Ncert Solutions Organism and Population Class 12 Notes & PDF

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Ncert Solutions Organism and Population Class 12 Notes

Chapter No13
ProvidingQuestions And Answers, Notes & Numericals PDF
Chapter Nameorganism and population
Medium English
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organism and population class 12 notes

Ncert Solutions Organism and Population Class 12 Notes & PDF

1. Keology is the study of the relationships of living organisms with the abiotic (physicochemical factors) and biotic components (other species) of their environment. It is concerned with four levels of biological organisation

(i) populations.

(ii) communities  

(iii) biomes

2. Variations in temperature, precipitation (rain and snow) and seasons account for the formation of major biomes such as deserts, rain forests and tundra.

3. Regional and local variations within each biome lead to the formation of a wide variety of habitats. Habitats are characterised by their (Abiotic components are temperature, water, soil, etc.

(ii) Biotic components are pathogens, parasites, predators, competitors, etc.

4. It is assumed that over a period of time, the organism, through natural selection, evolves adaptations to optimise its survival and reproduction in its habitat.

5. Major Abiotic Factors

(Temperature The average temperature on land varies seasonally, decreasing progressively from the equator towards the poles and from plains to the mountain tops. 

(a) It affects the structure and functions of enzymes and thus, the basal metabolism, activity and other physiological functions of the organism.

(b) Eurythermal organisms can tolerate and thrive in a wide range of temperatures.

(c) Stenothermal organisms tolerate only a narrow range of temperatures.

(ii) Water supports life on the earth. 

(a) The productivity and distribution of plants are also heavily dependent on water. 

(b) Even for aquatic organisms the water quality (chemical composition, pH) is important.

(c) Euryhaline organisms are tolerant of a wide range of salinities.

(d) Stenohaline organisms are restricted to a narrow range of salinity.

(e) Many freshwater animals cannot live for long in sen water and vice versa because of the osmotic problems they would face. 

(iii) Light is a source of energy, necessary for plants to produce food through photosynthesis. 

(a) Some plants are dependent on sunlight to meet their photoperiodic requirement for flowering. 

(b) For many animals too, diurnal and seasonal variations in light intensity and duration (photoperiod) act as cues for timing their foraging, reproductive and migratory activities.

(iv) Soil properties vary at different places depending on the climate, weathering process and process of soil development. 

(a) Characteristics of the soil such as soil composition, grain size and aggregation determine the percolation and water-holding capacity of the soil.

(b) Soil characteristics along with its pH. mineral composition and topography determine the vegetation in any area.

6. Responses to Abiotic Factor

The abiotic conditions of many habitats keep changing with time, but organisms try to maintain their constancy of theirs. internal environment, a process called homeostasis.

7. To maintain homeostasis, organisms adapt the following strategies: 

(1) Regulate All birds, mammals, a very few lower vertebrate and invertebrate species maintain homeostasis by physiological or behavioural means, which is known as thermoregulation and osmoregulation with respect to temperature or osmotic regulation.

(a) Most mammals, including humans, maintain a constant body temperature of 37°C. In summer, when the outside temperature is more than our body temperature, we sweat profusely. 

(b) Evaporating water from the sweat takes away the heat and body temperature comes down. 

(c) In winter, when the temperature is much lower than 37°C, muscle movement in the form of shivering, produces heat and raises the body temperature.

(ii) Conform All plants cannot maintain a constant internal environment. Their body temperature changes with the temperature of the surroundings. 

(a) In aquatic animals, the osmotic concentration of the body fluids changes with that of the surrounding water osmotic concentration. These animals and plants simply conform to the changes.

(b) They have not evolved to become regulators because thermoregulation is energetically expensive, especially for small animals like shrews and hummingbirds. 

(C) Small animals and birds tend to lose body heat very fast when it is cold outside; then they have to expend much energy to generate body heat through metabolism. This is the main reason why very small animals are rarely found in polar regions.

(iii) Migrate If stressful conditions are present in one location for a short duration, the organism moves away temporarily

from there to a more hospitable area and returns when the stressful period is over. Every winter the famous Keoladeo National Park (Bharatpur) in Rajasthan hosts thousands of migratory birds coming from Siberia and other extremely cold Northern regions.

