CBSE Class 12 Biology Revision Notes Chapter 13

Class 12 Biology Chapter 13 Notes: Organisms and Populations

Biology is a branch of science that deals with the study of life. It is divided into specialised fields covering organisms’ morphology, physiology, anatomy and origin. Ecology, as a branch of Biology, studies the relationship of organisms with biotic and abiotic factors present in the environment. 

In the Chapter, ‘Organisms and Populations’, students learn about the four levels of biological organisation: organisms, populations, communities and biomes. It also reveals the most significant physical factors of the environment, such as temperature, light, water and soil, that most organisms are adapted to.

Extramarks provides up-to-date preparation notes and study materials for students. Biology experts from the Extramarks team have prepared our Class 12 Biology Chapter 13 Notes. In Class 12 Biology Chapter 13 Notes, ‘Organisms and Populations’, students will learn about the complexity of the living world with simple definitions and concepts. The Notes help students understand what ecology is and points out its importance. 

Class 12 Biology Chapter 13 Notes, will answer all the ‘how-type’ and the ‘why-type’ questions scientifically as we observe them in the nature we live in. For example, how do bees know the flowers with nectar or why does a bird sing?

Ecology studies the relationships between living organisms and their environment. It is concerned with four levels of biological organisation.

• Organisms.
• Populations.
• Communities.
• Biomes.

Extramarks is a unique platform designed for students to provide them with high-quality study resources to help them to get excellent grades in the examinations. Students should visit the Extramarks website and prepare for their examination with the Chapter 13 Biology Class 12 Notes, developed by the Extramarks in-house subject experts.

Key topics covered in Class 12 Biology Chapter 13 Notes.

The crucial l topics summarised in Class 12 Biology Chapter 13 Notes are explained below:

• Organism and Its Environment.
• Populations.  

1. Organism and Its Environment:

Physiological ecology studies different organisms and their adaptations to their environment for survival and reproduction. The earth’s rotation and tilt of the axis cause annual variations that bring about precipitation. These together lead to the formation of important biomes such as deserts, rain forests and tundra.

The key elements that lead to variation in the physical and chemical conditions of different habitats are

• Temperature
• Water
• Light
• Soil

The biotic components, as stated in the Class 12 Biology Chapter 13 Notes, such as pathogens, parasites, predators and competitors also affect the habitat variation. Over time, the organisms go through natural selection and evolve adaptations that help them survive and reproduce. 

Each organism can tolerate a specific range of conditions, use diverse resources and perform a significant function in the ecosystem. These together create a  ‘niche’ in the ecosystem.  .

Class 12 Biology Chapter 13 Notes provide a detailed explanation of the significant role of the abiotic factors, how organisms respond to them and their adaptations.

1. Major Abiotic Factors:

Abiotic factors play a significant role in the adaptations of animals.

Temperature:

• Temperature is the most significant environmental factor in ecology.  
• The temperature on the land varies according to seasons. It decreases as we move from the equator towards the poles, from plains to mountain tops. 
• The temperature range is sub-zero level in polar areas, while in tropical deserts, it goes up to more than 50 degrees Celsius in summer. 
• Unique habitats such as thermal springs and deep-sea hydrothermal wells have temperatures that exceed hundred degrees Celsius. 
• The significant temperature plays a vital role in organisms’ enzyme kinetics and basal metabolism.
• If organisms can tolerate a wide range of temperatures, they are called eurythermal. A significant group of organisms are restricted to a narrow range of temperatures called stenothermal.

Water:

• The most crucial factor that influences the life of organisms is water. 
• Desert water limitation allows only a few species with unique adaptations to survive.
• The productivity and distribution of plants are influenced by water. 
• The salt concentration varies in water bodies; < 5% in inland waters, 30-35% in the sea and >100% in hypersaline lagoons.
• Euryhaline organisms can tolerate a wide range of salinities, while Stenohaline organisms tolerate a narrow range of salinities.

Light:

• Light is essential for all autotrophs as they can only produce their food in the presence of sunlight through photosynthesis. 
• In the forest, plants such as herbs and shrubs have adapted to photosynthesis even in low light conditions. The reason is that other trees overshadow them.
• Animals utilise the daily and seasonal variations in light intensity to understand reproduction, foraging and migration timings. 
• Deep in the oceans, the environment is very dark, and its inhabitants are oblivious of a source of light and energy called the sun. 
• Solar radiation is essential for life; however, UV radiation harms many organisms.

Soil:

• The nature of the soil varies in different places. The exhibition of different soil properties depends on the climate and the weathering process, whether the soil is transported or sedimentary.
• Soil composition, aggregation, and grain size decide the water holding capacity of the soil. Other factors include pH, topography and mineral composition.
• The type of soil determines the species of animal.

