CBSE Class 11 Biology – Photosynthesis in Higher Plants | Short Answer Type Questions with Answers (NCERT Based)

Course & Examination Details

Course: CBSE Class 11 Biology
Unit: Unit IV – Plant Physiology
Chapter: Chapter 13 – Photosynthesis in Higher Plants
Prescribed Textbook: NCERT
Board: CBSE

CBSE Board Examination Relevance

• Short answer questions usually carry 3–4 marks
• Test concept clarity, logical explanation, and NCERT terminology
• Require concise, structured, and diagram-supported (where applicable) answers

Section A: Light Reaction (Q1–Q15)

Q1. What is photosynthesis? Explain its significance.

Ans:
Photosynthesis is the process by which green plants synthesize carbohydrates from carbon dioxide and water using light energy in the presence of chlorophyll. It is significant because it provides food for all living organisms, releases oxygen essential for respiration, and maintains the carbon dioxide–oxygen balance in the atmosphere. Photosynthesis also forms the base of all food chains and supports life on Earth.

Q2. Describe the site of photosynthesis in higher plants.

Ans:
Photosynthesis occurs in chloroplasts present mainly in mesophyll cells of leaves. Light reactions take place in the thylakoid membranes or grana, while dark reactions occur in the stroma. This structural organization ensures efficient absorption of light energy and effective carbon fixation, making chloroplasts the functional units of photosynthesis.

Q3. What are photosynthetic pigments? Name them.

Ans:
Photosynthetic pigments are light-absorbing substances present in chloroplasts. They include chlorophyll a (primary pigment), chlorophyll b, and carotenoids. Chlorophyll a directly participates in photochemical reactions, while accessory pigments broaden the absorption spectrum and transfer energy to chlorophyll a, enhancing photosynthetic efficiency.

Q4. Explain the structure and role of a photosystem.

Ans:
A photosystem is a pigment–protein complex embedded in the thylakoid membrane. It consists of a light-harvesting complex and a reaction centre. The light-harvesting complex absorbs photons and transfers energy to the reaction centre, where photochemical reactions begin, leading to electron excitation and energy conversion.

Q5. Distinguish between Photosystem I and Photosystem II.

Ans:
Photosystem II has reaction centre P680 and participates in photolysis of water, releasing oxygen. Photosystem I has reaction centre P700 and helps in NADPH formation. PS II operates before PS I in non-cyclic electron flow. Both together convert light energy into chemical energy.

Q6. Explain photolysis of water and its significance.

Ans:
Photolysis of water occurs in Photosystem II, where water splits into oxygen, protons, and electrons using light energy. Oxygen is released into the atmosphere, electrons replace those lost by PS II, and protons contribute to proton gradient formation, driving ATP synthesis during photophosphorylation.

Q7. What is photophosphorylation? Describe its types.

Ans:
Photophosphorylation is the synthesis of ATP from ADP and inorganic phosphate using light energy. It occurs in two types: cyclic photophosphorylation involving only Photosystem I, producing ATP only, and non-cyclic photophosphorylation involving both PS I and PS II, producing ATP, NADPH, and oxygen.

Q8. Explain non-cyclic photophosphorylation.

Ans:
In non-cyclic photophosphorylation, electrons flow from Photosystem II to Photosystem I and finally to NADP⁺. This process produces ATP and NADPH and releases oxygen due to photolysis of water. It is the main pathway of light reactions in photosynthesis.

Q9. What are assimilatory powers of photosynthesis?

Ans:
Assimilatory powers refer to ATP and NADPH produced during light reactions. These energy-rich molecules are used in the dark reactions to fix carbon dioxide into carbohydrates. ATP provides energy, while NADPH provides reducing power for biosynthetic reactions.

Q10. Explain the role of electron transport chain in light reaction.

Ans:
The electron transport chain transfers excited electrons from PS II to PS I through carriers. Energy released during electron movement helps pump protons across thylakoid membranes, creating a proton gradient that drives ATP synthesis by chemiosmosis.

Q11. Why are accessory pigments important in photosynthesis?

