NCERT Solutions Class 12 Biology Chapter 11 Organism and population

Get your hands on the best NCERT Solutions for organism and population class 12 provided below by SimplyAcad,. These solutions are essential from exam point of view and help you strengthen your basics on Organisms and Populations.

NCERT textbook exercises are an extremely significant part of the exams, the paper questions are generally formed from these prescribed exercises by CBSE, therefore, completing them must be every students’ priority. Our subject experts have followed the recently updated syllabus of 2024-25 to avoid any misunderstanding. Scroll below to find the solutions organised in a detailed way for clarity and better understanding.

Access NCERT Answers Chapter 13: organism and population class 12 Question 1 to 3

Question 1

How is diapause different from hibernation?

Solution:

Diapause is a phase of suspended development to deal with undesirable conditions. Several species of Zooplankton and insects display diapause to pass through extreme climatic conditions while in their development stage. On the other hand, winter sleep or hibernation is a resting phase wherein animals escape winter by hiding in their shelters. They do so by entering a state of inactivity achieved by decreasing their metabolism. This process of hibernation is observed in squirrels, bats, and some rodents.

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Question 2

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

Solution:

The chances of survival of marine fish will reduce if placed in a freshwater aquarium, as their bodies are adapted to higher salt concentrations as provided by marine environments. In a freshwater environment, fishes fail to regulate the water which enters the body through the process of osmosis. Due to the presence of a hypotonic environment outside the fish’s body, water enters its body, which causes its body to swell, leading to the death of the marine fish.

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Question 3

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

Solution:

Thermophiles or Archaebacteria are ancient forms of bacteria that are present in deep-sea hydrothermal vents and hot water springs. They are able to withstand high temperature (exceeding 100 degrees C) as their bodies have adapted to these extreme environmental conditions. Such entities comprise specialized thermo-resistant enzymes that perform metabolic functions which do not get destroyed at these extreme temperatures.

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Access NCERT Answers Chapter 13: organism and population class 12 Question 4 to 6

Question 4

List the attributes that populations possess but not individuals.

Solution:

A group of entities belonging to the same species, residing in a specific geographical area at a particular time, together functioning as a unit can be termed a population.

Listed below are the attributes that a population exhibits:

• Natality or Birth rate: The ratio of live births in a particular area to the population of that area. The birth rate can be expressed as the number of individuals added to the population in terms of members of the population.
• Mortality or Death rate: The ratio of deaths in a region to the population of a region. The death rate can be expressed as the loss of individuals in terms of members of the population.
• Age distribution: The percentage of individuals of various ages in a given population. A population consists of individuals at any given time who are present in different age groups. Typically, an age pyramid can be used to depict the age distribution pattern.
• Sex ratio: The count of females or males per thousand individuals.
• Population density: The number of individuals in a population per unit at a particular time.

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Question 5

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

Solution:

If an adequate quantity of food resources is available to individuals in a population, it grows exponentially. The integral form of the exponential growth equation can be used to estimate the exponential growth, which is as follows:

 

$N_t = N_o e^{rt}$

Nt​=No​ert

Where:

• 
  $N_{\mathrm{t<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;\; font-size:\; 16px;">is\; the\; population\; density\; after\; ‘t’\; time.</span>}}$ 
• 
  $N_{\mathrm{o<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;\; font-size:\; 16px;">is\; the\; population\; density\; at\; time\; zero.</span>}}$ 
• 
  $\mathrm{e<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;">is\; the\; base\; of\; the\; natural\; logarithm\; =\; 2.71828.</span>}$ 
• 
  $\mathrm{r<span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;">is\; the\; intrinsic\; rate\; of\; natural\; increase.</span>}$ 

Let the current population density be ‘x’. The population density after three years will be 2x.

Substituting these values into the equation:

$2x = x \times e^{3r}$

Dividing both sides by x:

$2 = e^{3r}$

Taking the natural logarithm on both sides:

$\ln(2) = 3r$ $r = \frac{\ln(2)}{3}$ $r \approx 0.2311$

r≈0.2311

Therefore, the intrinsic rate of natural increase of the population is 0.2311.

Question 6

Name important defense mechanisms in plants against herbivory.

Solution:

A state of feeding on plants is known as herbivory. Many plants have evolved mechanisms, both chemical and morphological, to safeguard themselves against the act of herbivory. Listed below are the defense mechanisms of a few plants:

• Chemical defense mechanisms:
  • Caffeine, nicotine, opium, and quinine are some chemical substances that are produced in plants in response as part of their defense mechanism.
  • All parts of Calotropis weeds consist of lethal cardiac glycosides that prove fatal if consumed by herbivores.
• Morphological defense mechanisms:
  • Opuntia or cactus leaves are altered into thorns or sharp spines to prevent herbivores from feeding on them.
  • Margins of leaves in some plants are spiny, having sharp edges, preventing herbivores from feeding on them.
  • Sharp thorns with leaves are found in Acacia to prevent herbivores from feeding on them.

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Access NCERT Answers Chapter 13: organism and population class 12 Question 7 & 8

Question 7

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?

