Organisms And Populations Chapter 13 Class 12 Biology NCERT Textbook PDF

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NCERT Class 12 Biology Textbook Chapter 13 With Answer PDF Free Download

Organisms and Populations

Chapter 13: Organisms and Populations

13.1 Organism And Its Environment

Ecology at the organismic level is essentially physiological ecology which tries to understand how different organisms are adapted to their environments in terms of not only survival but also reproduction.

You may have learned in earlier classes how the rotation of our planet around the Sun and the tilt of its axis cause annual variations in the intensity and duration of the temperature, resulting in distinct seasons.

These variations together with annual variation in precipitation (remember precipitation includes both rain and snow) account for the formation of major biomes such as deserts, rain forests,s and tundra (Figure 13.1).

Regional and local variations within each biome lead to the formation of a wide variety of habitats. Major biomes of India are shown in Figure 13.2.

On planet Earth, life exists not just in a few favorable habitats but even in extreme and harsh habitats – scorching Rajasthan desert, perpetually rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, permafrost polar regions, high mountain tops, boiling thermal springs, and stinking compost pits, to name a few.

Even our intestine is a unique habitat for hundreds of species of microbes.

What are the key elements that lead to so much variation in the physical and chemical conditions of different habitats?

The most important ones are temperature, water, light, and soil.

We must remember that the Physico-chemical (abiotic) components alone do not characterize the habitat of an organism completely; the habitat includes biotic components also – pathogens, parasites, predators, and competitors – of the organism with which they interact constantly.

We assume that over a period of time, the organism had through natural selection, evolved adaptations to optimize its survival and reproduction in its habitat.

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NCERT Solutions Class 12 Biology Chapter 13 Organisms and Populations

1. How is diapause different from hibernation?


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 winters 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.

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


The chances of survival of marine fish will reduce if placed in a freshwater aquarium as their bodies are altered 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 their body which causes their body to swell leading to the death of the marine fish.

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?


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 temperatures (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 destructed at these extreme temperatures.

4. List the attributes that populations possess but not individuals.


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

Listed below are the attributes that a population exhibits:

  • Natality or Birth rate

It can be given by 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

It is 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

It can be given by the percentage of individuals of various ages in a given population. A population consists of individuals at any given time, and are present in different age groups. Typically, an age pyramid can be used to depict the age distribution pattern.

  • Sex ratio

It is the count of females or males per thousand individuals

  • Population density

It is given by the number of individuals in a population per unit area at a particular time.

6. Name important defense mechanisms in plants against herbivory.


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 defence mechanisms of a few plants:

Chemical defence mechanisms:

  • Caffeine, nicotine, opium, and quinine are some chemical substances that are produced in plants in response as part of their defence mechanism
  • All of the parts of Calotropis weeds consist of lethal cardiac glycosides that demonstrate to be 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 it
  • Margins of leaves in some plants are spiny, having sharp edges, preventing herbivores to feed on them
  • Sharp thorns with leaves are found in Acacia to prevent herbivores from feeding on them

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


An epiphyte or air plants are 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 the 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.

Organisms and Populations NCERT Textbook With Solutions PDF Free Download

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