Class 12 students can refer to Chapter Chapter 13 Organisms and Populations notes given below which is an important chapter in the class 12 Biology book. These notes and important questions and answers have been prepared based on the latest CBSE and NCERT syllabus and books issued for the current academic year. Our team of Business Studies teachers has prepared these notes for class 12 Biology for the benefit of students so that you can read these revision notes and understand each topic carefully.

Organisms and Populations Notes for Class 12 Biology

Objective Questions

Question. Temperature is the most ecologically relevant environmental factor. In which of the following habitats temperature can exceed 100° C ?
(a) Tropical desert
(b) Thermal springs
(c) Deep sea hydrothermal vents
(d) Both 2 and 3 

Question. Each of the following is an important factor in determining the productivity of fresh water lake except –
(a) Nutrients
(b) Depth
(c) Proximity to marine coast
(d) Temperature

Question. Which of the following statements is correct?
(a) Sometimes population density has more impact than the population size, and sometimes vice versa
(b) The carrying capacity of a habitat always remains the same
(c) Under normal conditions, births and deaths are the most important factors influencing population density, tl other 2 factors (immigration and emigrations) are important in special condition
(d) a and c

Question. Allen’s rule is related to –
(a) Hibernation
(b) Aestivation
(c) Migration
(d) Evolution

Question. The interaction is detrimental to both the species, in
(a) predation
(b) commensalism
(c) amensalism
(d) competition

Assertion & Reasoning Based MCQs

For question numbers 51-60, 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 : Aerenchyma is present in the leaves and petioles of hydrophytes.
Reason : Aerenchyma imparts buoyancy to the hydrophytes.
Answer : A

Question. Assertion : Mycorrhizal relation exists between Boletus and Pinus.
Reason : Mycorrhizal association is a symbiotic interaction.
Answer : B

Question. Assertion : Many mangrove plants possess high levels of organic solutes.
Reason : This is an adaptation to cope with the conditions of high salt concentration and osmotic potential.
Answer : A

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.
Answer : A

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.
Answer : A

Question. Assertion : The soil profiles of grassland,forest and desert biomes differ from each other.
Reason : Soil profile develops due to weathering process, accumulation of organic matter and leaching of mineral matter.
Answer : B

Question. Assertion : Phytoplanktons grow in abundance in the profundal zone of lake.
Reason : Profundal zone is the dark zone where light does not reach.
Answer : D

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.
Answer : D

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

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

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 match regarding organism Q and its habitat?
(a) Tundra – Polar bear
(b) Tropical rain forest – Deer
(c) Grassland – Bighorn sheep
(d) Desert – Camel

Question. The dominant plants in habitat where P lives could be
(a) Opuntia
(b) Nymphaea
(c) Deodar
(d) both (a) and (c).

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 statement is incorrect?
(a) Exponential growth occurs in organism such as lemmings.
(b) Logistic growth is more realistic.
(c) Exponential growth has two phases lag and log.
(d) In logistic growth, population passes well beyond the carrying capacity of ecosystem.

Question. Which of the following equations correctly represents Verhulst-Pearl logistic growth?
(a) dN/dt = rN(K − N)/K
(b) dN/dt = rN/K
(c) dN/dt = N(K N)/K
(d) dN/dt = r(K-N)K

Question. Study the population growth curves (A and B) in the given graph and select the incorrect option.

(a) Curve ‘A’ shows exponential growth, represented by equation dN/dt= rN. .
(b) Curve ‘B’ shows logistic growth, representedby equation dN/ dt = rN(K-N/K).
(c) Exponential growth curve is considered as more realistic than the logistic growth curve.
(d) Curve ‘A’ can also be represented by equation N_t = N₀e^rt.

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 : Aerenchyma is present in the leaves and petioles of hydrophytes.
Reason : Aerenchyma imparts buoyancy to the hydrophytes.

Question. Assertion : Many mangrove plants possess high levels of organic solutes.
Reason : This is an adaptation to cope with the conditions of high salt concentration and osmotic potential.

Question. Assertion : The soil profiles of grassland, forest and desert biomes differ from each other.
Reason : Soil profile develops due to weathering process, accumulation of organic matter and leaching of mineral matter.

