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

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

CBSE Class 12 Biology Chapter 13 Short 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, School Assessments

Short Answer Type Questions (60–80 Words Each)

(Prepared as per NCERT content and CBSE answer-writing standards)

Section A: Organism and Environment (Q1–Q8)

Q1. Explain the relationship between organisms and their environment.
Ans: Organisms and their environment are closely interrelated. The environment provides essential resources such as food, water, shelter, and suitable climatic conditions for survival. In return, organisms interact with and modify their environment through activities like respiration, feeding, and reproduction. Environmental factors strongly influence growth, distribution, and survival, making organisms dependent on their surroundings.

Q2. Define ecology and mention its scope.
Ans: Ecology is the branch of biology that studies interactions among organisms and between organisms and their physical environment. Its scope includes understanding distribution, abundance, adaptations, population dynamics, and interactions at different levels such as organisms, populations, communities, and ecosystems. Ecology helps explain how life is sustained on Earth.

Q3. What is meant by habitat and niche?
Ans: A habitat is the physical place where an organism lives, grows, and reproduces. A niche refers to the functional role of an organism in its environment, including its resource use and interactions. While habitat describes “where” an organism lives, niche explains “how” it survives and interacts within that habitat.

Q4. Why are organisms called open systems?
Ans: Organisms are called open systems because they continuously exchange energy, matter, and information with their environment. They take in nutrients, oxygen, and water and release waste products and heat. This constant interaction is essential for maintaining life processes such as metabolism, growth, and reproduction.

Q5. Explain environmental stress with an example.
Ans: Environmental stress refers to unfavorable conditions that reduce the growth, survival, or reproduction of organisms. Extreme temperatures, drought, or scarcity of food can cause stress. For example, prolonged drought creates water stress in plants, leading to reduced photosynthesis and stunted growth.

Q6. How do environmental factors affect species distribution?
Ans: Environmental factors such as temperature, water availability, light, and soil influence the survival and reproduction of species. Organisms can exist only within their tolerance limits. If environmental conditions exceed these limits, species cannot survive there, resulting in restricted distribution.

Q7. Differentiate between biotic and abiotic components of the environment.
Ans: Abiotic components are non-living factors such as temperature, water, light, and soil. Biotic components include all living organisms like plants, animals, and microorganisms. Abiotic factors influence the availability of resources, while biotic components interact with each other through various ecological relationships.

Q8. Why is ecology important for understanding life processes?
Ans: Ecology helps in understanding how organisms adapt, survive, and reproduce under different environmental conditions. It explains interactions among organisms and their environment, population growth patterns, and ecosystem stability. This knowledge is essential for conservation, resource management, and environmental protection.

Section B: Major Abiotic Factors (Q9–Q16)

Q9. Describe temperature as an important abiotic factor.
Ans: Temperature affects enzyme activity, metabolism, growth, and reproduction of organisms. Most organisms have an optimum temperature range for survival. Extremely high or low temperatures can be lethal. Temperature also determines species distribution, as organisms cannot survive beyond their tolerance limits.

Q10. What are eurythermal and stenothermal organisms?
Ans: Eurythermal organisms can tolerate a wide range of temperatures and are widely distributed. Stenothermal organisms can survive only within a narrow temperature range and have limited distribution. These categories reflect temperature tolerance of different species.

Q11. Explain the significance of water as an abiotic factor.
Ans: Water is essential for all life processes. Its availability influences the distribution of organisms, especially plants. Aquatic organisms depend completely on water, while terrestrial organisms show various adaptations to conserve it. Water scarcity creates stress conditions affecting growth and survival.

Q12. How does light affect living organisms?
Ans: Light is crucial for photosynthesis in plants and influences flowering and growth. In aquatic ecosystems, light availability decreases with depth, affecting plant distribution. Light also regulates biological rhythms and behavior in animals.

