Solutions-Class-12-Biology-Chapter-13-Organisms and Population-Maharashtra Board

Organisms and Population

Maharashtra Board-Class-12th-Biology-Chapter-13


Question 1. Multiple choice questions.

(1) Which factor of an ecosystem includes plants, animals and microorganisms?

(a) Biotic factor

(b) Abiotic factor

(c) Direct factor

(d) Indirect factor

Answer :

(a) Biotic factor

(2) An assembladge of individuals of different species living in the same habitat and having functional interactions is ................

(a) Biotic community

(b) Ecological niche

(c) Population

(d) Ecosystem

Answer :

(a) Biotic community

(3) Association between sea anemone and Hermit crab in gastropod shell is that of ................

(a) Mutualism

(b) Commensalism

(c) Parasitism

(d) Amensalism

Answer :

(b) Commensalism

(4) Select the statement which explains best parasitism.

(a) One species is benefited.

(b) Both the species are benefited.

(c) One species is benefited, other is not affected.

(d) One species is benefited, other is harmed.

Answer :

(d) One species is benefited, other is harmed.

(5) Growth of bacteria in a newly innoculated agar plate shows ................

(a) exponential growth

(b) logistic growth

(c) Verhulst-Pearl logistic growth

(d) zero growth

Answer :

(c) Verhulst-Pearl logistic growth

Question 2. Very short answer questions.

(1) Define the following terms :

(a) Commensalism

Answer :

Commensalism refers to the interaction between two species where one species benefits while the other is neither harmed nor benefited.

(b) Parasitism

Answer :

The interaction between two species in which one parasitic species derives benefit from the other host species by harming it, is called parasitism.

(c) Camouflage

Answer :

Camouflage is the disguising colouration or behaviour to merge with the surrounding so that prey or predator can remain hidden.

(2) Give one example for each :

(a) Mutualism

Answer :

Lichen, a mutualistic association between alga (cyanobacteria) and fungus, cannot survive independently. Alga synthesizes food through photosynthesis, while fungus absorbs moisture.

(b) Interspecific competition

Answer :

Leopard and lion competing for a same prey. Sheep and cow competing for grazing in the same land.

(3) Name the type of association:

(a) Clown fish and sea anemone

Answer :


(b) Crow feeding the hatchling of Koel

Answer :

Brood parasiytism

(c) Humming birds and host flowering plants

Answer :


(4) What is the ecological process behind the biological control method of managing with pest insects?

Answer :

  • Pest insects act as prey to predator birds or frogs.
  • The biological control method consists of releasing the predators in the farms so that they can control the pest population in the natural way.
  • This method releases predators to naturally control pest populations, thereby saving agricultural produce.
  • Frogs, being natural predators of locusts, also help control the pest population, thereby reducing the need for chemical pesticides.

Question 3. Short answer questions.

(1) How is the dormancy of seeds different from hibernation in animals?

Answer :

Dormancy and Hibernation in Seeds :

  • Seeds in dormancy lack metabolic activity and can only reappear with suitable moisture and sunlight.
  • Hibernation is a suspended state with reduced metabolic reactions.
  • Animal arousal after winter sleep is spontaneous and depends on ambient temperature.
  • Dormant seed doesn't show change unless planted or placed in moist conditions.

(2) If a marine fish is placed in a fresh water aquarium, will it be able to survive? Give reason.

Answer :

  • Marine fish have unique osmoregulation compared to freshwater fish.
  • Marine water has higher salinity than ion concentration, unlike freshwater.
  • Marine fish have unique mechanisms to cope with high saline environments.
  • Therefore, marine fish cannot survive in freshwater due to their unique osmoregulation mechanisms.

(3) Name important defense mechanisms in plants against herbivores.

Answer :

Herbivores feed on plants and thus they are predators for the vegetations.

Plants therefore have evolved variety of morphological and chemical defences against herbivores, which are as follows :

  • Cactus and acacia horns serve as morphological defenses against herbivores.
  • Many plants produce toxic and unpalatable chemicals that can affect the feeding herbivore.
  • Consumption of these chemicals can inhibit predator feeding, digestion, reproduction, or even kill it.
  • Calotropis plants produce toxic cardiac glycosides that can harm grazers.
    Secondary metabolites like nicotine, caffeine, quinine, strychnine, and opium act as defenses against grazers and browsers.

(4) An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?

Answer :

  • Orchid is an epiphyte. It gets the support from the mango tree. But it does not cause any harm to the mango tree.
  • Mango tree does not derive any benefit from this association. Therefore, this interaction is of type of commensalism.

(5) Distinguish between the following:

(a) Hibernation and Aestivation

Answer :

Hibernation Aestivation
Hibernation is winter sleep for some warm-blooded and cold-blooded animals. Aestivation is the type of summer sleep, shown by cold-blooded animals.
It is for the whole winter. It is of short duration.
Animals seek warmer places for hibernation. Animals search for the moist, shady and cool place to sleep.
Metabolic activities slowdown during hibernation. Metabolic activities of aestivators remain low during aestivation period.
Hibernation maintains body temperature and prevents internal damage.


Aestivation is a process that helps maintain body temperature by preventing excessive water loss, thus preventing internal body damage from high temperatures.
E.g. Bats, birds, manimals, insects, etc. show hibernation. E.g. Bees, snails, earthworms, salamanders, frogs, earthworms, crocodiles, tortoise, etc. show aestivation.