(iv) Suspend Bacteria, fungi and lower plants, make protective spores that help them survive unfavourable conditions. 

(a) Thick-walled spores remain dormant until favourable conditions return.

(b) Higher plants, seeds and some other vegetative reproductive structures also reduce their metabolic activity and undergo a state of dormancy. 

(c) They germinate to form new plants under favourable moisture and temperature conditions.

(d) Animals, if unable to migrate, might avoid stress by hibernation.

(e) Some snails and fish got into aestivation to avoid excessive heat and desiccation. 

(f) Many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development under unfavourable conditions. 

8. Adaptations The physiological, behavioural or morphological attributes or adjustments that enable the organism to survive and reproduce in its habitat are called adaptations. 

Some examples of adaptation are:

(i) Kangaroo rats in North American deserts

(a) Uses water produced through internal fat oxidation.

(b) Conserves water loss by concentrating its urine. 

(ii) Desert plants 

(a) Thick cuticles on their leaf surfaces.

(b) Stomata arranged in deep pits to minimise water loss.

(c) Stomata remains closed during day time.

(d) Leaves are reduced to spines to avoid stomatal loss of water (in Opuntia). 

(e) Photosynthesis took over by flattened stems.

(iii) Mammals from colder climates 

(a) Shorter ears and limbs to minimize heat loss

(b) Thick layer of fat (blubber) below their skin to reduce loss of body heat.

9. Example of Physiological Adaptations At high altitudes, the body does not get enough oxygen and people experience high-altitude sickness (nausea, fatigue and heart palpitations). But slowly one gets acclimatised as the body switches on a few physiological mechanisms, to compensate for low oxygen availability by

(i) increasing red blood cell production. 

(ii) decreasing the binding capacity of haemoglobin.

(iii) increasing breathing rate.

10. Examples of Biochemical Adaptations Special adaptations of enzyme proteins that can function even at very high or very low temperatures allow.

(i) Some microbes (archaebacteria) flourish in hot springs and deep-sea hydrothermal vents (at above 100°C). 

(ii) Many fish thrive in Antarctic waters (below zero temperatures).

11. Examples of Behavioural Adaptations

(i) Desert lizards absorb heat from the sun when their body temperature drops below normal, but move into shade when temperature starts increasing.

(ii) Some species burrow into the soil to hide and escape from above-ground heat.

(iii) Many birds migrate to different regions to escape the extreme cold and return when temperatures become normal.

12. Populations 

(i) Population attributes A population is constituted of organisms living area in groups in a well-defined geographic

(a) share or compete for similar resources.

(b) interbreed through sexual reproduction or reproduce asexually.

(ii) Examples of a population

(a) Rats in an abandoned dwelling. 

(b) Teakwood trees in a forest tract.

(c) Bacteria in a culture plate. 

(d) Lotus plants in a pond.

(iii) Population attributes that are not seen in an individual organism are

(a) natural selection operates at the population level to evolve the desired traits.

(b) a population has birth rates and death rates. In a population, these rates refer to per capita births and deaths, respectively.

(c) a population has a sex ratio (e.g., 60% of the population are females and 40 per cent males). 

(d) a population at any given time is composed of individuals of different ages.

13. Age pyramid refers to a graphical representation of the age distribution (per cent individuals of a given age or age group) plotted for the population.

(1) For the human population, the age pyramids generally show the Tage distribution of males and females in a combined diagram. 

(ii) The shape of the pyramids reflects the growth status of the population 

(a) Whether it is growing 

(b) Stable 

(c) Declining. 

(iii) The size of the population tells us a lot about its status in the habitat. Representation of age pyramids for the human population 

(iv) Effect of any environmental factor, e.g., predator or a pesticide application is evaluated in terms of change in the population size. Population size, called population density (designated as N), can be measured in numbers or in per cent cover or biomass.

14. Population Growth The size of a population for any species keeps changing with time, depending on various factors

(i) food availability

(ii) predation pressure 

(iii) weather conditions

15. The density of a population in a given habitat during a given period, fluctuates due to changes in four basic processes: 

(i) Natality refers to the number of births during a given period in the population. It leads to an increase in population density.

(ii) Mortality is the number of deaths in the population during a given period. It leads to a decrease in population density. 

