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Biology · Class 12 Science

Ch 11Organisms and Populations — Class 12 Biology, concept-first.

Our living world is fascinatingly diverse and complex. One way to make sense of that complexity is to study it at different levels of biological organisation — organisms, populations, communities, and biomes.

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Key concepts

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Population Level Properties

Think about a single person. You can describe them: tall, short, speaks Hindi, works as a teacher, has two children. Now think about a whole village or a whole country.

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Chapter contents

The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.

Introduction

Our living world is fascinatingly diverse and complex. One way to make sense of that complexity is to study it at different levels of biological organisation — organisms, populations, communities, and…

11.1

Populations

Individuals of a species are rarely found living entirely alone — most live in groups sharing a defined geographical area, competing for the same resources, and capable of interbreeding, which togethe…

11.1.1

Population Attributes

A population is more than just a collection of individuals. In nature, organisms of the same species rarely live in complete isolation.

11.1.2

Population Growth

A population's size is never fixed. It rises and falls constantly because of changes in food availability, predation pressure, and weather.

11.1.3

Life History Variation

Populations evolve to maximise their Darwinian fitness — essentially, their reproductive success, often measured by a high intrinsic rate of natural increase (the r value).

11.1.4

Population Interactions

In nature, no species lives alone. Even a plant that makes its own food cannot survive in isolation — it needs soil microbes to break down organic matter and return inorganic nutrients, and it needs a…

Summary

- Levels of organisation: Ecology is studied at the levels of organism, population, community, and biome.

Exercises

+Show 10 questions10 questions
  1. 11.1List the attributes that populations possess but not individuals.Free
  2. 11.2If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?Free
  3. 11.3Name important defence mechanisms in plants against herbivory.Free
  4. 11.4An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?Preview
  5. 11.5What is the ecological principle behind the biological control method of managing with pest insects?Preview
  6. 11.6Define population and community.Preview
  7. 11.7Define the following terms and give one example for each: (a) Commensalism (b) Parasitism (c) Camouflage (d) Mutualism (e) Interspecific com…Preview
  8. 11.8With the help of suitable diagram describe the logistic population growth curve.Preview
  9. 11.9Select the statement which explains best parasitism. (a) One organism is benefited. (b) Both the organisms are benefited. (c) One organism i…Preview
  10. 11.10List any three important characteristics of a population and explain.Preview

Exemplar Problems

Higher-order thinking / exemplar-style practice problems.

+Show 37 questions37 questions
  1. Q1If a population of 50 Paramoecium present in a pool increases to 150 after an hour, what would be the growth rate of population? (a) 50 per…Free
  2. Q2What would be the per cent growth or birth rate per individual per hour for the same population mentioned in the previous question (Question…Free
  3. Q3A population has more young individuals compared to the older individuals. What would be the status of the population after some years? (a)…Free
  4. Q4What parameters are used for tiger census in our country's national parks and sanctuaries? (a) Pug marks only (b) Pug marks and faecal pelle…Preview
  5. Q5Which of the following would necessarily decrease the density of a population in a given habitat? (a) Natality > mortality (b) Immigration >…Preview
  6. Q6A protozoan reproduces by binary fission. What will be the number of protozoans in its population after six generations? (a) 128 (b) 24 (c)…Preview
  7. Q7In 2005, for each of the 14 million people present in a country, 0.028 were born and 0.008 died during the year. Using exponential equation,…Preview
  8. Q8Amensalism is an association between two species where: (a) one species is harmed and other is benefitted (b) one species is harmed and othe…Preview
  9. Q9Lichens are association of: (a) bacteria and fungus (b) alga and bacterium (c) fungus and alga (d) fungus and virusPreview
  10. Q10Which of the following is a partial root parasite? (a) Sandal wood (b) Mistletoe (c) Orobanche (d) GanodermaPreview
  11. Q11Which one of the following organisms reproduces sexually only once in its life time? (a) Banana (b) Mango (c) Tomato (d) EucalyptusPreview
  12. Q12What is the interaction between two species called?Preview
  13. Q13What is commensalism?Preview
  14. Q14Name the association in which one species produces poisonous substance or a change in environmental conditions that is harmful to another sp…Preview
  15. Q15What is Mycorrhiza?Preview
  16. Q16What would be the growth rate pattern, when the resources are unlimited?Preview
  17. Q17What are the organisms that feed on plant sap and other plant parts called?Preview
  18. Q18Give a suitable example for commensalism.Preview
  19. Q19Define ectoparasite and endoparasite and give suitable examples.Preview
  20. Q20What is brood parasitism? Explain with the help of an example.Preview
  21. Q21The density of a population in a habitat per unit area is measured in different units. Write the unit of measurement against the following:…Preview
  22. Q22a. Label the three tiers 1, 2, 3 given in the above age pyramid. b. What type of population growth is represented by the above age pyramid?Preview
  23. Q23In an association of two animal species, one is a termite which feeds on wood and the other is a protozoan Trichonympha present in the gut o…Preview
  24. Q24Lianas are vascular plants rooted in the ground and maintain erectness of their stem by making use of other trees for support. They do not m…Preview
  25. Q25Define 'zero population growth rate'. Draw a age pyramid for the same.Preview
  26. Q26List any four characters that are employed in human population census.Preview
  27. Q27Give one example for each of the following types. a) Migratory animal b) Camouflaged animal c) Predator animal d) Biological control agent e…Preview
  28. Q28Fill in the blanks | Species A | Species B | Type of Interaction | Example | | --- | --- | --- | --- | | + | – | _______ | _______ | | + | +…Preview
  29. Q29Observe the set of 4 figures A, B, C and D and, answer the following questions i. Which one of the figures shows mutualism? ii. What kind of…Preview
  30. Q30Comment on the following figures: 1, 2 and 3: A, B, C. D, G, P, Q, R, S are speciesPreview
  31. Q31An individual and a population has certain characteristics. Name these attributes with definitions.Preview
  32. Q32The following diagrams are the age pyramids of different populations. Comment on the status of these populations.Preview
  33. Q33Comment on the growth curve given below.Preview
  34. Q34A population of Paramoecium caudatum was grown in a culture medium. After 5 days the culture medium became overcrowed with Paramoeium and ha…Preview
  35. Q35Discuss the various types of positive interactions between species.Preview
  36. Q36In an aquarium two herbivorous species of fish are living together and feeding on phytoplanktons. As per the Gause's Principle, one of the s…Preview
  37. Q37While living in and on the host species, the animal parasite has evolved certain adaptations. Describe these adaptations with examples.Preview

Sample & Board Papers

Sample papers and previous-year board questions for this subject.

+Show 4 questions4 questions
  1. Q1State 'two' observations made by German naturalist, Alexender von Humboldt during his extensive explorations in South American jungles. OR I…Preview
  2. Q2The population growth curve applicable for a population growing in a geometric fashion, when the resources are not limiting in the habitat w…Preview
  3. Q3(a) (i) Describe the population growth curve applicable in a population of any species in nature that has unlimited resources at its disposa…Preview
  4. Q4(a) Following are the responses of different animals to various abiotic factors. Describe each one with the help of an example. (i) Regulate…Preview