Solution:

An epiphyte, or air plant, is an entity growing on other plants. An orchid growing on the branch of a mango tree is an epiphyte. Such plants derive their nutrition and moisture from air, water, and rain or from the debris around them and not from the plant on which it is growing. Hence, the relationship between a mango tree and an orchid is an example of commensalism, wherein one species receives its benefits, and the other stays unaffected. In the given interaction, the orchid is benefitted, as it gets physical support from the mango tree, but the mango tree is unaffected.

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Question 8

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

Solution:

Predation is the ecological principle behind the biological control method of managing pest insects. Predation is referred to as the biological interaction between a predator and a prey wherein the predator feeds on the prey, thereby regulating the population of pest insects. Example: The Gambusia fish checks the mosquito larvae in water bodies.

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Access NCERT Answers Chapter 13: organism and population class 12 Question 9 & 10

Question 9

Distinguish between the following:

(a) Hibernation and Aestivation

(b) Ectotherms and Endotherms

Solution:

The differences are as follows:

(a) Hibernation and Aestivation

• Hibernation: Also known as winter sleep, it is a state of reduced activity observed in some entities to escape the extremely cold climatic conditions. Example: Squirrels and bears found in cold regions hibernate in winter.
• Aestivation: Also known as summer sleep, it is a state of reduced activity noticed in some entities to escape dehydration as a result of heat in summer. Example: Snails and fishes aestivate in summer.

(b) Ectotherms and Endotherms

• Ectotherms: The body temperature varies with their surroundings. They are cold-blooded entities. Examples: reptiles, fishes, amphibians.
• Endotherms: The body temperature remains constant. They are warm-blooded entities. Example: mammals and birds.

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Question 10

Write a short note on:

(a) Adaptations of desert plants and animals

(b) Adaptations of plants to water scarcity

(c) Behavioral adaptations in animals

(d) Importance of light to plants

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

Solution:

(a) Adaptations of desert plants and animals

• Desert plants: Some of the severe conditions typically found in deserts are scorching heat and water scarcity. Plants that are found in deserts have suitably adapted to this, as they have an extensive root system to tap the underground water. These plants possess sunken stomata on leaf surfaces and thick cuticles to decrease transpiration. A plant known as Opuntia has leaves that are completely altered into spines where photosynthesis is performed by the green stems. These plants of deserts have specialized pathways known as CAM or the C4 pathway to manufacture food which facilitates the stomata to stay closed during the day to decrease the water loss caused by the transpiration process.
• Desert animals: Lizards, kangaroo rats, and snakes are some animals found in deserts which have adapted appropriately to their habitat. For instance, the kangaroo rat inhabiting the Arizona Desert never drinks water in its life and has the potential to concentrate its urine to conserve water. Snakes and desert lizards have adapted in a way that they lounge in the sun early in the morning and burrow themselves in the sand in the afternoons to escape the heat of the day to prevent water loss.

(b) Adaptations of plants to water scarcity

Plants that are found in deserts have suitably adapted to water scarcity, as they have an extensive root system to tap the underground water. These plants possess sunken stomata on leaf surfaces and thick cuticles to decrease transpiration. A plant known as Opuntia has leaves that are completely altered into spines where photosynthesis is performed by the green stems. These plants of deserts have specialized pathways known as CAM or the C4 pathway to manufacture food which facilitates the stomata to stay closed during the day to decrease the water loss caused by the transpiration process.

(c) Behavioral adaptations in animals

Some entities that are affected by fluctuations in temperature experience adaptations, namely aestivation, hibernation, and migration, to escape environmental pressure and adapt to their natural habitat. These adaptations in the behavior of an entity are termed behavioral adaptations. For instance, ectothermic and a few endothermal animals display these adaptations.

Some cold-blooded animals, ectotherms such as amphibians, fish, and reptiles have their temperatures varying with their surroundings, as seen in desert lizards, where they lounge in the sun in the early hours of the day and burrow themselves in the sand when the temperature begins to rise at noon. Some warm-blooded animals, or endotherms, such as mammals or birds, escape the hot and cold weather conditions by hibernating in winter and aestivating in summer. They take shelter in burrows, caves, etc., to safeguard against these temperature variations.

(d) Importance of light to plants

The ultimate source of energy for plants is sunlight. Plants are referred to as autotrophic entities. They require light to perform photosynthesis. Light has a critical role in producing a photoperiodic response that takes place in plants. These plants respond to the changes in light intensity in different seasons to be able to meet their photoperiodic requirements for the process of flowering. Also, light has a significant role in aquatic habitats for the vertical distribution of plants in the sea.

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

Temperature is one of the most important ecological factors, varying from place to place and affecting the distribution of animals on the planet. Eurythermal animals can tolerate a range of temperatures, while stenothermal animals can withstand only a narrow range of temperatures. In order to suit their natural habitats, animals also undergo adaptations. For instance, animals in cooler regions have short limbs and ears, helping to prevent heat loss from the body. Animals in colder regions have thick fat layers underneath their skin and thick fur coats to prevent heat loss.

Water scarcity also expels animals to undergo several adaptations to familiarize themselves with their natural habitats. Lizards, kangaroo rats, and snakes are some animals found in deserts which have adapted appropriately to their habitat. For instance, the kangaroo rat inhabiting the Arizona Desert never drinks water in its life and has the potential to concentrate its urine to conserve water. Snakes and desert lizards have adapted in a way that they lounge in the sun early in the morning and burrow themselves in the sand in the afternoons to escape the heat of the day to prevent water loss