(b) Ectotherms and Endotherms

Answer :

Ectotherms Endotherms
Ectotherms do not have ability to generate heat in the body. Endotherms possess the ability to generate their own body heat.
Ectotherms depend on the environmental sources to heat their bodies. E.g sunlight. Endotherms do not depend upon outside

sources to generate heat.

Most ectotherms are confined to warmer parts of the world. Endotherms inhabit coldest parts of the earth.
Body temperature of ectotherms fluctuate according to ambient temperature. Body temperatures of endotherms remain constant and do not show fluctuations as per ambient temperatures.
Metabolic rate of ectotherms is low.

Eg. Amphibians and reptiles.

Metabolic rate of endotherms is high.

E.g. Mammals and birds

(c) Parasitism and Mutualism

Answer :

Parasitism Mutualism
Parasitism is the relationship where only one organism receive benefits, while the other is harmed in return. Mutualism is the relationship where both the organisms of distinct species are benefited.
Parasite cannot survive Without host but if the host is overexploited then parasite too dies. Both the species are dependent on each other for their benefits and survival.
Parasitism can be facultative or obligatory. Mutualism is obligatory relationship.
Parasitism is a negative interaction. Mutualism is a positive interaction.

(6) Write a short note on

(a) Adaptations of desert animals

Answer :

  • Desert animals are xerocoles, adapted for water conservation or heat tolerance.
  • They have low basal metabolic rates and rely on moisture from plants, rarely drinking water.
  • Desert animals like camel produce concentrated urine and dry dung.
  • Many hot desert animals are nocturnal, seeking shade or dwelling underground.
  • Smaller animals emerge from burrows at night.
  • In cold deserts, animals have warmer fur and insulating layers of fat.
  • Few desert reptiles can cool themselves by sweating, so they shelter during the day.
  • Some animals have bodies designed to save water, such as scorpions and wolf spiders with thick outer coverings.
  • Desert animals' kidneys concentrate urine, reducing water excretion.

(b) Adaptations of plants to water scarcity

Answer :

  • Desert plants have thick cuticles on their leaf surfaces and sunken stomata to minimize water loss through transpiration.
  • They have a special photosynthetic pathway called CAM-Crassulacean acid metabolism, allowing stomata to remain closed during the day.
  • Some plants, like Opuntia, have reduced leaves or spines to prevent transpiration.
  • The photosynthetic function is taken over by flattened stems called phylloclade.

(c) Behavioural adaptations in animals

Answer :

  • Desert lizards maintain body temperature through basking in the sun and moving into shade when temperature drops.
  • Snakes also bask during winter months due to their ectothermic nature.
  • Smaller animals use burrowing to adapt to temperature extremes.
  • Some species burrow into sand to hide from heat.
  • Birds and mammals migrate to cope with severe winter temperatures.

(7) Define Population and Community.

Answer :

  • Population : A population is a group of organisms from the same species that can interbreed and live together in a specific geographical area, sharing or competing for resources.
  • Community : Several populations of different species in a particular area makes a community.

Question 4. Long answer questions.

(1) With the help of suitable diagram describe the logistic population growth curve.

Answer :

  • Populations of any species have limited resources, leading to competition for resources. The most fit organisms succeed through survival and reproduction.
  • A habitat has enough resources to support a maximum number, but beyond a certain limit, the species' carrying capacity (K) is reached.
  • Population growth in limited resources shows phases: lag phase, acceleration phase, deceleration phase, and an asymptote when the density reaches the carrying capacity.
  • Verhulst-Pearl Logistic Growth, a plot of N relative to time (t), is considered a more realistic model due to finite resources for animal populations.
  • Logistic growth always shows a sigmoid curve.

(2) Enlist and explain the important characteristics of a population.

Answer :

Important characteristics of a population are as follows :

Natality, Mortality, Density, Sex ratio, Age distribution and age Pyramids depicting the same and Growth.

Natality :

  • Natality refers to the birth rate of a population, which can increase population density.
  • It can be crude birth rate, calculated by the number of births per 1000 population/year.
  • Specific birth rate is relative to a specific criterion like age.
  • Absolute natality is the number of births under ideal conditions with abundant resources.
  • Realized natality is the number of births under different environmental pressures, always greater than absolute natality.

Mortality :

  • Mortality is the death rate of a population, measured in deaths per 1,000 individuals per year.
  • A mortality rate of 9.5 in a population of 1,000 means 9.5 deaths per year or 0.95% of the total.
  • Absolute Mortality: The number of deaths under ideal conditions with abundant resources.
  • Realized Mortality: The number of deaths under environmental pressures.
  • Absolute mortality always outweighs realized mortality.

Density :

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

  • Natality i.e. Birth rate: Adds to initial population density.
  • Mortality: Decreases population density.
  • Immigration: Number of same species entering the habitat.
  • Emigration: Number of individuals leaving the habitat.

Natality and immigration increase population density, mortality and emigration decrease it.

Sex ratio : The sex ratio is the ratio of male to female individuals in a population, typically 1:1, and is considered the evolutionary stable strategy of ESS for each population, ensuring a balanced distribution.

Age distribution and age pyramid :

  • The age distribution and age pyramid are crucial for understanding human populations.
  • Each population consists of individuals of different ages, and the age distribution is plotted to create an age pyramid.
  • The pyramid divides the population into three age groups: Pre-reproductive (ages 0-14), reproductive (15-44), and post-reproductive (ages 45-85+).


Population Growth and Density Analysis :

  • Population growth leads to increased density.
  • Size and density are dynamic parameters.
  • Factors include food, predation pressure, and weather.
  • Density indicates population flourishing or decline.

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