CBSE Biology Chapter 11 Organism And Population Class 12 Notes: FREE PDF Download
PDF Summary - Class 12 Biology Organisms and Populations Notes (Chapter 11)
Organism and Its Environment
The branch of science that studies the relationship between the organism and the environment is called ecology. Various levels of the organization are:
(i) Organism: each individual belonging to the species.
(ii) Population: The group of organisms that are capable of interbreeding.
(iii) Communities: A combination of different populations combine together to form communities.
(iv) Biomes: A large number of flor and fauna found in a climatic zone.
Level of Organization
Major Abiotic Factor
Abiotic Factors
(i) 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 subzero 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.
(ii) 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.
(iii) 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.
(iv) 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.
Responses to Abiotic Factors
Various organisms respond differently towards various abiotic factors. The various abiotic factor responses are:
(i) Regulators:
Homeostasis is the ability of an organism to maintain constant body temperature, osmotic concentration and other physiological processes.
This property is observed in birds, mammals, and a few vertebrate and invertebrate species.
In humans, the body temperature is maintained at 37°C, which is an example of homeostasis.
During summer, when the temperature is high, the body sweats profusely to maintain body temperature, which is similar to the process of evaporation resulting in cooling.
In winter, when the outer temperature is low, the body saves energy to maintain the inner body temperature, making it warm.
Plants do not have the mechanism to maintain internal body temperature like animals.
Orgasmic Response
(ii) 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.
(iii) 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.
(iv) 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.
(v) Spores: There are certain microorganisms that include bacteria, fungi, etc to stop their growth during the unfavorable 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.
Adaptations
Adaptation is a feature that helps organisms survive or reproduce in their habitats.
Organisms usually adapt themselves according to the environment they live in.
For example, desert plants like Opuntia have thick cuticles, modified leaves into spines, and sunken stomata to reduce transpiration and undergo photosynthesis with the help of the CAM pathway.
Humans at higher altitudes like mountains and hills can experience altitude sickness, resulting in symptoms like nausea, shortness of breath, fatigue and heart palpitations.
However, humans can acclimatise themselves over time and increase the production of red blood cells to bind more oxygen and increase the rate of respiration.
Behavioral responses can also be observed in various animals based on environmental conditions.
Populations
Population Attributes
A group of individuals that have the ability to interbreed among themselves and produce fertile offspring is called a population.
Populations have various attributes, including birth rate and death rate.
Birth rate refers to the increase in population, while death rate refers to the decrease in population.
Other attributes of the population include sex ratio and age distribution, which can be represented as an age pyramid.
An age pyramid shows the status of a population in terms of its expanding, stable, or declining state.
An expanding population is characterised by more young individuals than old individuals.
A stable population has an equal number of young and old individuals.
A declining population has more old individuals than young individuals.
Age Pyramid
Population Growth
The population size of any species in a particular region can never be static, it changes over time depending upon the availability of the food resources, weather conditions, and pressure of the predators. There are four main reasons that result in the changes in the population density, they are:
(i) Natality: It is defined as the number of births in a given time.
(ii) Mortality: It is defined as the number of deaths in a given time.
(iii) Immigration: It is defined as the number of species of a population coming to a different habitat for a particular time.
(iv) Emigration: It is the defined as the number of species of a population moving out to N represents the population density which is given as:
${N}_{t+1}={N_t}+[(B+I)-(D+E)]$
Where the population density is represented by N at time t, the birth rate is represented by B+I while the death rate is represented by D+E.
Population Growth
Growth Models
(i) Exponential Growth
When the nutrients available are excessive in quantity then it results in the formation of exponential growth. If N is the size of the population, b is the birth rates (per capita birth not total births) and d is the death rate (per capita births not the total births), then during time period t, the increase or decrease in N, (dN/dt) will be-
$\dfrac{dN}{dt} = {(b – d)} \times {(N)}$
Let ${(b–d)} = { r}$, then
$\dfrac{dN}{dt} = {rN}$
Exponential Growth
In this equation, r is called the ‘intrinsic rate of natural increase’. The exponential also called the geometric growth pattern will result in the population forming a J-shaped curve. The end equation which is obtained bas been stated below:
${N_t} = {N_0}{e^rt}$
${N_t}$ = Population density after time t
${N_0}$ = Population density at time zero
r = intrinsic rate of natural increase
e = the base of natural logarithms.
(ii) Logistic Growth
When the nutrients and other resources available are limited then the growth of the population will be called the logistic growth or Verhulst-Pearl Logistic Growth. This condition will result in more competition among organisms. The presence of limited resources will result in a lag phase, then exponential growth followed by deceleration, and finally asymptote.
