CBSE Class 12 Biology Long Answer Questions – Organisms and Populations (NCERT Based)

Unit V: Ecology and Environment | Chapter 13: Organisms and Populations

CBSE Class 12 Biology Chapter 13 Long Answer Questions – Organisms and Populations (NCERT)

Course & Examination Details

• Course: CBSE Class 12 Biology
• Unit: Unit V – Ecology and Environment
• Chapter: Chapter 13 – Organisms and Populations
• Board: Central Board of Secondary Education (CBSE)
• Syllabus Base: Strictly as per NCERT Textbook
• Exam Relevance: CBSE Class 12 Board Examinations (5-mark questions)

Long Answer Type Questions (120–150 Words Each)

(Prepared strictly according to NCERT content and CBSE answer-writing standards)

Section A: Organism and Environment (Q1–Q4)

Q1. Explain the relationship between organisms and their environment.

Ans:
Organisms and their environment are closely interdependent and continuously interact with each other. The environment provides essential resources such as food, water, oxygen, shelter, and suitable climatic conditions required for survival and reproduction. In turn, organisms influence their environment through activities like respiration, feeding, excretion, and reproduction. Environmental factors such as temperature, water availability, light, and soil directly affect the distribution, growth, and abundance of organisms. Each organism can survive only within a specific range of environmental conditions known as its tolerance limits. Any change beyond these limits causes stress and may lead to migration, adaptation, or death. Thus, the environment shapes the life processes of organisms, while organisms contribute to maintaining ecological balance.

Q2. Define ecology and describe its scope.

Ans:
Ecology is the branch of biology that studies interactions among organisms and between organisms and their physical environment. Its scope is broad and includes the study of organisms, populations, communities, ecosystems, and the biosphere. Ecology explains how organisms adapt to environmental conditions, how populations grow and interact, and how energy flows through ecosystems. It also helps in understanding nutrient cycles, biodiversity patterns, and ecological balance. Ecological knowledge is essential for conservation of natural resources, wildlife protection, and sustainable development. By studying ecology, we gain insight into the functioning of ecosystems and the impact of human activities on the environment.

Q3. What is environmental stress? Explain its effects on organisms.

Ans:
Environmental stress refers to unfavorable conditions that reduce the growth, survival, or reproductive success of organisms. Stress occurs when environmental factors such as temperature, water availability, salinity, or light exceed the tolerance limits of organisms. Prolonged stress can lead to reduced metabolic activity, stunted growth, delayed reproduction, or even death. For example, drought creates water stress in plants, causing reduced photosynthesis and wilting. Similarly, extreme temperatures can disrupt enzyme activity in animals. To cope with stress, organisms adopt strategies like migration, suspension, regulation, or physiological and behavioral adaptations. Environmental stress plays a major role in shaping species distribution.

Q4. Explain how tolerance limits influence species distribution.

Ans:
Every organism has a specific range of environmental conditions within which it can survive, grow, and reproduce, known as tolerance limits. These limits apply to factors such as temperature, water availability, salinity, and light. When environmental conditions remain within tolerance limits, organisms thrive. However, if conditions exceed these limits, organisms experience stress and may not survive. As a result, species are distributed only in areas where environmental conditions fall within their tolerance range. For example, stenothermal organisms are restricted to narrow temperature zones, while eurythermal organisms have wider distribution. Thus, tolerance limits are key determinants of species distribution and abundance.

Section B: Major Abiotic Factors (Q5–Q9)

Q5. Describe temperature as an important abiotic factor affecting organisms.

Ans:
Temperature is one of the most important abiotic factors influencing life. It affects enzyme activity, metabolic rate, growth, reproduction, and survival of organisms. Most biochemical reactions occur efficiently within a narrow temperature range. Extremely high or low temperatures can denature enzymes and disrupt metabolism. Based on temperature tolerance, organisms are classified as eurythermal or stenothermal. Temperature also determines geographical distribution; for example, polar regions support limited life due to low temperatures. Seasonal temperature variations influence breeding, migration, and dormancy in many organisms. Thus, temperature plays a crucial role in regulating ecological processes and species distribution.

Q6. Explain the significance of water as an abiotic factor.

Ans:
Water is essential for all life forms as it participates in metabolic reactions, nutrient transport, and temperature regulation. Availability of water largely determines the distribution of organisms, particularly plants. Aquatic organisms depend entirely on water, while terrestrial organisms show various adaptations to conserve it. Water scarcity causes stress, leading to reduced growth and productivity. Plants are classified as hydrophytes, mesophytes, and xerophytes based on water availability. Salinity, another aspect of water, affects osmotic balance in organisms. Therefore, water availability and quality significantly influence survival, adaptation, and distribution of living organisms.

Q7. How does light influence living organisms?

Ans:
Light is a crucial abiotic factor, especially for photosynthetic organisms. It provides energy for photosynthesis, influencing plant growth and productivity. Light intensity, duration, and quality affect flowering and biological rhythms. In aquatic ecosystems, light penetration decreases with depth, limiting photosynthesis to the upper layers. Animals also depend on light to regulate daily and seasonal activities through biological clocks. Changes in light conditions can affect feeding, reproduction, and migration. Thus, light plays a vital role in shaping ecosystem structure and functioning.

Q8. Explain the role of soil as an abiotic factor.

Ans:
Soil influences the distribution of vegetation through its physical and chemical properties such as texture, structure, pH, mineral content, and water-holding capacity. Different soil types support different plant communities. Soil also affects nutrient availability and root penetration. Indirectly, soil influences animal distribution by determining vegetation patterns. For example, fertile alluvial soils support dense vegetation, while sandy soils support sparse plant growth. Thus, soil plays a significant role in shaping terrestrial ecosystems.

