Class 12 Biology

Organism Population


Question 1: How is diapause different from hibernation?

Answer: Diapause is the delay in development, while hibernation refers to minimizing the metabolism in order to survive adverse environmental conditions. Diapause is related to growth, while hibernation is related to metabolism.

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

Answer: A marine fish is adapted to live in highly saline water. its osmoregulation is suited to such conditions. It will not be able to maintain homeostasis in freshwater and will not survive in a freshwater aquarium.

Question 3: Define phenotypic adaptation. Give one example.

Answer: Some of the changes in an organism’s behavior, morphology and physiology in response to a unique environment is called phenotypic adaptation. Phenotypic adaptation may or may not be permanent. Example: cactus plants have sunken stomata to reduce transpiration to prevent water loss in extremely hot and dry conditions.

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

Answer: Most of the organisms can survive within a narrow range of temperatures, while very few can survive in a wide range of temperatures. Some microbes have the adaptations to withstand very high temperature and hence they survive and thrive in habitats with temperatures exceeding 100°C. Thermoacidophiles are examples of such microorganisms.

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

Answer: Birth Rate, Death Rate and Sex Ratio

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

Answer: We know that;

dN/dt = rN

Given; dN = 2 – 1 = 1, N = 1 and dt = 3
Or, 1/3 = r
Or, r = 0.33

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

Answer: Some plants have thorns to prevent grazing. Some plants have sticky resins, neurotoxic substances, etc. to prevent herbivory. Examples: Acacia, opium, etc.

Question 8: 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: This is an example of commensalism because in this relationship orchid plant is benefited but mango tree is neither harmed nor benefitted.

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

Answer: The concept of predation works behind the biological control method of managing with pest insects. The population of pest insects is managed with the help of biological control.

Question 10: Distinguish between the following:

(a) Hibernation and Aestivation

Answer: Hibernation happens during winter, while aestivation happens during summer. Bear shows the example of hibernation, while frog shows the example of aestivation.

(b) Ectotherms and Endotherms


These are cold blooded animals.These are warm blooded animals.
They cannot maintain their body temperature.They can regulate their body temperature.
Examples: fish and frogExamples: birds and mammals

Question 11: Write a short note on

(a) Adaptations of desert plants and animals

Answer: Adaptations of desert plants and animals are aimed at preventing water loss. For example; cactus plant has sunken stomata to minimize transpiration. These plants also have fleshy stems in order to conserve water. The rat kangaroo meets its all water needs by way of oxidation of fat inside the body. Many xerophytic animals have resorted to uricotelic mode of excretion in order to conserve water.

(b) Adaptations of plants to water scarcity

Answer: Following are some adaptations of xerophytic plants to live with water scarcity:

(c) Behavioural adaptations in animals

Answer: Following are the examples of behavioral adaptations in animals:

(d) Importance of light to plants

Answer: Following are the significance of light for plants:

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

Answer: Temperature has significant effect on metabolism of living beings. The level of thermal tolerance of species determines the geographical distribution of various species.

Eurythermal: Organisms which can tolerate and thrive in a wide range of temperatures are called eurythermal. Very few organisms come in this category.

Stentothermal: Organisms which can thrive in a narrow range of temperatures are called stentothermal. Most of the organisms belong to this category.

Question 12: List the various abiotic environmental factors.

Answer: Temperature, Light, Soil, Water

Question 13: Give an example for:

(a) An endothermic animal

Answer: Man

(b) An ectothermic animal

Answer: Lizard

(c) An organism of benthic zone

Answer: Lemon shark

Question 14: Define population and community.

Answer: Population: A group of members of a species living in a well defined geographical area is called population.

Community: A group of individuals of different species living in a well defined geographical area is called community.

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


Answer: The relationship between two organisms in which one organism is benefitted and another is neither harmed nor benefitted is called commensalism. Example: Egret and cattle.


Answer: The relationship in which the smaller organism lives on or inside a bigger organism (host) is called parasitism. In this case, the parasite is always benefitted while the host is always harmed. Example: tapeworm in human gut.


Answer: The ability of an organism to mix with its surroundings is called camouflage. Example: Stripes of tiger help it in hiding against long grasses.


Answer: The relationship between two organisms in which both are benefitted is called mutualism. Example: Lichen which is an association of fungi and algae.

Interspecific competition

Answer: Competition between members of two species is called interspecific competition. Example: Competition between goats and deer for grass.

Question 16: With the help of suitable diagram describe the logistic population growth curve.

Answer: In a habitat with limited resources, the population growth initially shows a lag phase. This is followed by phases of acceleration and deceleration, and finally an asymptote.

A plot of N in relation to time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth and is described by the following equation:


N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity

The logistic growth model is considered a more realistic growth model.

Question 17: Select the statement which explains best parasitism.

9a) One organism is benefited.
(b) Both the organisms are benefited.
(c) One organism is benefited, other is not affected
(d) One organism is benefited, other is affected.

Answer: (a) One organism is benefited

Question 18: List any three important characteristics of a population and explain.

Answer: Following are the three important characteristics of population:

Birth Rate: Number of live births per thousand members is generally taken as birth rate.

Death Rate: Number of deaths per thousand members is generally taken as death rate.

Sex Ratio: The ratio of females to males in a population is called sex ratio.