Question. Assertion : Mycorrhizal relation exists between Boletus and Pinus.
Reason : Mycorrhizal association is a symbiotic interaction.

Question. Assertion : Phytoplanktons grow in abundance in the profundal zone of lake.
Reason : Profundal zone is the dark zone where light does not reach.

Very Short Answer Type Questions

Question. Plants like Calotropis have evolved adaptations for defence against grazers. Explain.
Answer : In order to keep predators or herbivores away from grazing, certain plants like Calotropis produce highly poisonous cardiac glycosides that can make herbivores sick when eaten.
Hence, grazers like cattle, goats and sheep do not graze it. 

Question. When and why do some animals like frogs hibernate?
Answer : When animals are exposed to low temperatures, hibernation is necessary for cold-blooded animals like frogs to prevent their metabolic rate from getting slow down. 

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

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

Question. Give an example of drought escapers xerophytic plant.
Answer : Emphemerals are drought escapers xerophytic plants, for example: Tribulus terrestris.

Short Answer Type Questions

Question. If 8 individuals in a population of 80 butterflies die in a week, calculate the death rate of population of butterflies during that period.
Answer : Death rate is defined as the number of deaths per 1000 individuals of a population. Since, total number of butterflies = 80,
Number of butterflies that died = 8
Death rate = 8 /80= 0.1 butterflies per week.

Question. Differentiate between the mutualism and competition.
Answer :

Mutualism	Competition
(i) In mutualism both interacting species benefit from each other.	It is a rivalry between two or more organisms for resources.
(ii) Contact between the two organisms is obligatory to time and space.	Competition is of two types. Competition between individuals of the same species is called intraspecific competition while that between individuals of different species is termed as interspecific competition.

Question. Shark is eurythermal while polar bear is stenothermal. What is the advantage the former has and what is the constraint the later has?
Answer : Sharks being eurythermal can tolerate wide range of temperature variations and thus have wider distribution on earth, on the other hand, polar bear being stenothermal can tolerate only narrow range of temperature and is restricted to specific regions only.

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. A species is introduced into a new habitat with favourable environment. What kind of population growth does it undergo on a longterm basis? Describe briefly.
Answer : When a species is introduced into a new habitat with favourable environment (unlimited resources) it undergoes exponential growth. It has two phases-lag and log phase. In lag phase, species shows poor growth as trying to establish in the new environment. After its establishment, species shows maximum growth in the log or exponential phase, resulting in J-shape curve.

Question. Refer to the given graph and answer the following questions.

(a) What does the graph represent? Identify A, B and C.
(b) Differentiate between A and B.
(c) How organism C regulates their body functions?
Answer : (a) The given graph represents various ways of organismic response i.e., possibilities of living organisms to cope with stressful conditions. A represents the conformers, B represents regulators and C represents partial regulators.
(b) Differences between A (conformers) and B (regulators) are as follows:

Question. (a) How are herbs able to grow on forest floors?
(b) What are osmoconformers?
Answer : (a) Sciophytes or shade plants like herbs and shrubs survive under the shadow of big canopied trees or sun plants or heliophytes in forests as they are perfect shade tolerant
plants which show better growth in lower level of light intensity. They grow in a manner, that they are arranged in different strata according to their shade tolerance.
(b) Osmoconformers are those organisms which cannot maintain constant osmolarity of their body fluids and it varies according to their surrounding medium.

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. During a school trip to ‘Rohtang Pass’, one of your classmates suddenly developed ‘altitude sickness’. But, she recovered after sometime.
(a) Mention one symptom to diagnose the sickness.
(b) What caused the sickness?
(c) How could she recover by herself after sometime?
Answer : (a) Heart palpitation
(b) Sickness is due to low atmospheric pressure of high altitudes, as body does not get enough oxygen.
(c) After sometimes, body compensates for low oxygen availability by increasing red blood cell production, decreasing binding capacity of haemoglobin and by increasing breathing rate.

Question. (a) List any three ways of measuring population density of a habitat.
(b) Mention the essential information that can be obtained by studying the population of an organism.
Answer : (a) The different methods to study population size are as follows:
– Quadrat method : It is a method which involves the use of square of particular dimension to measure number of organisms. For example the number of Parthenium plants in a given area can be measured using the quadrat method.
– Direct observation: It involves counting of organisms. For example, in order to determine the number of bacteria growing in a petri dish, their colonies are counted.
– Indirect method : The number of fishes caught per trap gives the measure of their total density in a given water body.
(b) Whether competition of survival exists or not, whether the population is increasing or decreasing, natality, mortality, emigration and immigration.