Q9. Why are abiotic factors considered limiting factors?

Ans:
Abiotic factors are considered limiting factors because they restrict the growth, survival, and distribution of organisms when they exceed tolerance limits. Even if other conditions are favorable, the absence or excess of a single abiotic factor can limit survival. For example, lack of water limits plant growth in deserts, while low temperature restricts species in polar regions. Limiting factors determine the carrying capacity of environments and influence population size and distribution.

Section C: Responses to Abiotic Factors (Q10–Q14)

Q10. Explain regulation as a response to abiotic factors.

Ans:
Regulation is the ability of organisms to maintain a constant internal environment despite changes in external conditions. This process is known as homeostasis. Birds and mammals are typical regulators; they maintain constant body temperature, osmotic balance, and metabolic functions. Regulation requires high energy expenditure, as organisms must continuously adjust physiological processes. This strategy allows survival in fluctuating environments but is energetically expensive, making it unsuitable for many organisms.

Q11. Describe conformation with suitable examples.

Ans:
Conformation is a response where an organism’s internal conditions change with the external environment. Most plants, fishes, and reptiles are conformers. For example, body temperature in reptiles varies with ambient temperature. Conformation reduces energy expenditure because organisms do not actively regulate internal conditions. However, conformers are restricted to environments where conditions do not fluctuate beyond tolerance limits.

Q12. What is migration? Explain its ecological significance.

Ans:
Migration is the seasonal movement of organisms from unfavorable to favorable environments. It helps organisms avoid extreme conditions such as cold, heat, or food scarcity. Many birds migrate to warmer regions during winter to ensure survival and successful reproduction. Migration reduces environmental stress and enhances fitness, contributing to population stability.

Q13. Explain suspension as a survival strategy.

Ans:
Suspension is a strategy where organisms reduce metabolic activity during unfavorable conditions. Examples include hibernation in mammals, aestivation in reptiles, and diapause in insects. During suspension, growth and development slow down, conserving energy. This strategy allows organisms to survive extreme temperatures or resource scarcity until favorable conditions return.

Q14. Differentiate between regulation and conformation.

Ans:
Regulation maintains constant internal conditions regardless of external changes and requires high energy expenditure. Conformation involves changes in internal conditions according to the environment and requires less energy. Regulation is seen in birds and mammals, while conformation is common in plants, fishes, and reptiles.

Section D: Adaptations (Q15–Q18)

Q15. Define adaptation and explain its importance.

Ans:
Adaptation is a morphological, physiological, or behavioral feature that enhances an organism’s ability to survive and reproduce in a particular environment. Adaptations improve fitness and enable organisms to cope with environmental stress. They are genetically inherited and evolve over generations. Adaptations determine species distribution and ecological success.

Q16. Describe adaptations in desert plants.

Ans:
Desert plants show adaptations such as thick cuticles, reduced leaves or spines, deep root systems, and CAM photosynthesis. These features minimize water loss and allow survival in arid conditions. Such adaptations enable desert plants to conserve water and tolerate high temperatures.

Q17. Explain adaptations of aquatic animals.

Ans:
Aquatic animals possess streamlined bodies to reduce resistance, gills for respiration, and fins for movement. These adaptations help efficient swimming, respiration, and survival in water. Some also have buoyancy adaptations to maintain position in water.

Q18. Describe high-altitude adaptations in humans.

Ans:
Humans living at high altitudes show increased red blood cell count, higher hemoglobin concentration, and improved oxygen-carrying capacity. These adaptations help cope with low oxygen availability and maintain normal metabolic activity.

Section E: Population Attributes (Q19–Q22)

Q19. Define population and describe its main attributes.

Ans:
A population is a group of individuals of the same species living in a defined area at a given time. Population attributes include density, natality, mortality, age distribution, and sex ratio. These attributes describe group characteristics and help in understanding population dynamics.

Q20. Explain age distribution and age pyramids.

Ans:
Age distribution refers to the proportion of individuals in different age groups. Age pyramids graphically represent this distribution and indicate whether a population is expanding, stable, or declining. They help predict future population trends.

Q21. Why are population attributes not applicable to individuals?

Ans:
Population attributes describe collective characteristics of a group and cannot be applied to a single individual. An individual cannot have a birth rate or sex ratio, making these attributes meaningful only at the population level.

Q22. Explain the factors affecting population size.

Ans:
Population size is influenced by natality, mortality, immigration, emigration, and environmental conditions. Resource availability and environmental stress also play a significant role in determining population growth and stability.

Section F: Population Growth and Interactions (Q23–Q25)

Q23. Explain exponential and logistic population growth.

Ans:
Exponential growth occurs when resources are unlimited, resulting in a rapid increase in population size and a J-shaped curve. Logistic growth occurs under limited resources, where growth slows as population approaches carrying capacity, producing an S-shaped curve. Logistic growth is more common in nature.

Q24. What is carrying capacity? Explain its significance.

Ans:
Carrying capacity is the maximum population size an environment can support sustainably. It depends on resource availability and environmental conditions. Carrying capacity regulates population growth and prevents resource depletion.

Q25. Describe different types of population interactions.

Ans:
Population interactions include mutualism, competition, predation, parasitism, commensalism, and amensalism. These interactions influence survival, reproduction, and population size, helping maintain ecological balance.

CBSE Board Exam Tip

These 5-mark long answer questions are ideal for board exams, focusing on definitions, explanations, examples, and structured presentation using NCERT terminology.