operation
Answer : A

Question. July 11 is observed as 
(a) World Population Day
(b) No Tobacco Day
(c) World Environment Day
(d) World Health Day
Answer : A

Question. Keystone species deserve protection because these 
(a) are capable of surviving in harsh environmental conditions.
(b) indicate presence of certain minerals in the soil.
(c) have become rare due to overexploitation.
(d) play an important role in supporting other species.
Answer : D

Question. Which one of the following correctly represents an organism and its ecological niche ? 
(a) Vallisneria and pond
(b) Desert locust (Schistocerca) and desert
(c) Plant lice (aphids) and leaf
(d) Vultures and dense forest
Answer : C

Question. Carrying capacity is 
(a) the capacity of an individual to produce young ones.
(b) availability of resources in a given habitat to support a certain no of individuals of population, beyond which no further growth is possible.
(c) gene frequency from one generation to next.
(d) gene frequency in same generation.
Answer : B

Question. A lizard-like member of reptila is sitting on a tree with its tail coiled around a twig. This animal could be 
(a) Hemidactylus showing sexual dimorphism
(b) Varanus showing mimicry
(c) Garden lizard (Calotes) showing camouflage
(d) Chamaeleon showing protective colouration
Answer : D

Question. Which one of the following is a matching pair of certain organism(s) and the kind of association?
(a) Shark and sucker fish - Commensalism
(b) Algae and fungi in lichens - Mutualism
(c) Orchids growing on trees - Parasitism
(d) Cuscuta (dodder) growing on other - flowering plants - Epiphytism
Answer : B

Question. A population of 500 that experiences 55 births and 5 deaths during a one-year period. What is the reproductive rate for the population during the one-year period ? 
(a) 0.01/year
(b) 0.05/year
(c) 0.1/year
(d) 50/year
Answer : C

Question. The figure given below is a diagrammatic representation of response of organisms to abiotic factors. What do A, B and C represent respectively?

       A                 B                       C
(a) conformer    regulator      partial regulator
(b) regulator      partial          conformer 
                        regulator
(c) partial         regulator       conformer
    regulator
(d) regulator    conformer     partial regulator
Answer : D

Directions  : Each of these questions contains an Assertion followed by Reason. Read them carefully and answer the question on the basis of following options. You have to select the one that best describes the two statements.
(a) If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.
(c) If Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.

Question. Assertion : Thick cuticle is mostly present in disease resistant plants.
Reason : Disease causing agents cannot grow on cuticle and cannot invade the cuticle.
Answer : A

Question. Assertion : In sigmoid growth curve, population finally stabilizes itself.
Reason : Finally, the death rate increases than the birth rate. 
Answer : C

Question. Assertion : Tropical rain forests are disappearing fast from developing countries such as India.
Reason : No value is attached to these forests because these are poor in biodiversity.
Answer : C

Question. Assertion : Flora contains the actual account of habitat and distribution of plants of a given area.
Reason : Flora helps in correct identification.
Answer : B

Question. Assertion : Species are groups of potentially interbreeding natural populations which are isolated from other such groups.
Reason : Distinctive morphological characters are displayed due to reproductive isolation.
Answer : B

Question. Give example of an organism that enters ‘diapause’ and why. 
Answer : Bombyx mori (silk moth) is an insect that enters diapause due to some adverse environmental conditions such as drought, extreme temperature, reduced food availability; which, in turn, delays the overall development. The physiological and metabolic activities diminish at this particular time.

Question. Why are green algae not likely to be found in the deepest strata of the ocean? 
Answer : Green algae are photosynthetic in nature. In the deepest strata of the ocean i.e., benthic zone, light does not penetrate therefore, this zone is in perpetual darkness and photosynthetic organisms such as green algae cannot survive in this region.

Question. Mention how do bears escape from stressful time in winter. 
Answer : Bears undergo hibernation to escape from stressful time in winter. In hibernation, they seek a warm shelter and remain dormant, their respiration rate turns low and they consume stored food.

Question. How do seed bearing plants tide over dry and hot weather conditions?
Answer : Mesophytic seed bearing plants sometimes have to face hot and dry weather conditions. They survive such adverse conditions by forming underground perennating structures such as corms, rhizomes, tubers, etc. Xerophytic plants on the other hand have to face hot and dry conditions throughout the year. They show various adaptations like sunken stomata, fleshy organs, leaves reduced to spines, extensive root system, etc.

Question. How do snails escape from stressful time in summers? 
Answer : Snails undergo aestivation to escape from stressful time in summers.

Question.Why are some organisms called as eurythermals and some others as stenohaline?
Answer : Some organisms can tolerate a wide range of temperature variations, e.g., most mammals and birds. They are called eurythermals while other organisms live within narrow range of temperature because of their requirement of nearly constant tempeture throughout the year e.g., polar bear, lizards, amphibians, and are called stenothermals

Question. Why green plants are not found beyond a certain depth in the ocean? 
Answer : Green plants are photosynthetic and show autotrophic mode of nutrition. They require sunlight to photosynthesise. Light does not penetrate beyond a certain depth in oceans and hence very deep layers remain in perpetual darkness. In the absence of sunlight green plants cannot manufacture their food and hence cannot survive. Therefore, they are found in those regions of the ocean where sunlight is available.