Class 12 Biology Notes Chapter 13 are prepared by experienced faculty  who restructures and revises the content regularly while adhering to the CBSE guidelines and curriculum and have made the complicated concepts simple to understand. These notes are prepared in easy to understand language so that students can comprehend and strengthen their base and can answer any question no matter how tweaked they are and this enhances their confidence level to a great extent.

2. Responses To Abiotic Factors: 

As described in the Class 12 Biology Chapter 13 Notes, over a million years, species are believed to evolve into a stable environment that allows all biochemical reactions and physiological functions to enhance overall fitness with maximum  efficiency. The constancy could be in the form of optimum temperature and osmotic concentration of body fluids. In homeostasis, the organism maintains its internal environment despite varying external environmental conditions.

We will now look at how organisms cope with the varying external environments:

Regulate:

• Organisms maintain homeostasis by physiological means.
• All birds and mammals undergo thermoregulation and osmoregulation. 
• Most mammals regulate their body temperature similar to that of humans. 
• The average body temperature of a human being is 37 degrees Celsius.
• When the external temperature is too high in summer, we begin to sweat. Sweating causes evaporative cooling and hence cools down the body.
• In winter, the external environment is lower than 37 degrees Celsius. Hence, we begin to shiver to produce heat and raise the temperature.

Confirm:

• 99% of animals and plants cannot maintain a constant internal environment. Their body temperature changes with the atmospheric temperature. 
• The animals change their body temperature with the atmospheric temperature. 
• Osmotic concentration of the body fluids in aquatic animals changes concerning the ambient water osmotic concentration. Such animals and plants are called conformers.

Why were conformers not able to evolve to become regulators?

Thermoregulation expels a lot of energy into small animals like hummingbirds and shrews. Heat loss or heat game depends on the surface area of the animal. Small animals have less volume but the largest surface area. Thus, they tend to lose body heat quickly during winters. The animals use a lot of energy to produce heat through metabolism. For this reason, small animals cannot survive in polar regions.

Migrate:

• Migration is the temporary movement of an organism from a stressful habitat to a favourable habitat. The organisms then return once the difficult period is over. 
• Most birds migrate during winters to warmer areas over long distances. 
• Keoladeo National Park in Rajasthan hosts thousands of birds migrating from Siberia and other cold Northern regions.

Suspend:

• Bacteria, fungi and other small plants produce thick-walled pores that help them survive unfavourable conditions and germinate in a suitable environment.
• Seeds of higher plants also survive in periods of stress during dispersal. The seeds remain dormant and germinate at favourable temperatures and moisture to grow into new plants.
• If animals cannot migrate, they avoid stress by escaping in time. For  example, bears hibernate during winter while tiny snails and fish go into aestivation to avoid summer-related heat. Under stressful conditions, zooplankton enters the diapause-a stage of suspended development.

3. Adaptations of Plants and Animals: 

Adaptation is any morphological, physiological or behavioural feature that allows an organism to survive and reproduce in its habitat.  

• Many adaptations have evolved over the years in the genes of the organisms. For instance, the Kangaroo rat found in the North American desert meets its water requirements for internal fat oxidation when water is unavailable. The Kangaroo rat can also concentrate its urine to minimise water loss in excretory products.
• In deserts, many plants have thick cuticles on their leaf surfaces. The stomata are arranged in deep pits to minimise water loss through transpiration. Desert plants also have a particular photosynthetic pathway(CAM) that allows the stomata to remain closed in the daytime.
• The leaves in Opuntia are reduced to spines, and the flat stems carry over the photosynthetic function.
• Allen’s Rule is seen in Mammals from colder regions with shorter limbs to minimise heat loss.
• Aquatic mammals like seals have blubber below their skin that reduces the loss of body heat, just like an insulator.
• Altitude sickness. One experiences altitude sickness in places of high altitude. The body gets insufficient oxygen and hence shows nausea, fatigue and heart palpitations. The body compensates for low oxygen availability by increasing the production of RBCs and decreasing the binding capacity of haemoglobin. The breathing rate also increases.
• Many animals perform their physiological functions in a narrow range of temperatures. For Example, it is 37 degrees Celsius for humans, while few microbes can flourish in hot springs where temperatures exceed a hundred degrees Celsius. 
• Some organisms also show behavioural responses. For Example, desert lizards have the physiological ability to deal with high temperatures but manage to keep their body temperature relatively constant. 
• Some species follow specific ways to maintain body temperature: basking in the sun, absorbing heat when the temperature is below the comfort zone, shifting into the shade when the atmospheric temperature increases and burrowing into the soil to escape from the ground heat.