Ans:
Accessory pigments absorb light of different wavelengths that chlorophyll a cannot absorb efficiently. They transfer this energy to chlorophyll a, increasing the range of light usable for photosynthesis and protecting chlorophyll from photo-oxidation.

Q12. What is chemiosmosis in photosynthesis?

Ans:
Chemiosmosis is the process by which ATP is synthesized using a proton gradient across the thylakoid membrane. Protons accumulate inside thylakoids during light reactions and flow back through ATP synthase, driving ATP formation.

Q13. Explain the role of NADP⁺ in light reaction.

Ans:
NADP⁺ acts as the final electron acceptor in the light reaction. It accepts electrons and protons to form NADPH. This reduced molecule carries reducing power required for carbon fixation during the dark reactions.

Q14. Why does cyclic photophosphorylation occur?

Ans:
Cyclic photophosphorylation occurs when NADP⁺ is limited or when additional ATP is required. Electrons cycle back to Photosystem I, producing ATP without generating NADPH or oxygen, helping balance energy requirements.

Q15. How does light intensity affect light reaction?

Ans:
Increasing light intensity increases the rate of light reactions up to a saturation point. Beyond this point, other factors become limiting, and excessive light may damage chlorophyll through photo-oxidation.

Section B: Dark Reaction (Calvin Cycle) (Q16–Q30)

Q16. What is dark reaction? Why is it misleadingly named?

Ans:
Dark reaction refers to the light-independent phase of photosynthesis where carbon dioxide is fixed into carbohydrates using ATP and NADPH. It is misleadingly named because it does not occur in darkness but depends on products of light reaction.

Q17. Describe the Calvin cycle briefly.

Ans:
The Calvin cycle occurs in the stroma and involves three steps: carboxylation, reduction, and regeneration. CO₂ is fixed to RuBP, reduced to triose phosphates using ATP and NADPH, and RuBP is regenerated to continue the cycle.

Q18. Explain the role of RuBisCO enzyme.

Ans:
RuBisCO catalyses the fixation of carbon dioxide to RuBP in the Calvin cycle. It is the most abundant enzyme on Earth. However, it also shows oxygenase activity, leading to photorespiration, which reduces photosynthetic efficiency.

Q19. What is carboxylation in Calvin cycle?

Ans:
Carboxylation is the first step of Calvin cycle in which CO₂ combines with RuBP in presence of RuBisCO, forming two molecules of 3-phosphoglycerate, a three-carbon compound.

Q20. Explain the reduction phase of Calvin cycle.

Ans:
In the reduction phase, 3-phosphoglycerate is converted into glyceraldehyde-3-phosphate using ATP and NADPH. This step stores energy in carbohydrate form and leads to glucose synthesis.

Q21. What is regeneration phase of Calvin cycle?

Ans:
Regeneration involves conversion of remaining triose phosphates into RuBP using ATP. This ensures continuity of the Calvin cycle for further CO₂ fixation.

Q22. Why is RuBisCO considered inefficient?

Ans:
RuBisCO has low affinity for CO₂ and can bind oxygen, leading to photorespiration. This reduces carbon fixation efficiency, especially at high temperature and low CO₂ concentration.

Q23. What is photorespiration?

Ans:
Photorespiration is a process where RuBisCO binds oxygen instead of carbon dioxide, releasing CO₂ and consuming energy without producing sugars, thus reducing photosynthetic efficiency.

Q24. Why does photorespiration not occur in C₄ plants?

Ans:
C₄ plants concentrate CO₂ around RuBisCO in bundle sheath cells, preventing oxygen binding and eliminating photorespiration.

Q25. How many ATP and NADPH are required for one glucose molecule?

Ans:
To synthesize one glucose molecule, the Calvin cycle requires 18 ATP and 12 NADPH molecules.

Q26. Why is Calvin cycle important for plants?

Ans:
The Calvin cycle converts inorganic carbon dioxide into organic carbohydrates, providing food and energy for plant growth and metabolism.

Q27. Does Calvin cycle occur at night? Explain.

Ans:
Calvin cycle can occur at night if ATP and NADPH are available, but generally it slows down due to absence of light reaction products.

Q28. What is the first stable product of Calvin cycle?