(iii) Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during a specified time period. 

(iv) Emigration is the number of individuals in the population who exit or leave the habitat and go elsewhere during a specified time period.

16. Equation of population density N, No+[(B+I)-(D+E)] where, N, = population density at a time t B= birth rate I = immigration, D = death rate E= emigration, No = population in the beginning ball This equation shows that the population density will increase if the number of births plus the number of immigrants (B + 1) is more than the number of deaths plus the number of emigrants (D+E), otherwise it will decrease.

17. Growth Models tell us about the patterns of growth of a population with time.

(i) Exponential growth When resource (food and space) availability in the habitat is unlimited, each species has the ability to realise its full innate potential to grow in number. Then the population grows in an exponential or geometric fashion. 

(a) If in a population of size N, the birth rates (not total number but per capita births) are represented as b and death rates (again, per capita death rates) as d, then the increase or decrease in N during a unit time period

(b) The r in this equation is called the ‘intrinsic rate of natural increase’ and is a very important parameter chosen for assessing the impacts of any biotic or abiotic factor on population growth.

(c) Any species growing exponentially under unlimited resource conditions can reach enormous population densities in a short time.

(ii) Logistic growth

(a) No population achieves exponential growth for long, because resources are limited. 

(b) Eventually, the fittest’ individual will survive and reproduce. 

(c) In nature, a given habitat has enough resources to support the maximum possible number, beyond which no further growth is possible. This limit can be referred to as nature’s carrying capacity (K) for that species in that habitat.

(d) When N is plotted in relation to time t, the logistic growth shows a sigmoid curve and is also called Verhulst-Pearl logistic growth. It is given by the following equation: +11)

18. Life History

Variation Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (high r-value), in the habitat in which they live. (i) Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy.

(ii) Some organisms breed only once in their lifetime (Pacific salmon fish and bamboo) while others breed many times during their lifetime (most birds and mammals). 

(iii) Some produce a large number of small-sized offspring (Oysters and pelagic fishes) while others produce a small number of large-sized offspring (birds and mammals). 

(iv) It is suggested that life history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live.

19. Population Interactions

(i) In nature, animals, plants and microbes do not and cannot live without interacting in various ways to form a biological community.

(ii) Interspecific interactions arise from the interaction of populations of two different species. They could be beneficial, detrimental or neutral (neither harm nor benefit) to one of the species or both.

(iii) The possible outcomes of interspecific interactions are: 

(a) Mutualism Both species benefit. 

(b) Competition Both lose in their interactions with each other.

(c) Predation Only one species benefits, and the other gets killed. 

(d) Parasitism Only one species benefits, the other is harmed or loses life. 

(e) Commensalism One species is benefitted and the other is unaffected.

(f) Amensalism One species is harmed whereas the other is unaffected.

20. Predation

In this interaction, an animal called a predator, kills and consumes, the other weaker animal called prey. 

(i) Predation is nature’s way of transferring energy fixed by plants to higher trophic levels.

(ii) Role of predators 

(a) Predators keep prey populations under control and do not let them achieve very high population densities and cause ecosystem instability. also, help in maintaining species

(b) Predators’ diversity in a community by reducing the intensity of competition among competing prey species.

(iii) Prey defence mechanisms Prey species have evolved various defences to lessen the impact of predation.

(a) Some species of insects and frogs are cryptically coloured (camouflaged) to fool the predator. Some are poisonous and therefore avoided by predators. 

(b) The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical that it acquired in its caterpillar stage by feeding on a poisonous weed.

(c) Thorns (Acacia and cactus) are the most common morphological means of defence.

(d) Many plants produce and store chemicals that make the herbivore sick when they are eaten, inhibit feeding or digestion, disrupt its reproduction or even kill it. 

(e) The weed Calotropis grows in abandoned fields. It produces highly poisonous cardiac glycosides to avoid being grazed by cattle or goats.

(f) A wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.) are produced by them actually as defences against grazers and browsers.

21. Competition occurs due to limited resources between closely related species.

(i) It is a type of interaction, where both the species suffer. It may exist between some species interspecific competition or between individuals of different species (intraspecific competition).

