Advanced Study Material for Class 12 Biology: Chapter 11 Organisms and Populations
Review targeted academic resources with the CBSE Class 12 Biology Organisms and Populations Key Points. Built according to official CBSE standards for the 2026-27 term, these downloadable Class 12 Biology study materials support effective daily practice and deeper conceptual understanding for Chapter 11 Organisms and Populations.
Practice Class 12 Biology Resources: Chapter 11 Organisms and Populations
Navigate directly to the advanced Biology study materials using the digital viewer below. Each resource pack includes detailed conceptual summaries and solved practice questions, allowing students to instantly reinforce their learning.
CBSE Class 12 Biology Organisms and Populations Key Points. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.
Unit 10 Ecology – Chapter 13: Organisms and Populations (Key Points)
| S.No | Term | Explanation |
|---|---|---|
| 1 | Benthic zone | The bottom-most zone of a water body (e.g. the ocean floor), where the organisms called benthos live |
| 2 | Carrying capacity | The maximum number of organisms of a population that can be sustained by a given habitat or ecosystem |
| 3 | Ecological niche | The range of tolerance of a species, the resources it utilises and its functional role in an ecosystem |
| 4 | Aerenchyma | Tissue with air chambers in aquatic plants, which gives buoyancy to the plants |
| 5 | Eurythermal | Organisms which can tolerate and thrive in a wide range of temperatures |
| 6 | Stenothermal | Organisms restricted to a narrow range of temperatures |
| 7 | Habitat | The place where an organism lives and reproduces |
| 8 | Sexual deceit | The method by which the orchid Ophrys gets its flower pollinated: the flower resembles the female bee |
| 9 | Phytophagous | An animal feeding on plant sap and other parts of plants |
| 10 | Allen's rule | Mammals in colder climates have shorter ears and shorter limbs to minimise heat loss |
| 11 | Pseudocopulation | The male bee is attracted to what it perceives as a female and 'copulates' with the flower, e.g. the Mediterranean orchid Ophrys (sexual deceit), pollinated by bees. One petal of the flower resembles the female bee in size, colour and markings. This is a case of co-evolution |
| 12 | Brood parasitism | A bird (such as the cuckoo or koel) lays its eggs, which resemble the eggs of the host, in the nest of another bird (such as the crow) and lets the host incubate them |
| 13 | Competitive release | A species whose distribution is restricted to a small geographical area because of the presence of a competitively superior species is found to expand its distributional range when the competing species is removed. Examples: Connell's field experiments on the rocky sea coasts of Scotland, where the larger, competitively superior barnacle Balanus dominates and excludes the smaller barnacle Chthamalus; the Abingdon tortoise was eliminated by goats because of their greater grazing efficiency |
| 14 | Regulators | Organisms which maintain homeostasis by physiological and behavioural means |
| 15 | Conformers | Organisms whose body temperature or osmotic concentration of the body fluids changes with that of the surroundings |
| 16 | Age pyramid | The age distribution (percentage of individuals of a given age group) plotted for the population. The shape of the pyramid may be expanding, stable or declining |
| 17 | Natality | Birth rate: the number of births during a given period in the population, which increases the population |
| 18 | Mortality | Death rate: the number of deaths in the population during a given period, which decreases the population |
| 19 | Emigration | Individuals moving out of the habitat: the population decreases |
| 20 | Immigration | Individuals coming into the habitat from elsewhere: the population increases |
| 21 | Logistic growth | When the resources become limited, the population growth becomes S-shaped (sigmoid) and not exponential |
| 22 | Mutualism | Interaction in which both the species benefit |
| 23 | Amensalism | Interaction between two different species in which one is harmed and the other is neither benefited nor harmed |
| 24 | Predation | Interspecific interaction in which an animal (the predator) kills and consumes another, weaker animal (the prey) |
| 25 | Competition | Interaction in which both species suffer. It may exist between different species (interspecific competition) or between the individuals of the same species (intraspecific competition) |
| 26 | Gause's principle | The 'competitive exclusion principle': two closely related species competing for the same resource cannot coexist indefinitely, and the competitively inferior one will eventually be eliminated by the superior one |
| 27 | Resource partitioning, co-existence | The phenomenon in which species facing competition evolve mechanisms that promote co-existence rather than exclusion. MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist because of behavioural differences in their foraging activities |
| 28 | Parasitism | One species (the parasite) depends on another species (the host) for food and shelter, and in the process the host is damaged |