To  enjoy the maximum benefit of these resources and to know more about the adaptations of plants and animals, students are advised to register themselves at  Extramarks website and access the Class 12 Biology Chapter 13 Notes, at their convenience. 

2. Populations
3. Population attributes:

• A population is a group of individual species in a well-defined geographical area where they share or compete for primary resources and interbreeding.
• Inter breeding includes both sexual and asexual reproduction in a population.
• Some  examples of population include cormorants in a wetland, rats in an isolated building, teakwood trees in a forest, bacteria in a culture and lotus plants in a pond.
• A population consists of well-defined attributes while an individual organism does not.
• A population consists of birth and death rates, which refer to per capita births and deaths. These rates are expressed as a difference in numbers (increase or decrease) based on the members of the population.
• Another attribute feature of a population is the sex ratio, the ratio of males to females in a population.
• A population can also be composed of people of different ages. When the age distribution is plotted for the population, the resulting (percentage of a given age group) structure is called an age pyramid.
• The age pyramids outline the age distribution of males and females in a diagram. The shape of the pyramids reveals the growth status of the population-whether; it is growing, stable or declining.
• The size of the population reveals its status in the habitat. All ecological processes in a population are evaluated in terms of any change in the population size.  
• Population size is called population density and is designated as N.
• However, the total number is a problematic adoptable measure if the population is large and counting is impossible, for instance, the culture of bacteria in a Petri dish.
• For a few ecological investigations, knowing absolute population densities is futile. Regardless, relative densities serve the purpose equally well. For Example, the number of fish caught in a trap is a good enough measure of the total population density in the lake.

To enjoy the maximum benefit of these resources and to know more about the adaptations of plants and animals, ecology students are advised to register themselves at the Extramarks website and access the Class 12 Biology Chapter 13 Notes, at their convenience. 

2. Population Growth:

• The size of a population for any group of species, as stated in the Class 12 Biology Chapter 13 Notes, is a varying parameter. It changes with various factors like time, food availability, predation pressure and extreme weather. The density of a population in a particular habitat during a specific period fluctuates due to changes in four basic processes; natality and immigration, leading to an increase in population density, and mortality and emigration, which lead to a decrease:
• Natality: The number of births during a given period in the population.
• Mortality: The number of deaths in the population during a given period.
• Immigration: Several individuals of the same species have come into a habitat from a different habitat during the considered period.
• Emigration: The number of individuals of the population who have left their current habitat and gone to another habitat during the considered period.

If N is the population density at time t, then its density at time t +1 is Nt+1 = Nt+ [(B + I) – (D + E)]

Population density increases with the number of births and the number of immigrants. (B + I) is greater than the number of deaths and emigrants (D + E). Births and deaths are the most critical factors influencing population density.

Growth Models try to predict the population of an organism that reproduces based on fixed rules. Different animal populations in nature show some restraints on growth or behave the same way. Students can get more details about growth models in Class 12 Biology Chapter 13 Notes.

1. Exponential growth: 

The availability of resources such as food and space is significant for the unrestricted growth of a population. When resources in the habitat are not limited, each species can realise its innate potential fully to increase in number, as observed by Darwin during the development of his theory of natural selection. Then the population grows exponentially or geometrically. 

In a population size(N), the birth rates are represented as b and death rates as d, then the increase or decrease in N during a unit t (dN/dt) will be:

dN/dt = (b – d) × N

Let (b–d) = r, then

dN/dt = rN

The r in this equation is called the ‘intrinsic rate of natural increase’ and is a significant parameter chosen for assessing the impacts of any biotic or abiotic factor on population growth.

1. Logistic growth: 

There is no disposal of unlimited resources in the population of any species in nature to permit exponential growth. This brings forth competition between individuals for limited resources. Eventually, the ‘fittest’ of the individual survives and reproduces. A specific natural habitat has enough resources to support a maximum number, beyond which no further growth is possible. This limit is called nature’s carrying capacity (K) for that species in that habitat. A population growing in a habitat with limited resources initially shows a lag phase, followed by phases of acceleration and deceleration and ends with an asymptote. An N plot about time (t) results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth and can be described by the following equation:

dN/dt = rN(K-NK)

Where N = Population density at time t.

r = Intrinsic rate of natural increase.

K = Carrying capacity.

1. Life History Variation:

• When populations evolve to maximise their reproductive fitness, it is known as Darwinian fitness (high r-value) in the habitat in which they live.  
• Organisms evolve toward the most efficient reproductive strategy based on a particular set of selection pressures. Some species may breed only once in their lifetime, like bamboo, while others breed several times during their lifetime, for example, mammals. 
• On the other hand, some produce a large number of small-sized offspring, oysters for instance, while others produce a small number of large-sized offspring like birds.
• Ecologists state that life-history traits of organisms have evolved concerning the restrictions imposed by the abiotic and biotic components of the habitat in which they live. 