Long Answer Type Questions

Question. List the different ways by which organisms cope or manage with abiotic stresses in nature. Explain any three ways listed.
Answer : Living organisms cope with stressful conditions by various methods :
(i) Hibernation and aestivation : Hibernation is winter sleep in which animal passes the winter period in dormant condition in a warm place. Polar bears hibernate during winters.
Aestivation is summer sleep in which animal rests in a cool/ shady and moist place during extreme heat period. Ground squirrels of South-Western deserts undergo aestivation and lie in torpid state inside burrows during hot dry periods.
(ii) Camouflage : It is the ability to blend with the surroundings or background. It is protective to animals
which are preyed upon by others and it is also advantageous to predators as it eases predation, e.g., it is difficult to distinguish leaf like grasshopper from the surrounding foliage.
(iii) Mimicry : It is resemblance of one species with another in order to obtain advantage specially against predation. The species which is imitated is called model while the species which imitates is known as mimic. E.g., Viceroy butterfly mimics unpalatable, toxic Monarch butterfly.
(iv) Migration : The organisms can migrate temporarily from the unfavourable habitat to more favourable area and return when unfavourable period is over. Many animals, particularly birds, during winter undergo long-distance migrations to more favourable areas.
(v) Perennating structures : Various kinds of thick walled spores are formed in bacteria, fungi and lower plants which help them survive under unfavourable conditions. These germinate on return of suitable conditions.
(vi) Diapause : Under unfavourable conditions many zooplanktons in lakes and ponds are known to enter diapause i.e., a stage of suspended development.

Question. (a) Study the flow chart given below and complete the equation that follows by identifying 1, 2, 3 and 4.

N_{t + 1} = N_t + [(B + I) – (D + E)]
(b) Mention the different ways by which the population density of different species can be measured.
Answer : (a) In the given equation, 1, 2, 3 and 4 respectively are B, I, D and E. Therefore, the equation will be
N_{t + 1} = N_t + [(B + I) – (D + E)]
(b) Population density is defined as number of individuals of a species per unit area or per unit volume of environment.
Population density may be measured by :
(i) Numerical density calculated by number of individuals per unit area or volume. For example, if in a pond there were 20 lotus plants last year and through reproduction 8 new plants are added, taking the current population to 28, the birth rate will be calculated as 8/20 = 0.4 offspring per lotus per year.
(ii) Biomass density calculated as biomass per unit area or volume. For example if in an area, there are 200 Parthenium plants but only a single huge banyan tree, then the percent cover or biomass is more meaningful measure of the population size.
(iii) Abundance or absolute number of population. For ecological investigations, population density is measured as absolute population densities or relative densities. For example the tiger census in our National parks and tiger reserves is often based on pug marks and fecal pellets.

Question. (a) What is “population” according to you as a biology student?
(b) “The size of a population for any species is not a static parameter”. Justify the statement with specific reference to fluctuations in the population density of a region in a given period of time.
Answer : (a) According to me as a biology student, population is defined as the total number of interbreeding individuals of a species found in a geographical area who share and compete for similar resources.
(b) The population density is the number of individuals of a species per unit area/space at a given time. The size of a population (population density) is not a static parameter.
It keeps changing with time, depending upon a number of factors : abiotic and biotic, food availability, predation pressure, etc. The density of a population changes due to four basic processes:
(i) Natality : Number of births during a given period per unit population.
(ii) Mortality : Number of deaths in the population during a given period.
(iii) Immigration : Number of individuals of the same species moving inside a population during the time period.
(iv) Emigration : Number of individuals moving outside from a habitat during the time period.
Therefore, if N is the population density at time t, then its density at time t + 1 can be explained by the given equation :
N_t+1 = N_t + [(B + I) – (D + E)]
Where, B represents natality or number of births;
I represents number of immigrants;
D represents mortality or number of deaths;
E represents number of emigrants.
From the above equation it is clear that population density increases if the number of births plus the umber of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E). Otherwise it will decrease.