Q13. Explain the role of soil in determining vegetation.
Ans: Soil affects vegetation through its texture, structure, pH, mineral composition, and water-holding capacity. Different soil types support different plant communities. Soil indirectly affects animals by influencing the type of vegetation present in an area.

Q14. What is salinity and how does it affect organisms?
Ans: Salinity refers to the concentration of salts in water. It affects osmotic balance in organisms. Most freshwater organisms cannot survive in saline water, while marine organisms are adapted to high salinity conditions, limiting their distribution.

Q15. Why are abiotic factors called limiting factors?
Ans: Abiotic factors are called limiting factors because they restrict the growth, survival, and distribution of organisms when they exceed tolerance limits. For example, lack of water limits plant growth in deserts.

Q16. Explain the combined effect of abiotic factors on organisms.
Ans: Abiotic factors act together rather than independently. Temperature, water, light, and soil collectively influence metabolism, growth, and reproduction. A favorable combination supports survival, while unfavorable conditions create stress affecting organism distribution.

Section C: Responses to Abiotic Factors (Q17–Q24)

Q17. What is regulation in organisms?
Ans: Regulation is the ability of organisms to maintain constant internal conditions despite external environmental changes. Birds and mammals regulate body temperature and internal environment through physiological mechanisms, ensuring optimal functioning of enzymes and metabolic processes.

Q18. Explain conformation with an example.
Ans: Conformation is a response in which internal conditions of organisms change according to the external environment. Most plants, fishes, and reptiles are conformers. Their body temperature varies with surrounding temperature, reducing energy expenditure.

Q19. What is migration and why is it important?
Ans: Migration is the seasonal movement of organisms from unfavorable to favorable environments. It helps organisms escape harsh conditions, find food, and reproduce successfully. Many birds migrate to warmer regions during winter to ensure survival.

Q20. Describe suspension as a survival strategy.
Ans: Suspension is a response where organisms reduce metabolic activity during unfavorable conditions. Examples include hibernation in mammals, aestivation in reptiles, and diapause in insects. This strategy helps organisms conserve energy and survive extreme conditions.

Q21. Differentiate between regulation and conformation.
Ans: Regulation maintains constant internal conditions irrespective of external changes, while conformation involves changes in internal conditions with the environment. Regulation is energy-intensive and seen in birds and mammals, whereas conformation is common in plants and cold-blooded animals.

Q22. Why is regulation energetically expensive?
Ans: Regulation requires continuous energy expenditure to maintain constant internal conditions. Processes like thermoregulation, osmoregulation, and metabolic adjustments consume large amounts of energy, making regulation unsuitable for many organisms.

Q23. What is diapause?
Ans: Diapause is a form of suspension seen in insects where development is temporarily halted during unfavorable conditions. It helps insects survive extreme temperatures or scarcity of food until favorable conditions return.

Q24. How do organisms cope with seasonal variations?
Ans: Organisms cope with seasonal variations through migration, suspension, regulation, or conformation. These responses help them survive periods of extreme temperature, food scarcity, or water shortage.

Section D: Adaptations (Q25–Q32)

Q25. Define adaptation and state its significance.
Ans: Adaptation is a morphological, physiological, or behavioral trait that enhances an organism’s ability to survive and reproduce in a particular environment. Adaptations increase fitness and enable organisms to cope with environmental stress.

Q26. Describe adaptations in desert plants.
Ans: Desert plants have thick cuticles, reduced leaves or spines, and deep root systems to reduce water loss. Many follow the CAM pathway to minimize transpiration, allowing survival in arid conditions.

Q27. Explain adaptations of aquatic animals.
Ans: Aquatic animals have streamlined bodies to reduce resistance, gills for respiration, and fins for movement. These adaptations enable efficient swimming, respiration, and survival in water.

Q28. What are high-altitude adaptations in humans?
Ans: Humans living at high altitudes have increased red blood cell count and higher hemoglobin affinity for oxygen. These adaptations help compensate for low oxygen availability in mountainous regions.