Question. Mention any two activities of animals which get cues from diurnal and seasonal variations in light intensity. 
Answer : Most animals are active during a particular period of the day, e.g., butterflies and most birds are active during day time hence called diurnal whereas few animals like rat, cockroaches and birds like owl are active during night hence called nocturnal. Various activities like flowering in plants and migration in birds and animlas are affected by seasonal variations.

Question. How does camouflage help an insect?
Answer : Camouflage is the ability of an organism to blend with the surroundings or background. It is the most common type of adaptation in animals. It is advantageous to insects whether they act as prey or predator. Prey insects can easily hide from their predators and escape from them whereas predator insects can also hide from their prey and remain unnoticed so that easily attack their prey.

Question. When and why do some animals like frogs hibernate? 
Answer : Hibernation or winter sleep is quite common in ectothermal (cold blooded) animals like frog. This is because they are unable to regulate their body temperature which changes with the fluctuating external environment. Such animals pass the winter period in dormant condition, resting in warm places. Hibernation is necessary for these animals to prevent their metabolic rate from slowing down to levels which could be harmful to them.

Question. Which one of the two, stenothermals or eurythermals, show wide range of distribution of earth and why?
Answer : Eurythermal organisms are those organisms which can tolerate wide range of temperature variations and thus show wide distribution on earth.

Question. When and why do some animals like snails go into aestivation? 
Answer : When animals like snails are exposed to very high temperatures, they go into aestivation to avoid the heat of summer.

Question. Why is the polar region not a suitable habitat for tiny humming birds? 
Answer : Polar region is not a suitable habitat for humming bird. Because of larger surface area relative to volume it tends to lose heat very fast in comparision to large sized animals hence it will have to spend more energy in maintaining its body temperature.

Question. Between amphibians and birds, which will be able to cope with global warming? Give reason.
Answer : Birds are better able to cope with global warming then amphibians because birds are eurythermal and can tolerate wide range of temperature whereas amphibians are stenothermal and cannot tolerate much fluctuation in the ambient temperature.

Question. How do herbs and shrubs survive under the shadow of big canopied trees in forests?
Answer : Herbs and shrubs survive under the shadow of big canopied trees in forests as they are perfect shade tolerant plants showing better growth in lower level of light intensity and are arranged in different strata according to their shade tolerance.

Question. List any two physiological responses that help you to gradually get acclimatised to high altitudes when you go from the planes.
Answer : (i) Enhancement in RBCs production to compensate low oxygen availability
(ii) Increase in breathing rate.

Question. When an organism is called a ‘conformer’?
Explain with the help of an example.
Answer : The organism in which osmotic concentration of body fluids and body temperature changes according to ambient conditions is called conformer. E.g. in aquatic animal Asterias, the osmotic concentration of body fluids changes according to the osmotic concentration of the surrounding water.

Question. Write the normal body temperature of humans.
How is it maintained during summers?
Answer : Normal body temperature of humans is about 37°C. Humans maintain constant body temperature by homeostasis. During summer, when external temperature rises, we begin to sweat profusely. As sweat evaporates, cooling of body occurs.

Question. How do desert lizards cope with temperature variations in their environment? Explain.
Answer : Desert lizards lack the physiological ability that mammals have to deal with the high temperature. They keep their body temperature fairly constant by behavioural means. They enjoy in the sun and absorb heat when their body temperature drops below the comfort zone, but move into shade when the surrounding temperature starts increasing. Some species are capable of burrowing into the soil to hide and escape from too much heat.

Question. Bear hibernates whereas some species of zooplanktons enters diapause to avoid stressful external conditions. How are these two ways different from each other?
Answer : Bears undergo hibernation during winters to escape extreme cold. It is characterised by low body temperature, slow breathing and heart rate and low metabolic rate. However, diapause is a stage of suspended development or growth occurring in many insects and other invertebrates during which metabolism is greatly decreased. Diapause is often triggered by seasonal changes and regulated by inborn rhythm.

Question. Why do we experience shivering during winters when the temperature is very low?
Answer : When the ambient temperature is very low, our body starts shivering. It is an exercise that raises body temperature and helps to maintain constant internal body temperature at about 37°C, by mechanism of homeostasis.

Question. Humming birds live among the bushes in tropics while penguins live on icebergs. They cannot survive if their habitats are reversed. Justify.
Answer : Hummingbird is a small animal that has large surface area as compared to volume. In colder environment, it will lose heat very fast and will have to spend more energy in maintaining internal body temperature as compared to large sized animals. It is because of this reason small sized animals like humming bird do not occur in polar regions. Penguins found in polar regions have narrow and acuminate wings as compared to broader wings of birds of warmer areas and possess thick fur, subcutaneous fat and small extremities which help in conservation of heat so if they inhabit warm tropical areas they won’t be able to dessipate extra heat and will suffocate and die.

