Key Topics Explained: Organisms and Populations Class 12 Biology 2025-26
FAQs on Organisms and Populations: NCERT Class 12 Biology Chapter 13 Free PDF 2025-26
1. What are the most frequently asked 5-mark questions from Organisms and Populations for the CBSE 2025-26 board exam?
For the CBSE board exams, some of the most important long-answer (5-mark) questions from this chapter involve explaining concepts with examples and diagrams. Key areas to focus on are:
- Explain the different types of population interactions (like predation, competition, mutualism, commensalism) with at least two suitable examples for each.
- Describe the logistic and exponential population growth models. A question may ask you to draw the J-shaped and S-shaped curves and explain why the logistic model is considered more realistic.
- Illustrate and explain the three types of age pyramids (expanding, stable, and declining) and what they signify about a population's status.
- Discuss the various morphological, physiological, and behavioural adaptations of organisms to different environmental conditions, such as high temperature or water scarcity.
2. How should one structure an answer for a 3-mark question on population interactions to score full marks?
To secure full marks for a 3-mark question on population interactions in the Class 12 Biology exam, your answer must be precise and well-structured. Follow these steps:
- Define the Interaction: Start with a clear, one-sentence definition of the specific interaction asked (e.g., Parasitism).
- State the Effect: Clearly mention the effect on the two interacting species using signs (+ for benefit, - for harm, 0 for neutral). For example, for parasitism, the interaction is (+ / -).
- Provide a Specific Example: Give a clear and accurate example as mentioned in the NCERT syllabus. For instance, for parasitism, you can mention 'Cuscuta on a hedge plant' or 'human liver fluke'. Avoid vague examples.
3. What types of 1-mark or MCQ questions are expected from the topic of population attributes?
For the 2025-26 board exam, 1-mark questions or MCQs from population attributes typically test your knowledge of key definitions and terms. Important questions can be based on:
- Definitions of natality (birth rate), mortality (death rate), immigration, and emigration.
- The concept of carrying capacity (K) and what it represents.
- Identifying the type of age pyramid (e.g., triangular) when a description of age groups is given.
- The formula for population density and population growth.
4. What are the key differences between commensalism and mutualism that are crucial for exams?
From an exam perspective, the critical difference lies in the outcome for the two interacting species.
- In Mutualism, both species involved in the interaction derive benefits. The relationship is denoted as (+ / +). A classic example is the relationship between fungi and algae in lichens.
- In Commensalism, only one species benefits, while the other is neither harmed nor benefited. This relationship is denoted as (+ / 0). A key example is an orchid growing as an epiphyte on a mango tree, where the orchid gets support but the mango tree is unaffected.
5. What are the important diagrams from Chapter 13, Organisms and Populations, that frequently appear in CBSE board papers?
Students should practise the following diagrams from this chapter as they are frequently asked in board exams:
- The S-shaped (Sigmoid) curve representing logistic population growth, clearly labelling the carrying capacity (K).
- The J-shaped curve representing exponential population growth.
- The three types of age pyramids: Expanding (Triangular), Stable (Bell-shaped), and Declining (Urn-shaped).
- The graph showing organismic response to abiotic factors, illustrating regulators, conformers, and partial regulators.
6. Why is the logistic growth model considered a more realistic representation of population growth in nature?
The logistic growth model is considered more realistic because it acknowledges that resources in any given habitat are finite, not unlimited. It incorporates the concept of carrying capacity (K), which is the maximum population size that the environment can sustain. The exponential model assumes infinite resources, leading to an unrealistic, indefinite J-shaped growth curve. In contrast, the logistic model's S-shaped curve shows that as the population (N) approaches the carrying capacity (K), its growth rate slows down due to environmental resistance, which is a more accurate depiction of what happens in most natural populations.
7. How can a student avoid common mistakes when explaining ecological adaptations in an exam?
To avoid common errors when answering questions on ecological adaptations, students should:
- Be Specific: Instead of a general statement like 'animals in cold climates have thick fur', provide a specific example, e.g., 'Polar bears have a thick layer of fat called blubber and thick fur for insulation against extreme cold'.
- Categorise Adaptations: When asked for adaptations, try to provide examples of different types if possible—morphological (e.g., spines in desert plants), physiological (e.g., kangaroo rat's ability to produce concentrated urine), and behavioural (e.g., desert lizards basking in the sun).
- Link Adaptation to Pressure: Always explicitly connect the adaptation to the specific environmental challenge it helps overcome. This demonstrates a deeper understanding beyond simple memorisation.
8. Beyond definitions, how is Gause's 'Competitive Exclusion Principle' applied in case-based questions?
In a case-based question, you would apply Gause's 'Competitive Exclusion Principle' to analyse a scenario. The principle states that two closely related species competing for the same limited resources cannot coexist indefinitely, and the competitively inferior one will be eliminated. To answer, you must:
1. Identify the two competing species from the case study.
2. Pinpoint the specific limited resource they are competing for (e.g., food, space).
3. Predict the outcome based on the principle, i.e., the eventual elimination of one species. Alternatively, you can discuss how coexistence is possible through mechanisms like resource partitioning, where species adapt to use the resource differently to avoid direct competition.
