NCERT Solution Class 12 Biology Chapter 13 Organisms and Population

NCERT Solutions, Question Answer and Mind Map for Class 12 Biology Chapter 13, “Organisms and Population,” is a comprehensive study material package designed to help students understand the dynamics of populations and communities.

NCERT Solutions provide detailed explanations and answers to the questions presented in the chapter. The solutions cover all the topics in the chapter, including population growth, population interactions, and community ecology. They also provide tips on how to answer different types of questions, including short answer, long answer, and multiple-choice questions.

The question-answer section of the chapter covers a wide range of topics, from the concept of population density and growth to the different types of interactions between populations. It also includes questions on the ecological niche, competition, predation, and symbiosis.

The mind map provides a visual representation of the key topics covered in the chapter, allowing students to understand the connections between different concepts and ideas. The mind map covers the different factors that affect population growth, such as birth rate, death rate, immigration, and emigration. It also includes the different types of population interactions, such as competition, predation, and mutualism.

NCERT Solution Class 12 Biology Chapter 13 Organisms and Population with Mind Map PDF Download

Ecology: Ecology is the branch of biology that deals with the interactions among organisms and between the organism and its physical (abiotic) environment. Study of ecology is important to strike a balance between development and maintenance of natural environmental and biotic communities, use and conservation of resources, solve local, regional, and global environmental problems.

Organism and Its Environment:

The branch of science that studied the relationship between the organism and the environment is called ecology.

Various levels of the organization are:

  • Organism: each individual belonging to the species. 
  • Population: The group of organisms that are capable of interbreeding.
  • Communities: A combination of different populations combine together to form communities.
  • Biomes: A large number of flora and fauna found in a climatic zone.
NCERT Solution Class 12 Biology Chapter 13 Organisms and Population – Class 12 Biology Chapter 13

Major Abiotic Factor:

The most important biotic factor that affects the environment, as well as organisms, is the temperature. The temperature around the poles is lesser than the temperature found around the equator. The temperature of the polar region ranges from sub zero and increases up to >500C in tropical deserts in summer. The temperature will affect the metabolism rate and physiology of the body as it affects enzyme kinetics. Those organisms that can tolerate a high range of temperature are called eurythermal. E.g., dog, cat, red algae, etc while a large number of organisms tolerate only a narrow range of temperature called stenothermal. E.g., python, crocodile, penguin. The temperature tolerance of organisms depends upon the type of geographical area they are found.

The other important abiotic factor is water on which life depends. The area where the amount of water is less are called deserts where only organisms that have special adaptations will be able to survive. In the aquatic organism, the composition and pH of the water are very important. The range of salinity for some organisms is quite high called euryhaline, e.g., green crab and molly fish while in some organisms the salinity level tolerance is very low called stenohaline e.g., haddock, goldfish, etc. The organisms of seawater are found to be less adaptive to the marine water and vice versa due to their different osmotic environment.

The other important abiotic factor is light which is useful for the process of photosynthesis and can be observed in the case of autotrophs. The main source of light, well known, is the sun. The requirement of light intensity varies from organism to organism as some organisms require high light intensity while some organisms require low light intensity. There are various types of plants categorized on the basis of the light intensity required, the short-day plants and long-day plants. In animals, the small fluctuation of light will affect the various plant activities that include migration, reproduction, and foraging. The quality of the spectrum of solar radiation is quite important for life. The spectrum consists of ultraviolet radiation which is very harmful to the organisms while the different colors of the visible spectrums are not available for marine organisms found at various depths of the oceans.

The other important abiotic factor affecting organisms and their population is soil. The soil nature and features vary depending upon the type of climate, and the process of weathering, the development of soil, transportation of soil, or whether it is sedimentary. There are various parameters that affect the type of soil that are pH, minerals present in the soil, and the topography. Apart from this, other parameters such as pH, mineral composition, and topography depend upon the type of vegetation and animals present.

Organisms: An organism refers to a contiguous living system that lives in an environment and has the ability to adapt and retain certain structure and behaviour. It includes fungi, bacteria, plants, animals, and humans. An organism collectively forms a population. The population forms a community which operates the ecosystem. The ecosystem consists of both biotic and abiotic factors.

Major abiotic factors which lead to variation in the physical and chemical conditions of different habitat are temperature, water, light, and soil.

Responses to Abiotic Factors:

Various organisms respond differently towards various abiotic factors.

