Organisms and Populations Class 12 Biology Exam Questions

Please refer to Biology For Organisms and Populations Class 12 Biology Exam Questions provided below. These questions and answers for Class 12 Biology have been designed based on the past trend of questions and important topics in your class 12 Biology books. You should go through all Class 12 Biology Important Questions provided by our teachers which will help you to get more marks in upcoming exams.

Class 12 Biology Exam Questions Organisms and Populations

Objective Questions

Question. Find out the false one –
(a) Texture of soil depends upon of size of mineral particles
(b) Soil aeration is inversely proportional to water holding capacity
(c) Waterlogged soil does not suit plants as it creates anaerobic environment
(d) Availability of minerals in soil is independent of soil pH

Question. Find out the correct match with reference to their habitat –
(a) Mango tree – Canada
(b) Snow leopards – Kerela forest
(c) Tuna fish – Temperate latitudes in oceans
(d) Lion – Gujarat

Question. The famous ‘Australian havoc’ is associated with which of the following invasive species :-
(a) Nile pearch
(b) Princkly pear cactus
(c) Red fox
(d) Rabbit

Question. Mark the incorrect statement.
(a) Many fishes thrive in Antarctic water where temperature is always below zero .
(b) Microbes can survive in hot springs where temperature exceeds 100°C.
(c) Fishes can survive even at a depth where pressure exceeds 100 atm.
(d) Desert lizards have marvelous physiological ability to survive scorching heat of desert.

Question. What four factors define population growth?
(a) Births; deaths, immigration, emigration
(b) Survivorship, age-specific mort ty, fecundity, death rate
(c) Mark-recapture, census, quadrate sampling, transects
(d) Age-specific birth rate, metapopulation structure, quadrate, ecotone

Case Based MCQs

Case I : Read the following passage and answer the questions from given below.
Organism P has thick lips and tongue so that it can easily feed on the commonly available spiny plants. Organism Q has thick layer of insulating fat under the skin. It was strong hooves to walk steadily on steep surfaces and lives in burrows during winters. Organism R has bright colours and sticky pads on its fingers and toes. It lives on trees.

Question. Which of the following is correct habitat for organisms P regarding its adaptation?
(a) Grassland biome
(b) Desert biome
(c) Tropical rainforest
(d) Tropical deciduous forest

Question. Which of the following is incorrect regarding organisms R’s habitat?
(a) The vegetation shows stratification
(b) Epiphytic growth is rich
(c) Standing crop is highest
(d) Deep rooted shrubs are common due to abundant sunlight

Question. Organisms P, Q and R respectively most likely occur in

(a) F, B and A
(b) C, A, E
(c) A, F and C
(d) B, D and A.

Case II : Read the following passage and answer the questions from given below.
Growth of a population with time shows specific and predictable patterns. Two types of growth
patterns of population are exponential and logistic growth. When resources in the habitat are
unlimited each species has the ability to realise fully its innate potential to grow in number. Then
the population grows in exponential fashion.
When the resources are limited growth curve shows an initial slow rate and then it accelerates
and finally slows giving the growth curve which is sigmoid.

Question. Which of the following equations correctly represents the exponential population growth curve?
(a) dN/dt = rN
(b) dN/dt=rN(K −N) K
(c) N_t = N₀e^rt
(d) Both (a) and (c)

Question. The population growth is generally described by the following equation:
dN/dt = rN (K-N)/K
What does ‘r’ represent in the given equation?
(a) Population density at time ‘t’
(b) Intrinsic rate of natural increase
(c) Carrying capacity
(d) The base of natural logarithm

Assertion & Reasoning Based MCQs

Two statements are given-one labelled Assertion and the other labelled Reason.
Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
(a) Both assertion and reason are true and reason is the correct explanation of assertion.
(b) Both assertion and reason are true but reason is not the correct explanation of assertion.
(c) Assertion is true but reason is false.
(d) Assertion is false but reason is true.

Question. Assertion : Microclimate generally differs from the prevailing regional climatic conditions.
Reason : Microclimate represents the climatic conditions that prevail at local scale or in areas
of limited size.

Question. Assertion : Ectotherms are able to remain active under cold conditions.
Reason : Ectotherms are unable to maintain a constant internal temperature.

Question. Assertion : With increase in population size, environmental resistance tends to increase.
Reason : This is a nature’s way to check the expression of biotic potential.

Question. Assertion : Heliophytes, generally have low photosynthetic, respiratory and metabolic
activities.
Reason : Heliophytes are the sun adapted plants which are adapted to high intensity of light.

Question. Assertion : Mimicry is the resemblance of one organism to another.
Reason : Mimicry may be protective or aggressive.