Logistic Growth
The curve of logistic growth will be S-shaped or sigmoid in shape. The equation below shows Verhulst-Pearl Logistic Growth:
$\dfrac{dN}{dt} = rN \left( \dfrac{k-N}{K} \right)$
N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity.
Population Interactions
The interaction between different organisms can be divided into two categories that are, interspecific interactions and intraspecific interactions. When the interaction occurs between the same species then it is called Intraspecific interaction while the interaction occurring between different organisms then it is called interspecific interaction.
(i) Predation: The interaction in which the prey is killed by the predator. This is important for maintaining the species of the prey. In the case of the plants, herbivores act like the predators. In some plants, various adaptations are found that help them to protect themselves from the prey. The adaptation may include thorns as a defense or defensive agents like caffeine, nicotine, quinine, etc, that are also used commercially.
(ii) Competition: The interaction between the organism where both the organism compete for various resources that include food, water, shelter, etc. the organism may be of the same species or may be of different species. This interaction occurs when the resources are limited. According to the Gause’s principle of Competitive Exclusion. The two species that are closely related will compete for the same resources and cannot co-exist so the one which is inferior will be eliminated during competition.
(iii) Parasitism: It is an interaction where one species depends upon the other species resulting in benefitting the one species and harming the other species. Those parasites that feed on the outer surface of the host organism are called ectoparasites. For example, marine fishes consist of ectoparasitic copepods. In the case of birds, there is Brood parasitism in which the birds lay eggs in the nest of the host while the host incubates them.
(iv) Commensalism: It is an interaction between species where it is observed that one of the species is benefitted while the other species neither get any benefit nor any harm. For example, an orchid growing as an epiphyte on the branch of a mango.
(v) Mutualism: It is the interaction where both the species will benefit each other. Examples are lichens (mutual and symbiotic relationship between algae and fungi), and mycorrhiza (symbiotic relationship between fungi and the roots of the higher plants).
(vi) Amensalism: It is the interaction between the organism in which one species is harmed while the other species is neither benefited nor harmed.
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Section−A (1 Mark Questions)
1. Name the type of interaction that exists between:
(i) Orchid growing on a mango branch
(ii) Fig flower pollinated by a wasp
Ans. (i) Commensalism
(ii) Mutualism
2. What is Allen's rule?
Ans. Allen’s rule states that mammals from colder climates generally have shorter ears and limbs to minimise heat loss.
3. Define stenothermal and eurythermal organisms.
Ans. Eurythermal organisms: these organisms can tolerate and thrive in a wide range of temperatures.
Stenothermal organisms: these organisms are restricted to a narrow range of temperatures.
4. Why do clownfish and sea anemones pair up? What is this relationship called?
Ans. Clownfish live in the tentacles of sea Anemone and get protection from predators. This relationship is called commensalism.
5. What is meant by nature’s carrying capacity (K)?
Ans. Carrying capacity (K) is the maximum population size of the species that the environment can sustain indefinitely, given the food, habitat, water, and other necessities available in the environment.
Section−B (2 Mark Questions)
6. Why do predators avoid eating Monarch butterflies?
Ans. The Monarch butterfly is highly distasteful (bitter) to its predator (bird) because of a special chemical present in its body. The butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.
7. Distinguish between ectotherms and Endotherms.
Ans. Ectotherms are those animals whose body temperature changes and matches that of the environment in which they are living, whereas endotherms are those animals whose body temperature is maintained relatively constant by physiological regulation.
8. How do humans regulate body temperature at 37℃?
Ans. Humans maintain their body temperature at 37℃. In summer, when the outside temperature is more than our body temperature, humans sweat profusely. This results in evaporative cooling, i.e., the body cools as the sweat evaporates, a mechanism similar to a desert cooler. In winter, when the temperature is much lower than 37℃, the body starts to shiver. This is a kind of exercise that produces heat and raises the body temperature.
9. How do kangaroo rats adapt themselves to survive in water-scarce conditions?
Ans. Kangaroo rats show several adaptations-
They oxidise internal fat to meet 90% of water requirements (metabolic water).
To prevent water loss, they live in burrows during the day and produce concentrated urine and dry faeces.
10. What is population density? How will you measure population density in the following cases?
(i) fish in a lake
(ii) tiger census in a national park
(iii) a single huge banyan tree with a large canopy.
Ans. Population density is the number of individuals of a population (species) found in a unit area or space at a given time.