1. Population Interactions:

Class 12 Biology Chapter 13 Notes highlights the importance of interspecific interactions. Interspecific interactions arise from the interaction of two populations belonging to different species. They could be beneficial, detrimental or neutral. A ‘+’ sign is assigned for beneficial interaction, a ‘-‘ sign for detrimental and 0 for neutral interaction. 

In mutualism, both species benefit, while in competition, both interactions are lost with each other. One species benefits from parasitism and predation while the other species is harmed. In commensalism, one species is benefitted while the other is neither harmed nor benefitted. Commensalism involves the interaction where one species is harmed while the other is unaffected.

To  enjoy the maximum benefit of these resources and to know more about the adaptations of plants and animals, ecology  and above stated phenomenons, students are advised to register themselves at the Extramarks website and access the Class 12 Biology Chapter 13 Notes, at their convenience.

Predation:  

• Predation is a way of transferring energy fixed by plants to higher trophic levels in nature.
• Predators help regulate a balance of species diversity in a community by decreasing the competition among competing prey species.
• The prey may become extinct if the predator is overexploited.
• Moreover, the prey species have developed defences to protect themselves from predation.
• Chemical substances that we extract from plants for commercial use (nicotine, caffeine, quinine, opium) are produced by them as defences against grazers.

Competition:

• Competition is best defined when the fitness of one species is higher in the presence of another species.
• Darwin was convinced that interspecific competition that includes the survival of the fittest is essential in organic evolution. 
• Competition generally occurs when closely related and unrelated species compete for the same limited resources.
• In Gause’s ‘Competitive Exclusion Principle, two closely related species competing for the same resources cannot co-exist, causing the elimination of the inferior species.

Parasitism:

• Parasites have evolved to be host-specific, where both the host and the parasite co-evolve.
• The life cycles of parasites often involve one or two intermediate hosts or vectors to carry out the parasitisation of their primary host. 
• For instance, to complete its life cycle, the human liver fluke depends on two intermediate hosts, a snail and a fish.

Commensalism:

• Interactions in which a single species is benefitted, and the other is neither harmed nor benefited are known as commensalism. Examples include an orchid growing on a mango branch, barnacles on the back of a whale, cattle egret, and grazing cattle on farms.

Mutualism:

• Both the interacting species are benefitted. 
• Lichens represent mutualism between a fungus and algae or cyanobacteria.
• The associations between fungi and the roots of higher plants in mycorrhizae.
• Mutualism saw in plant and animal relationships.

Class 12 Biology Chapter 13 Notes: Exercise Questions and  Solutions

The exercise questions and answer solutions covered in Class 12 Biology Chapter 13 Notes are prepared by experienced subject experts from our Extramarks. . These questions and solutions are an outstanding aid to help students revise the chapter quickly and easily. It also clarifies their concepts and helps to clear their doubts  and score well in exams. The ecology concepts are well laid out for students to learn effortlessly with accurate, up to date and authentic notes prepared by subject experts in collaboration with the research and development team (R&D)  and then it is reviewed by the quality check team (QC) before it’s shared with the students. . The solutions comprise all essential topics covered in the chapter. 

NCERT Exemplar for Class 12 Biology

NCERT exemplar for Class 12 Biology for chapter-‘Organisms And Population’ covers all essential topics and provides comprehensive exercise  for practice to enhance  students’ knowledge and strengthen their conceptual understanding. The exemplar books are excellent study materials for adequate preparation for boards and any other entrance examinations.  Regular tests and revisions make them aware of their mistakes through guided practice and help to achieve high grades in their exams.

Key features of Class 12 Biology Chapter 13 Notes

Here are some crucial features of Class 12 Biology Chapter 13 Notes:

• The study materials at Extramarks are well prepared with comprehensive coverage of all topics from NCERT textbook and NCERT exemplar. Students need to first be thorough with NCERT books and then upgrade their revision with additional study materials provided by Extramarkers
• The notes are prepared by subject experts with decades of teaching experience. The chapter notes cover all topics with detailed explanations and also include a set of important questions and formulas for quick reference.
• The notes are prepared pointwise and in a simple format to help students study and understand long/complex concepts quickly and easily. 
• Extramarks is one of the best websites for students to enhance their board exam preparation with subject experts and authentic, accurate and reliable study materials.   The credibility  Extramarks has built over the years is trusted by students and teachers across the country.