Very Short Answer Type Questions

Question. Soil horizons A and B represent solum. Why?
Answer : Only A and B horizons of soil represents solum or true soil because they have weathered products of the parent rock. 

Question. Name the type of interaction that exists between barnacles and whale.
Answer : Commensalism 

Question. In a population, per capita birth rate is 0.025 and per capita death rate is 0.008 during a unit time period. What is the value of intrinsic rate of natural increase, ‘r’ for the population?
Answer : Intrinsic rate of natural increase, r = Per capita birth rate – Per capita death rate = 0.025 – 0.008 = 0.017. 

Question. Give one example where population estimation of an organism is done indirectly without actually counting the organism.
Answer : Sometimes population size is indirectly estimated without actually counting them, for example, tiger census in our National parks and tiger reserves is often based on pug (animal’s foot print) marks and faecal pellets. 

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. 

Short Answer Type Questions

Question. What type of interaction is seen when koel lays eggs in crow’s nest? 
Answer : Koel or cuckoo laying its eggs in crow’s nest is an example of brood parasitism. In brood parasitism, the parasitic bird lays its eggs in the nest of its host, and the host incubates them.Here, the eggs of the parasitic bird have evolved to resemble the host’s egg (in size and colour) so as to avoid being detected.

Question. Draw and explain expanding age pyramids of human population. Why is it so called?
Answer : An age pyramid for expanding human population is as follows:

Pyramid with broad base or triangular shape indicates a rapidly expanding population with a high percentage of prereproductive individuals followed by reproductive then postreproductive individuals. Thus, in rapidly growing population, birth rate is high and population keeps growing.

Question. What does the given age pyramid signify about the status of a population? (The bar at the base represents pre-reproductive individuals.)

Answer : Given is a bell-shaped age pyramid which signifies that the population is stable. Such age pyramid is formed when the number of pre-reproductive and reproductive individuals is almost equal and the post-reproductive individuals are comparatively fewer. It implies that the population is neither decreasing nor increasing, instead is maintained at a stable level.

Question. Explain why very small animals are rarely found in polar region.
Answer : Very small animals have large surface area to body volume ratio. It results in excessive heat loss from exposed body surface. Such a great extent of heat loss makes it impossible for very small animals to survive in cold polar regions. Large animals have small surface area to body volume ratio, reducing heat loss and making temperature maintenance easier for them. This effect of temperature on the absolute size of an animal and the relative proportions of various body parts is also known as Bergmann’s rule.

Question. Why do clown fish and sea anemone pair up? What is this relationship called? 
Answer : Mutualism is the interaction between clown fish and sea anemone. The clownfish lives among the stinging tentacles of sea anemone and gets protection from its predators.
Clownfish has a slimy mucus covering that protects it from the poisonous tentacles of sea anemone. Also clown fish makes its meals from the anemone’s leftover. In return clown fish helps anemone in catching its prey by luring other fish towards it.
It also eats the dead tentacles keeping the anemone and the area around it clean.

Question. Why the plants that inhabit a desert are not found in a mangrove? Give reasons.
Answer : Plants inhabiting desert (xerophytes) are not found in mangroves, because xerophytic plants are adapted to dry and hot environment. They possess various physical modifications to tolerate extreme water scarcity and heat, like extensive root system, succulent organs, leaf reduced to spine, etc. Mangrove
swamp is a region of vegetation where soil is highly saline and water logged. Only halophytes can survive in such regions as they possess aerial roots called pneumatophores through which gaseous exchange occurs. Roots of xerophytes are positively geotropic and will suffocate and die in such badly aerated soil
ultimately leading the whole plant to death.

Question. Define the following:
(a) Ecological niche
(b) Gause’s competitive exclusion principle
(c) Mimicry
Answer : (a) Ecological niche (Grinnel, 1917) is a specific part of habitat occupied by individuals of a species which is circumscribed by its range of tolerance, range of movement, microclimate, type of food and its availability, shelter, type of predator and timing of activity.
(b) Gause’s competitive exclusion principle states that two or more species with similar niche requirements cannot coexist indefinitely in the same area and one of the two gets eliminated.
(c) Animals develop strategies to live better in their environment. Mimicry is the resemblance of one species with another in order to obtain advantage, especially against predation.

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. 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 solublisation 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. When you go for a trek/trip to any high altitude places, you are advised to take it easy
and rest for the first two days. Comment, giving reasons.
Answer : Atmospheric pressure is low at higher altitudes as compared to plains. When we go for a trek/trip on high altitude, then due to low atmospheric pressure our body does not get enough oxygen, as a result of which we experience nausea, fatigue and heart palpitation (altitude sickness). But by taking rest for first two days, body gets acclimatised to high altitude conditions. The body compensates low oxygen availability by increasing red blood cell production, decreasing binding capacity of haemoglobin and increasing breathing rate. Hence, we automatically stop experiencing altitude sickness.