(ii) Some totally different species could also compete for the same resources, e.g., in some shallow South African lakes, visiting flamingoes and resident fishes compete for their common food and zooplankton.

(iii) In interspecific competition, the feeding capacity of one species might be reduced due to the interfering and inhibitory presence of the other, species although the resources are plenty, e.g., when goats were introduced to Galapagos Islands, the Abingdon tortoise became extinct within a decade due to the excellent grazing efficiency of goats.

(iv) Competitive release states the phenomenon of a species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species. It is found to expand its distribution range dramatically when the competing species are experimentally removed.

(v) Gause’s competitive exclusion principle states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually by the superior one. 

(vi) Resource partitioning refers to the phenomenon in which species facing competition might evolve, mechanisms that promote co-existence rather than exclusion.

MacArthur showed that five closely related species of warblers, living on the same trees, were able to avoid competition and co-exist due to behavioural differences in their foraging activities. 

22. Parasitism It is the mode of interaction between two species in which one species (parasite) depends on the other species (host) for food and shelter. In this process, the parasite damages the host and benefits itself.

(i) Adaptive features in parasites 

(a) The parasite has evolved in a way that both host and parasite tend to co-evolve.

(b) It does not have unnecessary sense organs. 

(c) Adhesive organs or suckers are also present.

(d) Digestive system not present. 

(e) Reproductive capacity is high.

(ii) Life cycles of some parasites are complex, where one or more intermediate hosts or vectors to facilitate parasitisation are present, e.g.,

(a) Human liver fluke depends on two intermediate hosts, a snail and a fish to complete its life cycle. 

(b) Malarial parasite (Plasmodium) needs a vector (mosquito) to complete its life cycle.

(iii) Types of Parasites Parasites can be of two types, ie, ectoparasites and endoparasites. 

(a) Ectoparasites feed on the external surface of the host organism, e.g., lice on humans and ticks on dogs, marine fish infested with ectoparasitic copepods, Cuscuta, a parasitic plant growing on hedge plants. 

(b) Endoparasites live inside the host body at sites (liver, kidney, lungs, red blood cells, etc). The life cycles of endoparasites are more complex because of their extreme specialisation. Their morphological and anatomical features are greatly simplified while emphasising their reproductive different potential. 

(iv) Brood parasitism is a phenomenon in which one organism (parasite) lays its eggs in the nest of another organism.

eg., Cuckoo bird lays its eggs in the nest of its host and lets 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 them as foreign eggs. 23. Commensalism This is the interaction in which one species benefits without affecting the other, e.g.,

(i) An orchid growing as an epiphyte on a mango branch. 

(ii) Barnacles growing on the back of a whale.

(iii) The cattle egret foraging close to the cattle that stir up and flush out insects from the vegetation. 

(iv) Sea anemone that has stinging tentacles and the clown fish that lives among them to get protection from predators.

24. Mutualism This interaction confers benefits on both the interacting species, e.g.,

(i) Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.

(ii) The mycorrhizae are associations between fungi and the roots of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while the plant in turn provides the fungi with energy-yielding carbohydrates. 

(iii) Plants offer nectar, juicy and nutritious fruits to animals that help pollinate their flowers and disperse their seeds.

25. Amensalism defines the interaction between two different species in which one species is harmed and the other is neither benefited nor harmed, e.g., The mould Penicillium secretes penicillin, which kills bacteria but, the mould is unaffected.

organisms and populations neet questions -Exercises

Question 1. How is diapause different from hibernation?

Answer Diapause is a stage of suspended development that some organisms like zooplankton in lakes and ponds, adopt to survive under unfavourable conditions. Hibernation is just a behavioural adaptation to avoid extreme cold when an organism slows down its metabolic activities but does not suspend it.

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

Answer No, a marine fish placed in an aquarium containing freshwater, will not be able to survive in freshwater because its body system is adapted to function normally in a narrow range of salinity. It cannot survive in the low salinity of the fresh water.

Question 3. Define phenotypic adaptation. Give one example.

Answer Phenotypic adaptations are useful changes in morphology and physiology in response to changing environmental conditions. They disappear when environmental conditions return to normal. Temporary dry conditions make the plants shed their leaves, which grow back again once the water availability becomes normal.