The various abiotic factor responses are:

  • Regulators: They are those organisms that are capable of maintaining homeostasis and regulation resulting in constant body temperature, osmotic concentration, etc. This property can be observed in the case of the birds and mammals along with few vertebrates and invertebrate species. In the case of humans, the body temperature is maintained at 370C resulting in homeostasis. In the summer season when the temperature is very high then the body sweats profusely in order to maintain the body temperature which is similar to the process of evaporation resulting in cooling. In the winter season, the outer temperature is very low so the body saves continuously to maintain the inner body temperature making it warm. In the case of plants, this mechanism of maintaining the internal body temperature is absent.
  • Conformers: They are those organisms that are unable to regulate their body temperature. Their body releases or absorbs heat that results in an increase or decrease in body temperature resulting in the process of thermoregulation which is an energetic process. In the case of small animals, the surface-to-volume ratio is larger so the heat of the body can be released quickly, thus, the animals are absent in the polar region. The process of evolution will result in various benefits.
  • Partial Regulators: They are those species that are capable of regulation but only up to a certain limit depending upon the environmental conditions. The organisms simply undergo confirmation when they cross this limit.
  • Migration: The movement of animals from one place to another depending upon their requirements. For example, the migratory birds that come every winter from Siberia to Keoladeo National Park (Bharatpur) in Rajasthan due to the stressful conditions in their habitat.
  • Spores: There are certain microorganisms that include bacteria, fungi, etc to stop their growth during the unfavourable conditions of the environment. As in the winter season, the animals undergo winter sleep called hibernation while in summer they undergo summer sleep called aestivation.


The feature that helps the organism to survive or to reproduce in their habitats is called adaptation. It is observed that the organisms usually adapt themselves according to the environment they live in. For example, in the case of the desert plants like Opuntia, they have thick cuticles, leaves modified into spines, and sunken stomata so as to reduce the rate of transpiration and undergo photosynthesis with the help of the CAM pathway. while in the case of the higher altitudes like mountains and hills, humans have shown altitude sickness resulting in nausea, short breaths, fatigue, heart palpitations, etc. But after some time they acclimatize themselves according to the environment and results in the higher production of red blood cells so that more oxygen can bind to them and increases the rate of respiration. There are certain behavioural responses that can be observed in various animals based on the environmental conditions.


Population is defined as a group of individuals or organisms of any species living in a well–defined geographical area, at a specific time with the capability of interbreeding. For example, population of deer in a forest.

Population Attributes:

  • Birth rate: Total number of individuals born in a given period of time.
  • Death rates: Total number of deaths in a period of time.
  • Sex Ratio: Total number of females and males per 1000 individuals.
  • Age pyramid: A plot of age distribution.
NCERT Solution Class 12 Biology Chapter 13 Organisms and Population – Class 12 Biology Chapter 13

Population Growth:

Population growth refers to the increase in the number of individuals in a population. This depends on various factors such as weather, food availability, predator pressure, etc.

The population density changes due to the following factors:

  • Natality: The number of births in a population in a given time period.
  • Mortality: The number of deaths in a population in a given time period.
  • Emigration: The number of individuals who moved to some other habitat in a certain time period.
  • Immigration: The number of individuals who have come into the habitat from elsewhere in a certain period of time.
NCERT Solution Class 12 Biology Chapter 13 Organisms and Population – Class 12 Biology Chapter 13

Population Growth Models:

The population growth models include:

  • Exponential Growth: In the limited supply of food, the population follows an exponential growth.
  • Logistic Growth: When the resources are finite and become limited sooner or later, the population growth is said to be logistic.
NCERT Solution Class 12 Biology Chapter 13 Organisms and Population – Class 12 Biology Chapter 13

exponential growth equation as:

Nt = N0 ert


Nt = Population density after time t

N0 = Population density at time zero

r = intrinsic rate of natural increase

e = the base of natural logarithms (2.71828)

Population Interactions:

This refers to the interaction between different populations. There are various modes of population interaction.

These include:

  • Predation: This is a type of interaction in which an organism kills and feeds on another organism. The one who kills is known as the predator and the one who is killed is the prey.
  • Competition: This is the type of biological interaction between different animals or species in which both are harmed.
  • Parasitism: Parasitism is a type of interaction between species in which the parasite lives inside the body of another organism and cause harm to it.
  • Commensalism: In this type of interaction one organism benefits while the other is neither benefitted nor harmed.
  • Mutualism: In this type of interaction, both the species or organisms are benefitted from each other.
NCERT Solution Class 12 Biology Chapter 13 Organisms and Population – Class 12 Biology Chapter 13

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