Ans:
The first stable product of Calvin cycle is 3-phosphoglyceric acid, a three-carbon compound.

Q29. Why is ATP required in dark reaction?

Ans:
ATP provides energy for reduction of PGA and regeneration of RuBP, making carbon fixation possible.

Q30. What links light and dark reactions?

Ans:
ATP and NADPH produced during light reactions link the two phases by providing energy and reducing power for carbon fixation.

Section C: C₃, C₄ Pathways & Factors Affecting Photosynthesis (Q31–Q50)

Q31. Explain C₃ pathway.

Ans:
C₃ pathway is the Calvin cycle where CO₂ fixation results in formation of 3-PGA as first stable product. It occurs in mesophyll cells and is common in most plants like wheat and rice.

Q32. Explain C₄ pathway briefly.

Ans:
C₄ pathway involves initial CO₂ fixation into a four-carbon compound by PEP carboxylase in mesophyll cells. The Calvin cycle occurs in bundle sheath cells, increasing efficiency.

Q33. What is Kranz anatomy?

Ans:
Kranz anatomy is a special leaf structure in C₄ plants where bundle sheath cells surround vascular bundles and contain RuBisCO.

Q34. Compare efficiency of C₃ and C₄ plants.

Ans:
C₄ plants are more efficient as they avoid photorespiration, perform better at high temperature and light intensity, and show higher productivity.

Q35. Why does C₄ pathway require more ATP?

Ans:
Additional steps of CO₂ transport and regeneration of PEP require extra ATP, making C₄ pathway energy-intensive.

Q36. Explain Blackman’s law of limiting factors.

Ans:
Blackman’s law states that the rate of photosynthesis is controlled by the factor present in minimum amount, even if other factors are favourable.

Q37. How does CO₂ concentration affect photosynthesis?

Ans:
Increasing CO₂ concentration increases photosynthesis up to a saturation level, after which the rate remains constant.

Q38. Explain the effect of temperature on photosynthesis.

Ans:
Photosynthesis increases with temperature up to an optimum range, beyond which enzyme activity declines and photorespiration increases in C₃ plants.

Q39. How does water availability affect photosynthesis?

Ans:
Water deficiency causes stomatal closure, reducing CO₂ entry and lowering photosynthetic rate.

Q40. Explain the role of light quality in photosynthesis.

Ans:
Red and blue wavelengths are most effective for photosynthesis as they are strongly absorbed by chlorophyll.

Q41. What are internal factors affecting photosynthesis?

Ans:
Internal factors include chlorophyll content, leaf anatomy, stomatal opening, enzyme activity, and protoplasmic factors.

Q42. Why is CO₂ usually the limiting factor?

Ans:
Atmospheric CO₂ concentration is low compared to plant requirement, making it the most common limiting factor.

Q43. How does leaf area affect photosynthesis?

Ans:
Larger leaf area provides more surface for light absorption and gas exchange, increasing photosynthetic capacity.

Q44. Why is photosynthesis reduced under water stress?

Ans:
Water stress leads to stomatal closure, reduced CO₂ uptake, and decreased enzyme activity, lowering photosynthesis.

Q45. How does chlorophyll content affect photosynthesis?

Ans:
Higher chlorophyll content increases light absorption, enhancing photosynthetic rate.

Q46. Why is photosynthesis considered an anabolic process?

Ans:
It synthesizes complex organic molecules like glucose from simple inorganic substances using energy.

Q47. How does oxygen concentration affect photosynthesis?

Ans:
High oxygen concentration promotes photorespiration in C₃ plants, reducing photosynthetic efficiency.

Q48. Why are C₄ plants adapted to tropical regions?

Ans:
They efficiently photosynthesize at high temperature, light intensity, and low CO₂ conditions.

Q49. What is the ecological significance of photosynthesis?

Ans:
Photosynthesis sustains ecosystems by providing food, oxygen, and regulating carbon cycle.

Q50. Why is photosynthesis called the backbone of life?

Ans:
It provides energy, food, and oxygen for all living organisms, making life on Earth possible.

Best Suited For

• CBSE Class 11 Annual Examinations
• NCERT-based short-answer practice
• Conceptual clarity and revision