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

Answer These microbes have enzymes and proteins that do not get denatured at high temperatures and hence can catalyse metabolic reactions.

Question 5. List the attributes that populations but not individuals. 

Answer: A population has the following attributes that an individual does possess. not possess 

  • (i) Birth rates and death rates
  • (ii) Sex ratio
  • (iii) Population density
  • (iv) Age distribution 
  • (v) Population growth.

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

Answer The intrinsic rate of increase (r) of the population is measured by the formula log 20.7931-0.264 or 26.4% t 3

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


(i) Thorns, prickles, spines, etc., on stems and leaves that can injure or kill the grazer.

(ii) Very smooth, sticky or slippery leaves.

(iii) Hair on leaves with irritants. 

(iv) Defensive compounds like resins, wax, and lignins released onto the plant’s surface acts as repellants. 

(v) Alkaloids, terpenoids, cyanogenic glycosylated, etc., are poisonous chemicals released by plants.

Question 8. An orchid plant is growing on the branch of a mango tree. How do you describe this interaction between the orchid and the mango tree?

Answer This is commensalism on the part of an orchid. As mango trees are neither harmed nor benefitted. 

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

Answer: The ecological principle behind the biological control method of managing pest insects is to keep their population in check by using their natural predators and parasites.  It is called summer sleep.

Animals rest in a cool, moist, shady place. It lasts for daytime as nights are cooler and animals will come out.

Endotherm These are warm-blooded animals. They are capable of maintaining their body

Question 10. Distinguish between the following

(a) Hibernation and Aestivation 

(b) Ectotherms and Endotherms

Answer (a)

image 8
image 9

Question 11. Write a short note on 

(a) Adaptations of desert plants and animals.

(b) Adaptations of plants to water scarcity.

(c) Behavioural adaptations in animals.

(d) Importance of light to plants. 

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



(a) Adaptations of desert plants

(1) Thick cuticles on their leaf surface.

(ii) Stomata arranged in deep pits to minimise water loss. 

(iii) Stomata remain closed during day time.

(iv) Special photosynthetic pathway (CAM). 

(v) Leaves are reduced to spines to avoid stomatal loss of water (in Opuntia).

(vi) Photosynthesis is taken over by flattened stems. 

Adaptations of desert animals

(i) Very little water requirement. Are able to use internal metabolism as a source of water.

(i) Produce concentrated urine and solid faeces, 

(iii) Have mechanisms to prevent loss of water in breathing.

(Iv) Burrowing habit to avoid day heat. 

(b) Same as 

(a) for plants.

(c) Behavioural adaptations in animals to avoid stressful or harmful conditions are as follows 

(1) Aestivation

(ii) Camouflage

(iii) Migration

(iv) Hibernation

(v) Mimicry. 

(d) Importance of light to plants

(i) The length and extent of light determines the growing processes of plants, e.g., seed germination, growing, budding, blooming flowering and the production of seed.

(ii) To grow and develop, plants need sunlight as a source of energy to make their food (photosynthesis). The amount of light and intensity influences the rate of photosynthesis. 

(II) The time for which light is available (photoperiodism) and its intensity affects the flowering and thus, the reproduction in sexually reproducing plants.

(e) Effect of temperature or water scarcity on adaptations of animals In conditions of high temperature or water scarcity, the adaptations will be such that help the animals to

(i) reduce water loss from the body in breathing and excretion. 

(ii) utilise water generated internally through metabolic activity.

In cold conditions, the adaptation would be such that will help the animal to conserve body heat.

keep their body warm by developing thick fur. insulating the body by depositing fat under the skin.

avoid cold conditions by migration or hibernation.

produce antifreeze proteins and solutes in excessively cold conditions.

Question 12. List various abiotic environmental factors.


(i) Temperature

(Ii) Water-salinity pH 

(iii) Light wavelength, intensity, duration, etc.

(iv) Soil-its quality, pH, aeration, hydration, etc. 

(v) Topography-slope, altitude, plains.

Question 13. Give an example for 

(a) An endothermic animal

(b) An ectothermic animal 

(c) An organism of benthic zone


(a) An endothermic animal-Monkey, human and deer.

(b) An ectothermic animal-Frog, lizard and chameleon.