Population density= Number of individuals/Space
The population density in the following cases:
(i) The number of fish caught per trap is a good enough measure of the total population density in the lake.
(ii) The tiger census in our national parks and tiger reserves is often based on pug marks and faecal pellets.
(iii) percent cover or biomass.
11. Define the following.
(i) Mutualism
(ii) Competition
Ans. (i) Mutualism- It is an interaction between two organisms of different species where both partners benefit and help each other in survival. Examples- lichens, mycorrhiza, etc.
(ii) Competition- It is a process in which the fitness of one species (measured in terms of its ‘r’, the intrinsic rate of increase) is significantly lower in the presence of another species.
Benefits of Class 12 Biology Revision Notes
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The notes are prepared by experts in such a way that students can get all the important concepts of the chapter easily.
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The notes are available in a pdf format, hence, students can refer to them at any time - offline.
Importance of Class 12 Biology Organisms And Populations Notes
These notes simplify complex topics such as population growth, ecological adaptations, and interactions between organisms, making them easier for students to understand.
They highlight key concepts like population dynamics, natality, mortality, and carrying capacity, which are crucial for grasping how populations function in ecosystems.
The notes help students review important ecological principles and terms, providing a clear understanding of how organisms adapt and survive in different environments.
With well-organised content and examples, the notes aid in quick revision, especially before exams, ensuring that students focus on the most important points.
They include solved examples and practice questions, helping students apply the concepts learned in real-life ecological situations.
By focusing on exam-relevant topics, the notes ensure students are well-prepared for both board exams and competitive exams like NEET.
These notes simplify complex topics such as population growth, ecological adaptations, and interactions between organisms, making them easier for students to understand.
They highlight key concepts like population dynamics, natality, mortality, and carrying capacity, which are crucial for grasping how populations function in ecosystems.
The notes help students review important ecological principles and terms, providing a clear understanding of how organisms adapt and survive in different environments.
With well-organised content and examples, the notes aid in quick revision, especially before exams, ensuring that students focus on the most important points.
They include solved examples and practice questions, helping students apply the concepts learned in real-life ecological situations.
By focusing on exam-relevant topics, the notes ensure students are well-prepared for both board exams and competitive exams like NEET.
Tips For Learning the Class 12 Biology Chapter 11 Organisms And Populations
Begin by understanding key ecological terms like ecology, population, species, and the role of abiotic and biotic factors, as they form the foundation of this chapter.
Focus on population interactions such as mutualism, predation, competition, and parasitism. Understanding these relationships is crucial for grasping the dynamics of populations.
Use diagrams to visualise concepts like population growth curves, ecological pyramids, and adaptation strategies. Practise drawing them to help retain the information.
Study population growth models, including exponential and logistic growth, and understand factors like birth rate, death rate, and carrying capacity.
Focus on adaptations and how organisms survive in various environments. Understanding physiological, behavioural, and structural adaptations is key.
Regularly practise answering questions, including those based on population dynamics and ecological interactions, to strengthen your grasp of the concepts.
Relate the topics to real-world examples, such as human population growth, predator-prey relationships, and how environmental factors affect species distribution.
Revise regularly to retain important terms, formulas, and concepts, and apply them to both theoretical and practical problems.
Conclusion
Class 12 Biology Chapter 11: Organisms and Populations, students explore how organisms interact with their environment and each other. This chapter covers important concepts like ecological adaptations, population dynamics, and species interactions. By understanding these principles, students gain insight into the complex relationships that maintain ecological balance. The chapter highlights key factors like birth rates, death rates, and environmental resistance that influence population growth. Overall, mastering this chapter provides a deeper understanding of ecology and helps students apply these concepts in real-world situations, crucial for exams and future studies in biology.
FAQs on Organisms And Populations Class 12 Notes: CBSE Biology Chapter 11
1. What are the essential core concepts to focus on for a quick revision of the Organisms and Populations chapter?
For a complete and quick summary of this chapter, focus on these core concepts as per the CBSE 2025-26 syllabus:
- Ecological Hierarchy: The levels from organism to biome.
- Abiotic Factors: Key environmental factors like temperature, water, light, and soil and their influence.
- Responses to Abiotic Factors: How organisms cope through regulation, conformity, migration, and suspension (hibernation/aestivation).
- Population Attributes: Concepts like natality (birth rate), mortality (death rate), sex ratio, and age pyramids.
- Population Growth Models: The difference between exponential and logistic growth, and the concept of carrying capacity (K).
- Population Interactions: The six main types – predation, competition, parasitism, mutualism, commensalism, and amensalism.