Question. How does the human body maintain constant temperature both in summers and winters? Explain. 
Answer : When the ambient temperature is very low, our body starts shivering. It is an exercise that raises body temperature and helps to maintain constant internal body temperature at about 37°C, by mechanism of homeostasis.

Question. Name the interaction in each of the following:
(a) Ascaris worms living in the intestine of human
(b) Sucker fish attached to the shark
(c) Smaller barnacles disappeared when Balanus dominated in the coast of Scotland
(d) Wasp pollnating fig inflorescence
Answer : (a) Endoparasitism
(b) Commensalism
(c) Competition
(d) Mutualism

Question. What does S-shaped pattern of population growth represent? How is J-shaped pattern different from it and why? 
Answer : S-shaped pattern of population growth represents logistic growth, in which population grows in sigmoid fashion as resources are limited. Population growth is slow initially, then becomes very rapid and finally slows down as environmental resistance increases.
J-shaped pattern shows exponential growth. In this, population increases rapidly and at the peak, population growth ceases abruptly due to environmental resistance.

Question. Name and explain the type of interaction that exists in mycorrhizae and between cattle egret and cattle. 
Answer : Mycorrhiza is a mutualistic interaction between fungus and roots of higher plants. The root provides food and shelter to the fungus. The fungus helps the plant in solubilisation and absorption of minerals, water uptake and protection against pathogenic fungi.
The egret and grazing cattle in close association is an example of commensalism. Commensalism is the interaction in which one organism is benefitted and other organism is neither harmed nor benefitted. The egrets always forage close to where the cattle are grazing because the cattle, as they move, stir up and flush out from the vegetation insects that otherwise might be difficult for the egrets to find and catch.

Question. Predation is usually referred to as detrimental association. State any three positive roles that a predator plays in an ecosystem. 
Answer : Predators play important role in ecosystem. These are discussed as follows:
(i) Maintaining prey population : In nature, the population of predator is quite small as compared to that of the prey. The prey has high reproductive potential. If, for some time, the prey population is allowed to grow without predation, then it would grow beyond the carrying capacity of the environment. The predator keeps the population of the prey under check so that an equilibrium is maintained. Example, the prickly pear cactus introduced in Australia in the early 1920’s caused havoc by spreading rapidly into millions of hectares of rangeland. Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced into the country.
(ii) Maintaining species diversity : Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species. Example, in the rocky intertidal communities of the American Pacific Coast, the starfish Pisaster is an important predator.
When all the starfish were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year because of interspecific competition.
(iii) Vegetation : Predation helps in growth of vegetation all over the globe by restricting population of herbivores.

Question.Explain parasitism and coevolution with the help of one example of each. 
Answer : Parasitism is the interspecific interaction where one of species (called parasite) depends on the other species (host) for food and shelter and damages the host. E.g., malarial parasite in blood cells of humans. Coevolution in parasitism refers to the process in which parasite evolves mechanism to interact and neutralise the mechanism evolved by the host to reject or resist parasite.

Question. Differentiate between mutualism, parasitism and commensalism. Provide one example for each of them.
Answer : Differences between mutualism, commensalism and parasitism are as follows:

Question. Egrets are often seen along with grazing cattle. How do you refer to this interaction? Give a reason for this association.
Answer :  The egret and cattle grazing in close association is an example of commensalism. Commensalism is the interaction in which one organism is benefitted and other organism is neither harmed nor benefitted. The egrets always forage close to where the cattle are grazing because the cattle, as they move, stir up and flush out from the vegetation insects that otherwise might be difficult for the egrets to find and catch.

Question. (a) What is “r” in the population equation given below:
dN/dt = rN
(b) How does the increase and the decrease in the value of ‘r’ affect the population size?
Answer : (a) In the population equation dN/dt = rN , ‘r’ is the intrinsic rate of natural increase. dt
(b) It is a very important parameter chosen for assessing impacts of any biotic or abiotic factor on population growth. Its value depends upon the birth rates and death rates.

Question. In a pond there were 40 lotus plants. After a year the number rose to 56. Calculate birth rate of a lotus plant. 
Answer : Initial number of lotus plants = 40
Current population = 56
No. of lotus plants added = 16
Birth rate = 16/40 = 0.4 lotus plants per year

Question. Name the interaction in each of the following:
(a) Cuckoo lays her eggs in the crow’s nest.
(b) Orchid grows on a mango tree.
(c) Ticks live on the skin of dogs.
(d) Sea anemone is often found on the shell of hermit crab.
Answer : (a) Brood parasitism
(b) Commensalism (c) Ectoparasitism (d) Commensalism

Question. Name the interaction in each of the following:
(a) Cuscuta growing on a shoe flower plant
(b) Mycorrhizae living on the roots of higherplants
(c) Clown fish living among the tentacles of sea anemone 
Answer : (a) Parasitism
(b) Mutualism
(c) Mutualism