(c) An organism of benthic zone-Corals and sponges.

Question 14. Define population and community.

Answer Population It is a group of individuals of the same species that 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 exhibits interaction and interdependence through trophic relationships. 

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

(a) Commensalism 

(b) Parasitism

(c) Camouflage

(d) Mutualism

(e) Interspecific competition


(a) Commensalism is an interaction between two species, where one is benefitted and the other remains unaffected. e.g., Clownfish and sea anemones. The clown fish gets protection from predators which stay away from the 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 benefitted and the other is harmed. e.g.. Cuscuta, a parasite 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 an organism with the surroundings due to similar colour, marking and shape so, as to avoid predators. e.g., Leaf-like insects such as grasshoppers. 

(d) Mutualism The interaction between two species in which, both organisms are benefitted to maintain the life process is called mutualism. e.g., Lichens and fungi help in the 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. eg, The Abingdon tortoise in the Galapagos islands became extinct within a decade after goats were introduced on the Island, apparently due to the greater browsing efficiency of the goats.

Question 16. With the help of a suitable logistic population growth curve.

Answer Logistic population growth curve

(i) The resources become limited and the population can grow exponentially at a certain point in time, so no

(ii) This growth model is realistic. 

(iii) Every ecosystem or environment has limited resources to support a particular maximum number of individuals called its carrying capacity (K)

(iv) When Nis is plotted in relation to time t, the logistic growth curve shows the sigmoid curve and is also called. Verhulst-Pearl logistic growth. It is given by the following equation: 

Population growth curve ‘a’ when responses are not limiting the growth, plot is exponential, ‘b’ when responses are limiting the growth, plot is logistic, Kis carrying capacity

image 11

Question 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, and another is affected.

Answer (d)

Question 18. List any three important characteristics population and explain. Answer The three important characteristics of a population are 

(1) Population density The number of individuals of a species per unit area or volume is called population density.

where, PD = Population density N = Number of individuals in a region S=Number of unit area in a region

(ii) Birth rate is expressed as the number of births per 1000 individuals of a population per year. (i) Death rate is expressed as the number of deaths per 1000 individuals of a population per year.

organism and population ncert Questions

Question 1. What is the interaction between two species called? 

Answer Interaction between two species is called interspecific interaction. These could be beneficial, detrimental or neutral to one of the species or both.

Question 2. What is commensalism?

Answer Commensalism is an interaction where one species is benefitted and the other is unaffected. e.g.. An orchid growing as an epiphyte on a mango branch.

Question 3. Name the association in which one species produces a poisonous substance or a change in environmental conditions that is harmful to another species.

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

Question 4. Why do high-altitude areas have brighter sunlight and lower temperatures as compared to the plains? from the sun and particle-free air.

Answer Brighter sunlight is because of reduced distance Lower temperatures are because of low atmospheric pressure.

Question 5. What is homeostasis?

Answer Homeostasis is the tendency of the organism to maintain a constant internal environment despite varying external environment conditions.

Question 6. Define aestivation.

Answer Aestivation is a behavioural adaptation to avoid desiccation when an organism slows down its metabolic 

Question 7. What is diapause and its significance?

Answer It is a stage of suspended development that some organisms like zooplankton in lakes and ponds, adopt to survive under unfavourable conditions.

Question 8. What are the organisms that feed on plant sap and other plant parts called?

Answer The organisms that feed on plant sap and other plant parts are called phytophagous.

Question 9. What is high altitude sickness? Write its symptoms.

Answer High altitude sickness is experienced by people going to high altitudes, where oxygen concentrations are low and the body system reacts by developing the symptoms like nausea, headache and heart palpitations.

Question 10. Define ectoparasite and endoparasite as suitable examples

Answer Ectoparasite feed on the external surface of the host e.g., Lice on humans and ticks on dogs. Many marine fish are infested with ectoparasitic copepods. Endoparasites live inside the host body at different sites (liver, kidney lungs, red blood cells, etc.).

The morphological and anatomical features of endoparasites are greatly simplified while emphasising their reproductive potential.

Question 11. What is brood parasitism? Explain with the help example.

