What traumatic event does Zitkala-Sa describe at the school?

The forcible cutting of her long hair

Why was the cutting of her hair so humiliating to Zitkala-Sa?

Among her people, shingled hair was worn by cowards and mourners

Differentiate between regulators and conformers.

Regulators maintain a constant internal environment (homeostasis) through physiological means, whereas conformers allow their internal environment to change with the external environment.

What is meant by carrying capacity?

Carrying capacity (K) is the maximum number of individuals of a population that a given environment can support with its available resources.

Write the equation for exponential population growth.

The exponential growth is given by \\( \\frac{dN}{dt} = rN \\), where r is the intrinsic rate of natural increase and N is the population size.

Write the equation for logistic population growth.

Logistic growth is given by \\( \\frac{dN}{dt} = rN\\left(\\frac{K-N}{K}\\right) \\), where K is the carrying capacity.

What is brood parasitism? Give an example.

Brood parasitism is where a parasitic bird lays its eggs in the nest of a host, which raises the young; the cuckoo (koel) laying eggs in the crow's nest is an example.

State the competitive exclusion principle.

Gause's principle states that two closely related species competing for the same limited resources cannot coexist indefinitely; the inferior competitor is eventually eliminated.

How do desert plants adapt to conserve water?

They have thick cuticles, sunken stomata, CAM photosynthesis, and modified leaves (spines) to reduce transpiration, as seen in Opuntia.

Give an example of mutualism.

The relationship between a fig tree and its specific pollinator wasp, where the wasp pollinates the fig and uses the fruit to lay eggs.

What are the different types of age pyramids?

Expanding (triangular, growing population), stable (bell-shaped) and declining (urn-shaped, dwindling population) age pyramids.

Define commensalism with an example.

Commensalism is an interaction where one species benefits and the other is unaffected; an orchid growing on a mango tree branch is an example.

Organisms and Populations

Class 12 Biology · Biology · Chapter 11

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CBSE Class 12 · Biology

Genetics + Evolution: Unit VII, 20 marks

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CBSE Class 12 · Biology

Genetics + Evolution: Unit VII, 20 marks

Marks booster + cheatsheet: key concepts in 2 lines each, common traps
16 marks: Reproduction, Unit VI
12 marks each: Units VIII and IX
10 marks: Ecology, Unit X
13 chapters, with page numbers
Common traps, corrected

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Summary

This chapter introduces ecology at the levels of the organism and the population. It explains how organisms interact with their abiotic environment, with major factors being temperature, water, light and soil, and how these define habitats and biomes. Organisms respond to environmental stress by being regulators (maintaining homeostasis), conformers, by migration or by suspending activities through adaptations such as hibernation, aestivation, diapause and the formation of resistant stages. Adaptations, structural, physiological and behavioural, that fit organisms to their environment are described with examples like the kangaroo rat, desert plants, allen's rule and altitude sickness. At the population level, attributes such as population density, birth and death rates, sex ratio, age pyramids and dispersion are explained. Population growth is modelled by exponential growth (J-shaped curve, with intrinsic rate of natural increase r) and logistic growth (S-shaped curve constrained by carrying capacity K). The chapter then discusses population interactions, classifying them as mutualism, competition, predation, parasitism, commensalism and amensalism, with examples such as the predator-prey relationship, brood parasitism in the cuckoo, plant-pollinator mutualism in figs and wasps, and Gause's competitive exclusion principle.

Organism and its environmentAdaptations to abiotic factorsPopulation attributesPopulation growth modelsPopulation interactionsMutualism, predation, parasitism and competition

Key terms

Population

A group of individuals of the same species living in a defined geographical area and capable of interbreeding.

Carrying capacity (K)

The maximum population size that the resources of an environment can sustain.

Logistic growth

Population growth that follows an S-shaped (sigmoid) curve, limited by carrying capacity.

Mutualism

An interaction in which both interacting species benefit.

Commensalism

An interaction in which one species benefits and the other is neither harmed nor benefited.

Homeostasis

The maintenance of a constant internal environment by an organism despite varying external conditions.

Extra questions & answers

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The maximum population size an environment can sustain is its:

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