CBSE Class 11 Biology – Photosynthesis in Higher Plants | Long 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 Significance

• Long answer questions generally carry 5 marks
• Test conceptual depth, explanation, and structure
• Answers must be NCERT-specific, logical, and well-organised

Section A: Light Reaction (Q1–Q8)

Q1. Explain the process of photosynthesis and its overall 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 occurs in chloroplasts and involves conversion of solar energy into chemical energy. The overall reaction results in formation of glucose and release of oxygen. Photosynthesis is significant because it forms the base of all food chains, supplies oxygen for aerobic respiration, and maintains atmospheric balance of oxygen and carbon dioxide. It also supports growth of plants and sustains life on Earth directly or indirectly.

Q2. Describe the structure and function of chloroplast in photosynthesis.

Ans:
Chloroplasts are double-membrane-bound organelles present in mesophyll cells of leaves. The inner membrane encloses a fluid-filled stroma, which contains enzymes for the Calvin cycle. Embedded within the stroma are stacks of thylakoids called grana. Thylakoid membranes contain photosystems, pigments, and electron carriers responsible for light reactions. Light energy is absorbed in the grana, producing ATP and NADPH. The stroma uses these products to fix carbon dioxide into carbohydrates. Thus, chloroplasts are the structural and functional sites of photosynthesis.

Q3. Explain the organisation and role of photosystems in light reaction.

Ans:
Photosystems are pigment–protein complexes embedded in thylakoid membranes. Each photosystem consists of a light-harvesting complex and a reaction centre. The light-harvesting complex contains chlorophyll and accessory pigments that absorb light energy and transfer it to the reaction centre. Photosystem II has reaction centre P680 and initiates photolysis of water. Photosystem I has reaction centre P700 and facilitates reduction of NADP⁺ to NADPH. Together, both photosystems convert light energy into chemical energy during light reactions.

Q4. Describe photolysis of water and its importance.

Ans:
Photolysis of water is the light-driven splitting of water molecules during photosynthesis. It occurs at Photosystem II in the thylakoid membrane. Light energy splits water into oxygen, protons, and electrons. Oxygen is released into the atmosphere as a by-product. Electrons replace those lost by Photosystem II, maintaining electron flow, while protons contribute to proton gradient formation essential for ATP synthesis. Photolysis is important because it supplies electrons for light reactions and is the source of oxygen evolved during photosynthesis.

Q5. Explain cyclic and non-cyclic photophosphorylation.

Ans:
Photophosphorylation is the synthesis of ATP using light energy. In cyclic photophosphorylation, only Photosystem I participates. Electrons cycle back to the same photosystem, producing ATP only, without NADPH or oxygen formation. In non-cyclic photophosphorylation, both Photosystem II and Photosystem I operate. Electrons flow from water to NADP⁺, producing ATP, NADPH, and oxygen. Non-cyclic photophosphorylation is the main pathway during photosynthesis, while cyclic photophosphorylation supplements ATP requirement when needed.

Q6. Explain the role of electron transport chain in light reactions.

Ans:
The electron transport chain consists of a series of electron carriers embedded in the thylakoid membrane. Excited electrons from Photosystem II pass through these carriers to Photosystem I. During this movement, energy is released and used to pump protons into the thylakoid lumen, creating a proton gradient. This gradient drives ATP synthesis by chemiosmosis through ATP synthase. The electron transport chain thus links light absorption to ATP formation, making energy available for carbon fixation.

Q7. What are assimilatory powers? Explain their role.

Ans:
Assimilatory powers refer to ATP and NADPH produced during light reactions of photosynthesis. ATP provides energy required for biosynthetic reactions, while NADPH supplies reducing power needed to reduce carbon dioxide into carbohydrates. These molecules are transported to the stroma, where they are utilised in the Calvin cycle. Without assimilatory powers, carbon fixation cannot occur, making light reactions essential for completion of photosynthesis.

Q8. Why are accessory pigments important in photosynthesis?

Ans:
Accessory pigments such as chlorophyll b and carotenoids absorb light of wavelengths that chlorophyll a cannot absorb efficiently. They transfer this absorbed energy to chlorophyll a, increasing the range of light usable for photosynthesis. Accessory pigments also protect chlorophyll from photo-oxidation by dissipating excess light energy. Thus, they enhance photosynthetic efficiency and protect the photosynthetic apparatus.

Section B: Dark Reaction (Calvin Cycle) (Q9–Q15)

Q9. Explain the Calvin cycle and its phases.

Ans:
The Calvin cycle is the light-independent phase of photosynthesis occurring in the stroma of chloroplasts. It consists of three phases: carboxylation, reduction, and regeneration. In carboxylation, CO₂ combines with RuBP in presence of RuBisCO, forming 3-PGA. During reduction, 3-PGA is converted into triose phosphates using ATP and NADPH. In regeneration, RuBP is regenerated using ATP to continue the cycle. The Calvin cycle converts inorganic carbon into carbohydrates essential for plant growth.