Question. Water is very essential for life. Write any three features both for plants and animals which enable them to survive in water scarce environment.
Answer : Water is very essential for life. Plants and animals show modifications according to availability of water in the area. Some of the adaptations seen in plants which enable them to survive water scarce environment are as follows:
– Plants of hot deserts are adapted to survive in dry conditions of soil and high temperatures. The plant which evade dry conditions are known as ephemerals. Some plants have deep tap root which are capable of absorbing water from deep soil e.g., Prosopis, Acacia, etc.
– In case of cacti and succulents, the presence of fleshy leaves and stems to store water (succulence) is an adaptation to dry environment. In cacti, leaves are reduced to spines, where stems are modified to fleshy structures.
– Many tropical plants, particularly grasses which grow in hot and arid climates possess C4 pathway of photosynthesis. So, these plants perform better in low soil water environments. Such plants, use less water to achieve higher rates of photosynthesis.
Some of the adaptations seen in animals which enable them to survive water scarce environment are as follows:
– 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.
– The Kangaroo rats conserves water by excreting solid urine and can live from birth to death without even drinking water. Loss of water is minimised by producing nearly solid urine and faeces.
– The camels show tolerance to wide fluctuations in body temperature and are able to maintain blood stream moisture even during extreme heat stress.

Question. Some organisms suspend their metabolic activities to survive in unfavourable conditions.
Explain with the help of any four examples.
Answer : To tide over unfavourable conditions, some organisms suspend their metabolic activities. These are discussed as follows :
(i) Bacteria, fungi and lower plants develop thick walled spores, which germinate during suitable conditions.
(ii) Polar bears go into hibernation during winter season to escape cold.
(iii) Some snails and fish undergo aestivation to avoid summer related problems like heat and dessication.
(iv) During unfavourable conditions, zooplanktons in lakes and ponds are known to enter diapause, i.e., stage of suspended development.

Question. Name and explain the kind of interaction in the following:
(i) Algae and fungi in lichens
(ii) Hermit crab and sea anemone
(iii) Head louse and humans
Answer : (i) Mutualism is found between algae and fungi in lichen.
Lichen is a composite entity which is formed jointly by an alga and a fungus. The fungus provides water, minerals and shelter to the alga. In return alga provides food to the fungus.
(ii) Interaction between sea anemone and hermit crab is considered as an example of mutualism (or as
protocooperation by some). Sea anemone uses hermit crab as a portable home and is able to find more food. Hermit crab in turn gets protection from its enemies. Thus it is a mutually beneficial association. Recent studies reveal that it is obligate mutualism not proto-cooperation.
(iii) Parasitism is found between head louse and humans. Head louse is an ectoparasite that sucks the blood of man or feed on living tissues of head’s skin.

Long Answer Type Questions

Question. Study the table given below in regard to  population interactions and answer the questions that follow :

[Note: (+) plus = beneficial interaction; (–) minus = detrimental interaction; (0) zero = neutral interaction]
(i) Identify the interactions.
(ii) Explain each one of them.
Answer : (i) (a) = Amensalism
(b) = Parasitism/Predation
(c) = Competition
(d) = Mutualism
(e) = Commensalism
(ii) (a) Amensalism is an association between two species that is detrimental to one of the species but has no effect on the other. A common example of amensalism is the release of chemical toxins by plants that can inhibit the growth of the other plant species (allelopathy).
(b) Parasitism is an association in which one organism (the parasite) lives o n or in the body of another organism (host), from which it obtains its nutrients. It is an one sided relationship in which parasite is benefitted and host is harmed.
(c) Competition is a rivalry between two or more organisms for obtaining the same resources. It may be between individuals of same species (intraspecific) or different species (interspecific).
(d) Mutualism is an interaction between two organisms of different species where both the partners are benefitted and are obligatory to each other.
(e) Commensalism is the interaction between two individuals of different species in which one is benefitted while other remains unaffected.

Question. (a) Compare, giving reasons, the J-shaped and S-shaped models of population growth of a species.
(b) Explain “fitness of a species” as mentioned by Darwin.
Answer : (a) The comparison between J-shaped or exponential growth and S-shaped or logistic growth is as follows :

(b) The fitness, according to Darwin, refers ultimately and only to reproductive fitness. Hence, those who are better fit in an environment, leave more progeny than others. These, therefore, will survive more and hence are selected by nature.
He called it natural selection and implied it as a mechanism of evolution.