| 29 | Endoparasite | A parasite that lives inside the host's body, e.g. Plasmodium |
| 30 | Ectoparasite | A parasite that feeds on the external surface of the host organism for food and shelter, e.g. lice, copepods, ticks and Cuscuta |
| 31 | Diapause | In unfavourable conditions, many species in lakes and ponds enter a stage of suspended development called diapause. It occurs in both summer and winter, e.g. zooplankton in lakes and ponds |
| 32 | Hibernation | Winter (or summer) sleep to evade stress or unfavourable conditions, e.g. winter sleep in bears; frogs also hibernate |
| 33 | Prey defence mechanisms | Insects and frogs are camouflaged. The monarch butterfly is distasteful to birds, as it accumulates a special chemical from the poisonous weed it feeds on. Plants have thorns or spines (Acacia, cactus). Plants produce highly poisonous chemicals like cardiac glycosides, nicotine, caffeine, quinine, strychnine and opium as a defence against grazers and browsers |
| 34 | Phenotypic adaptation | Non-genetic, temporary, non-heritable changes in the physical or morphological characteristics, behaviour or physiology in response to changes in the environment. E.g. mountain sickness at high altitude gradually disappears by an increase in RBCs and the breathing rate |
| 35 | Endothermic | Warm-blooded: the animal regulates its body temperature, e.g. monkey |
| 36 | Ectothermic | Cold-blooded: the body temperature changes with that of the environment |
| 37 | Ecology | The branch of science that deals with the interactions among organisms, and between the organisms (biotic) and their physical (abiotic) environment |
| 38 | Population | The sum total of all the individuals of a species in a specific geographical area |
| 39 | Species | A group of individuals of one or more populations which resemble each other and interbreed among themselves successfully |
| 40 | Biotic community | The assemblage of all the populations of different species present in an area that interact among themselves. The three types are the plant, animal and microbial communities |
| 41 | Ecosystem | The sum total of the biotic and abiotic components of a particular geographical area, integrated through the exchange of energy and the recycling of nutrients |
| 42 | Biome | A large unit of the environment, which consists of a major vegetation type and its associated fauna in a specific climatic zone |
| 43 | Biosphere | All the ecosystems of the world taken together |
| 44 | Factors of the environment | Abiotic (temperature, water, light, soil) and biotic (pathogens, parasites, predators and competitors) |
| 45 | CAM | Crassulacean Acid Metabolism, a photosynthetic pathway in which the stomata remain closed during the day |
| 46 | Adaptation of desert plants | A waxy coating (thick cuticle) on the leaf, the CAM pathway (stomata close during the day), and spines in place of leaves: all these reduce water loss |
| 47 | Adaptation of mammals in a cold climate | Shorter ears and limbs (Allen's rule), and a thick layer of fat (blubber) below the skin, which insulates and reduces the loss of heat |
| 48 | Adaptation of desert lizards | They absorb heat from the sun when the body temperature drops below the comfort zone, and move into the shade when the ambient temperature rises above the comfort level |
| 49 | Adaptation of humans at high altitude | Compensation for the low oxygen by an increase in RBCs and total haemoglobin, and an increased breathing rate (vital capacity) |
| 50 | Population density | The number of individuals of a species per unit area or volume: PD = N/S, where N is the number of individuals and S is the area |
| 51 | Birth rate | The number of births per 1000 individuals of a population |
| 52 | Death rate | The number of deaths per 1000 individuals of a population |
| 53 | Sex ratio | The number of females per 1000 males in a population at a given time |
| 54 | Exponential growth | Common where the resources (food and space) are not limited. \( \frac{dN}{dt}=(b-d)N=rN \), where r is the intrinsic rate of natural increase |
Important Questions for NCERT Class 12 Biology Organisms and Populations
Ques. Certain characteristic demographic features of developing countries are
(a) high fertility, low or rapidly falling mortality rate, rapid population growth and a very young age distribution
(b) high fertility, high density, rapidly rising mortality rate and a very young age distribution
(c) high infant mortality, low fertility, uneven population growth and a very young age distribution
(d) high mortality, high density, uneven population growth and a very old age distribution.
Answer: A
Ques. What is a keystone species?
(a) A species which makes up only a small proportion of the total biomass of a community, yet has a huge impact on the community’s organization and survival.
(b) A common species that has plenty of biomass, yet has a fairly low impact on the community’s organization.
(c) A rare species that has minimal impact on the biomass and on other species in the community
(d) A dominant species that constitutes a large proportion of the biomass and which affects many other species.
Answer: A
Ques. The maximum growth rate occurs in
(a) stationary phase
(b) senescent phase
(c) lag phase
(d) exponential phase.