Q10. Describe the role and limitations 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, RuBisCO has a dual nature, acting both as a carboxylase and oxygenase. Its oxygenase activity leads to photorespiration, which reduces photosynthetic efficiency by releasing CO₂ and consuming energy. RuBisCO also has low affinity for CO₂, making it inefficient under high temperature and low CO₂ conditions.

Q11. What is photorespiration? Why is it considered wasteful?

Ans:
Photorespiration is a process in which RuBisCO binds oxygen instead of carbon dioxide, leading to the release of CO₂. It occurs mainly in C₃ plants under high temperature and low CO₂ concentration. Photorespiration is considered wasteful because it consumes energy and releases fixed carbon without producing ATP or sugars. This reduces photosynthetic efficiency and plant productivity.

Q12. Why does photorespiration not occur in C₄ plants?

Ans:
C₄ plants possess a CO₂-concentrating mechanism. CO₂ is initially fixed by PEP carboxylase in mesophyll cells and transported to bundle sheath cells, where the Calvin cycle occurs. High CO₂ concentration around RuBisCO prevents oxygen binding, eliminating photorespiration. This adaptation makes C₄ plants more efficient, especially under high temperature and light intensity.

Q13. How are light and dark reactions interdependent?

Ans:
Light and dark reactions are interdependent. Light reactions produce ATP and NADPH, which are essential for carbon fixation in dark reactions. Dark reactions utilise these molecules to reduce CO₂ into carbohydrates. In turn, dark reactions regenerate ADP, inorganic phosphate, and NADP⁺ required for light reactions. Thus, both phases operate in coordination to complete photosynthesis.

Q14. Calculate energy requirement for synthesis of one glucose molecule.

Ans:
To synthesise one molecule of glucose, six molecules of CO₂ must be fixed. The Calvin cycle requires 18 ATP and 12 NADPH molecules for this process. ATP provides energy, while NADPH supplies reducing power. This energy requirement highlights the importance of efficient light reactions for sustaining carbon fixation.

Q15. Why is dark reaction also called biosynthetic phase?

Ans:
The dark reaction synthesises carbohydrates from carbon dioxide using ATP and NADPH. Since it involves formation of complex organic molecules from simple inorganic substances, it is termed the biosynthetic phase of photosynthesis.

Section C: C₃, C₄ Pathways & Factors Affecting Photosynthesis (Q16–Q25)

Q16. Explain the C₃ pathway of photosynthesis.

Ans:
The C₃ pathway, also known as the Calvin cycle, is the most common pathway of carbon fixation. The first stable product is 3-phosphoglycerate, a three-carbon compound. CO₂ fixation occurs in mesophyll cells with RuBisCO as the primary enzyme. Photorespiration is common in C₃ plants, reducing efficiency under high temperature and low CO₂ conditions.

Q17. Describe the C₄ pathway and its advantages.

Ans:
The C₄ pathway involves initial fixation of CO₂ into a four-carbon compound by PEP carboxylase in mesophyll cells. CO₂ is then released in bundle sheath cells for the Calvin cycle. This pathway prevents photorespiration and increases photosynthetic efficiency under high temperature and light intensity, though it requires extra ATP.

Q18. What is Kranz anatomy?

Ans:
Kranz anatomy refers to a special leaf structure in C₄ plants where vascular bundles are surrounded by bundle sheath cells rich in chloroplasts. Mesophyll cells encircle these bundle sheath cells, facilitating efficient CO₂ concentration and preventing photorespiration.

Q19. Compare C₃ and C₄ plants.

Ans:
C₃ plants form 3-PGA as first product and show photorespiration, making them less efficient. C₄ plants form OAA as first product, lack photorespiration, and are more efficient under high temperature and light conditions. C₄ plants require more ATP but have higher productivity.

Q20. Explain Blackman’s law of limiting factors.

Ans:
Blackman’s law states that when a process is influenced by several factors, its rate is limited by the factor present in the least amount. In photosynthesis, even if light and temperature are favourable, low CO₂ concentration can limit the rate.

Q21. Discuss the effect of light on photosynthesis.

Ans:
Light intensity, quality, and duration affect photosynthesis. Rate increases with light intensity up to saturation. Red and blue wavelengths are most effective. Excess light may damage chlorophyll.

Q22. Explain the role of carbon dioxide in photosynthesis.

Ans:
Carbon dioxide is a raw material for photosynthesis. Increasing its concentration increases photosynthetic rate up to saturation. CO₂ is often the limiting factor under natural conditions.

Q23. How does temperature affect photosynthesis?

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

Q24. How does water availability affect photosynthesis?

Ans:
Water deficiency causes stomatal closure, reducing CO₂ entry and lowering photosynthesis. It also affects enzyme activity and chlorophyll synthesis indirectly.

Q25. Explain the ecological significance of photosynthesis.

Ans:
Photosynthesis sustains ecosystems by providing food and oxygen, forming the base of food chains, and maintaining atmospheric balance of gases. It regulates the global carbon cycle and supports life on Earth.

Best Suited For

• CBSE Class 11 Annual Examinations
• NCERT-based 5-mark answer practice
• Conceptual clarity and structured writing