Q29. How do fishes adapt for respiration?
Ans: Fishes possess gills that efficiently extract dissolved oxygen from water. A counter-current flow mechanism enhances oxygen absorption, enabling survival in aquatic environments.

Q30. Explain thermal adaptations in animals.
Ans: Animals in cold regions have thick fur, fat layers, and compact bodies to reduce heat loss. Desert animals have light-colored bodies and behavioral adaptations to avoid heat.

Q31. Are adaptations permanent? Explain.
Ans: Adaptations are genetically inherited and evolve over generations. They are permanent traits that improve survival but may become disadvantageous if environmental conditions change drastically.

Q32. How do plant adaptations influence their distribution?
Ans: Plant adaptations determine their ability to survive in specific environments. Xerophytes dominate deserts, hydrophytes thrive in water bodies, and mesophytes grow in moderate conditions, influencing vegetation patterns.

Section E: Population Attributes (Q33–Q40)

Q33. Define population and population density.
Ans: A population is a group of individuals of the same species living in a defined area at a given time. Population density refers to the number of individuals per unit area or volume.

Q34. Explain natality and mortality.
Ans: Natality is the rate at which new individuals are added to a population by birth, while mortality is the rate of death. Both determine population size and growth.

Q35. What is age distribution in a population?
Ans: Age distribution refers to the proportion of individuals in different age groups—pre-reproductive, reproductive, and post-reproductive. It indicates population growth trends.

Q36. Describe age pyramids.
Ans: Age pyramids graphically represent age distribution in a population. Expanding pyramids indicate growth, stable pyramids show equilibrium, and declining pyramids reflect decreasing population.

Q37. What is sex ratio and why is it important?
Ans: Sex ratio is the proportion of males to females in a population. It influences reproductive potential and future population growth.

Q38. Why are population attributes not applicable to individuals?
Ans: Population attributes like birth rate, death rate, and sex ratio describe group characteristics and cannot be applied to a single individual.

Q39. How is population density measured in plants?
Ans: In plants, population density is often measured by percent cover or biomass rather than number, especially when counting individuals is difficult.

Q40. What factors influence population size?
Ans: Population size is influenced by natality, mortality, immigration, emigration, and environmental conditions such as resource availability.

Section F: Population Growth and Interactions (Q41–Q50)

Q41. Explain population growth.
Ans: Population growth refers to the change in population size over time. It depends on birth rate, death rate, immigration, and emigration, and reflects environmental conditions.

Q42. Describe exponential population growth.
Ans: Exponential growth occurs under unlimited resources. Population size increases rapidly, producing a J-shaped curve. It is rare in nature due to resource limitations.

Q43. Explain logistic population growth.
Ans: Logistic growth occurs when resources are limited. Growth slows as population approaches carrying capacity, producing an S-shaped curve commonly seen in nature.

Q44. What is carrying capacity?
Ans: Carrying capacity is the maximum population size that an environment can support sustainably without resource depletion.

Q45. Differentiate between J-shaped and S-shaped growth curves.
Ans: J-shaped curves represent exponential growth with unlimited resources, while S-shaped curves represent logistic growth under resource limitation.

Q46. What is mutualism? Give an example.
Ans: Mutualism is an interaction where both species benefit. For example, pollinators obtain nectar while plants achieve pollination.

Q47. Explain competition as a population interaction.
Ans: Competition occurs when organisms compete for limited resources such as food or space. Both competing species are negatively affected.

Q48. Describe predation and its ecological significance.
Ans: Predation involves one organism feeding on another. It helps regulate population size and maintain ecological balance.

Q49. What is parasitism?
Ans: Parasitism is an interaction where one organism benefits at the expense of another. Parasites depend on hosts for nutrition and shelter.

Q50. Explain commensalism with an example.
Ans: Commensalism is an interaction where one species benefits while the other is neither harmed nor benefited, such as epiphytes growing on trees.

CBSE Exam Note

These short answer questions (2–3 marks) are ideal for board exams, focusing on clarity, NCERT terminology, and structured presentation.