- Adaptations: Morphological, physiological, and behavioural changes for survival.
2. For revision, what are the four major abiotic factors and their primary effects on organisms?
A quick recap of the major abiotic factors includes:
- Temperature: This is the most ecologically relevant factor. It affects the kinetics of enzymes and thus the metabolic activity and geographical distribution of organisms (eurythermal vs. stenothermal).
- Water: Life is unsustainable without water. Its availability determines the productivity and distribution of organisms. For aquatic organisms, the chemical composition and pH are crucial.
- Light: It is essential for photosynthesis in autotrophs. Light intensity and duration also affect animal activities like foraging, migration, and reproduction.
- Soil: The nature and properties of soil (composition, pH, mineral content) determine the type of vegetation and the animals that can be supported in an area.
3. How can you quickly summarise the difference between exponential and logistic growth for revision?
Here is a simple summary to differentiate the two population growth models:
- Exponential Growth (J-shaped curve): Occurs when resources like food and space are unlimited. The population grows at a geometric rate, leading to a steep, J-shaped curve. The rate of increase depends on the intrinsic rate of natural increase (r).
- Logistic Growth (S-shaped curve): Occurs when resources are limited. The growth shows a lag phase, followed by rapid acceleration, and finally slows down as it approaches the environment's carrying capacity (K). This results in a sigmoid or S-shaped curve.
4. How are the concepts of population growth and carrying capacity (K) interlinked in this chapter?
Population growth describes how a population's size changes over time. Carrying capacity (K) is a critical concept that links to this growth; it represents the maximum population size that a specific environment can sustainably support given its limited resources. When a population is small, it may grow exponentially. However, as it approaches the carrying capacity (K), its growth rate slows down due to increased competition for resources, eventually leading to the S-shaped logistic growth pattern where the population size stabilises around K.
5. What is a simple method to remember the six key population interactions for revision?
A helpful revision strategy is to use a notation system where '+' denotes a benefit, '-' denotes harm, and '0' denotes no effect. The six interactions are:
- Mutualism (+, +): Both species benefit. Example: Lichens (algae and fungi).
- Competition (-, -): Both species are harmed as they compete for the same limited resources.
- Predation (+, -): One species (predator) benefits, while the other (prey) is killed.
- Parasitism (+, -): One species (parasite) benefits, while the other (host) is harmed but usually not killed immediately.
- Commensalism (+, 0): One species benefits, while the other is unaffected. Example: An orchid growing on a mango tree.
- Amensalism (-, 0): One species is harmed, while the other is unaffected.
6. Why is understanding the difference between eurythermal and stenothermal organisms important for revision?
Understanding this distinction is key to revising the topic of adaptation and species distribution. Eurythermal organisms can tolerate a wide range of temperatures, allowing them to have a wider geographical distribution. In contrast, stenothermal organisms are restricted to a narrow temperature range and are found only in specific, stable thermal environments. This concept helps explain why certain species are found in particular biomes and how they respond to climate changes.
7. Why are very small animals typically not found in polar regions? How does this concept connect to homeostasis?
This is a classic application of the concept of homeostasis and thermoregulation. Small animals have a larger surface area to volume ratio compared to larger animals. This means they lose body heat very rapidly to the cold surroundings. Maintaining a constant internal body temperature (homeostasis) in such extreme cold would require an enormous amount of metabolic energy, which is not sustainable. Therefore, very small animals are rarely found in polar regions as they cannot efficiently regulate their body temperature.
8. What are some common misconceptions to avoid when revising concepts like population interactions and growth models?
To ensure conceptual clarity during revision, avoid these common pitfalls:
- Confusing competition types: Remember that competition is not just between different species (interspecific), but also occurs within the same species (intraspecific), which is often more intense.
- Misinterpreting growth models: Don't assume population growth is always exponential. The logistic model is more realistic as it accounts for limited resources and carrying capacity (K).
- Mixing up interactions: Be clear on the difference between mutualism (+/+), where both benefit, and commensalism (+/0), where only one benefits and the other is neutral.
9. Which diagrams are most effective for a rapid visual summary of this chapter?
For a quick and effective visual revision, focusing on the following diagrams is highly recommended:
- Age Pyramids: The three types (expanding, stable, and declining) provide a quick summary of a population's status.
- Population Growth Curves: The J-shaped curve for exponential growth and the S-shaped (sigmoid) curve for logistic growth are fundamental to understanding population dynamics.
- Organism Response Graph: A graph showing the responses of regulators, conformers, and partial regulators to external abiotic conditions.