Answer: D
Ques. Mycorrhiza is an example of
(a) symbiotic relationship
(b) ectoparasitism
(c) endoparasitism
(d) decomposers.
Answer: A
Ques. Two opposite forces operate in the growth and development of every population. One of them is related to the ability to reproduce at a given rate. The force opposite to it is called
(a) fecundity
(b) environmental resistances
(c) biotic control
(d) mortality.
Answer: B
Ques. Which type of association is found in between entomophilous flower and pollinating agent?
(a) Mutualism (b) Commensalism
(c) Co-operation (d) Co-evolution
Answer: D
Ques. Two different species cannot live for long duration in the same niche or habitat. This law is
(a) Allen’s law
(b) Gause’s hypothesis
(c) Dollo’s rule
(d) Weismann’s theory.
Answer: B
Ques. In a population, unrestricted reproductive capacity is called as
(a) biotic potential (b) fertility rate
(c) carrying capacity (d) birth rate.
Answer: A
Ques. In which of the following plant sunken stomata are found?
(a) Nerium (b) Hydrilla
(c) Mango (d) Guava
Answer: A
Ques. An interesting modification of flower shape for insect pollination occurs in some orchids in which a male insect mistakes the pattern on the orchid flower for the female species and tries to copulate with it, thereby pollinating the flower. This phenomenon is called
(a) pseudopollination (b) pseudoparthenocarpy
(c) mimicry (d) pseudocopulation.
Answer: D
Ques. The abundance of a species population, within its habitat, is called
(a) relative density (b) regional density
(c) absolute density (d) niche density.
Answer: D
Ques. Which one of the following pairs is correctly matched?
(a) Parasitism - intraspecific relationship
(b) Uricotelism - aquatic habitat
(c) Excessive perspiration - xeric adaptation
(d) Streamlined body - aquatic adaptation
Answer: D
Ques. Mycorrhiza exhibits the phenomenon of
(a) parasitism (b) symbiosis
(c) antagonism (d) endemism.
Answer: B
Ques. Association of animals when both partners are benefitted
(a) colony (b) mutualism
(c) commensalism (d) ammensalism.
Answer: B
Ques. Which one is true?
(a) Commensalism when none of the interacting populations affect each other.
(b) Symbiosis when the interaction is useful to both the populations.
(c) Symbiosis when neither populations affects each other.
(d) Commensalism when the interaction is useful to both the populations.
Answer: B
Ques. The relation between algae and fungi in a lichen is
(a) symbiosis (b) parasitism
(c) commenalism (d) protocooperation.
Answer: A
Ques. A mutually beneficial association necessary for survival of both partners is
(a) mutualism/symbiosis
(b) commensalism
(c) amensalism
(d) both (a) and (b).
Answer: A
Ques. Competition for light, nutrients and space is most severe between
(a) closely related organism growing in different niches
(b) closely related organisms growing in the same area/niche
(c) distantly related organisms growing in the same habitat
(d) distantly related organisms growing in different niches.
Answer: B
Free study material for Biology
CBSE Class 12 Biology Study Material: Chapter 11 Organisms and Populations
Quick Revision Material for Chapter 11 Organisms and Populations
Review targeted study resources for Chapter 11 Organisms and Populations tailored for Class 12 learners. Utilizing these structured notes and quick-revision tools ensures complete alignment with current CBSE evaluation standards.
Expert Notes and Solved Exam Questions
Built using official NCERT guidelines for Class 12 Biology, these materials provide reliable academic support. Integrating past examination questions and step-by-step solutions helps students understand official CBSE grading criteria.
Additional Study Resources for Class 12
Wrap up your chapter revision by testing your knowledge against standard objective question formats. Explore our full library of free, up-to-date sample papers and online assessments to secure top grades in your CBSE examinations.
FAQs
Our advanced study package for Chapter 11 Organisms and Populations includes detailed concepts, diagrams, Mind Maps, and explanation of complex topics to ensure Class 12 students learn as per syllabus for 2026 exams.
The Mind Maps provided for Chapter 11 Organisms and Populations act as visual anchors which will help faster recall during high-pressure exams.
Yes, teachers use our Class 12 Biology resources for lesson planning as they are in simple language and have lot of solved examples.
Yes, You can download the complete, mobile-friendly PDF of the Biology Chapter 11 Organisms and Populations advanced resources for free.
Yes, our subject matter experts have updated the Chapter 11 Organisms and Populations material to align with the rationalized NCERT textbooks and have removed deleted topics and added new competency-based questions.