Answer Brood parasitism is the phenomenon in which an organism (parasite) lays eggs on the nest of another organism’s host. e.g.. A cuckoo (koel) bird lays its eggs in the nest of its host and lets the host incubate them. The eggs of the parasitic bird 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.

organism and population class 12

Question 1. If a freshwater fish is placed in an aquarium containing seawater, will the fish be able to survive? Explain giving reasons.

Answer No, a freshwater fish placed in an aquarium containing seawater, will not be able to survive in seawater. Because its body system is adapted to function normally in a narrow range of salinity and it cannot survive in the high salinity of the sea.

Question 2. Why do all the freshwater organisms have contractile vacuoles whereas the majority of marine organisms lack them?

Answer Contractile vacuole helps in osmoregulation. Because the cellular environment of a freshwater organism is hypertonic, the water diffuses inside the cell constantly and gets collected in the contractile vacuole, which squeezes the extra water out of the cell periodically. Thus, keeping the internal environment constant.

Question 3. 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 all colours of the visible components of the spectrum do not enter the depths of water.

Question 4

(a) Label the three tiers 1, 2, and 3 given in the above age pyramid. 

(b) What type of population growth is represented by the above age pyramid?


1-Pre-reproductive stage.

 2-Reproductive stage.

3-Post-reproductive stage

The above age pyramid represents expanding type of population growth.

Question 5. In an association of two animal species, one is a termite that feeds on wood and the other is a protozoan Trichonympha present in the gut of the termite. What type of association do they establish?

Answer The type of association they establish could be either of these Commensalism If the protozoan is living inside the gut without affecting the termite negatively or positively Parasitism If the protozoan is detrimental to the termite.

Question 6. Lianas are vascular plants rooted in the ground and maintain the erectness of their stem by making use of other trees for support. They do not maintain direct relations with those trees, Discuss the type of association the lianas have with the trees.

Answer The type of association, the lianas have with the trees is commensalism because the plant gets the support of the tree without affecting the tree. 

Question 7. Give the scientific names of any two microorganisms inhabiting the human intestine.

Answer The scientific names of two microorganisms inhabiting the human intestine are Escherichia coli and Lactobacillus

Question 8. 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 defence agent


(a) Migratory animal-American buffalo and dolphin

(b) Camouflaged animal-Grasshopper and Chameleon 

(c) Predator animal – Lion

(d) Biological control agent- Myxoma virus to kill European rabbit and Gambusia fish to check the growth of mosquito larvae

(e) Phytophagous animal-Insects (beetle, butterfly, etc.) 

organisms and populations class 12 ncert

Question 1. A population of Paramecium caudatum was grown in a culture medium. After 5 days the culture medium became overcrowded with Paramecium 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 Initially, after a lag phase, the population will grow in an exponential manner as the nutrients and space will be abundant When the food sources get depleted, the population density starts to decrease and ends in an asymptote phase then the population density reaches the carrying capacity (maximum number of individuals of a population or species that a given environment can sustain indefinitely). The population shows a pattern of logistic growth giving an S-shaped curve.

Question 2. While living in and on the host species, the animal parasite has evolved certain adaptations. Give examples.

Answer Parasites have evolved special adaptations such as,

(i) The loss of unnecessary sense organs-Lice, mites and fleas don’t have wings. 

(ii) Presence of adhesive organs or suckers to cling on to the host-In tapeworms and leeches.

(iii) Loss of digestive system-Tapeworm.

(iv) High reproductive capacity-Roundworm produces large progeny.

Question 3. Does light factor affect the distribution of organisms? Write a brief note giving suitable examples of either plants or animals. 

Answer Plants require sunlight for photosynthesis. Therefore, light is an animal. an important factor that affects the distribution of plants. e.g..

(i) Many species of small plants (herbs and shrubs) growing in forests are adapted to photosynthesise optimally under very low light conditions so they will be seen distributed in shady areas under tall, canopied trees.

(ii) Many plants in the shade will grow vertically to gain access to light. These plants will appear to have smaller leaves and are smaller than others of the same species of the same age found in conditions with better sunlight.

(iii) Large-sized trees will be present in areas that get abundant sunlight. 

(iv) Plants dependent on sunlight to meet their photoperiodic requirement for flowering, will try to be distributed in areas, where this requirement is being met for their reproductive success.

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