Photosynthesis
Short Questions
1. What is photosynthesis?
Answer: Photosynthesis is the process by which green plants convert CO₂ and H₂O into glucose using solar energy, releasing O₂.
2. Where does photosynthesis occur in a plant cell?
Answer: Photosynthesis occurs in the chloroplasts, mainly in the mesophyll cells of leaves.
3. What is the primary pigment involved in photosynthesis?
Answer: Chlorophyll a is the primary pigment involved in photosynthesis.
4. What is the role of carotenoids in photosynthesis?
Answer: Carotenoids absorb light and transfer energy to chlorophyll, protecting it from photo-oxidation.
5. What are the two main phases of photosynthesis?
Answer: The two main phases are the light reaction and the dark reaction (Calvin cycle).
6. What is the source of oxygen released in photosynthesis?
Answer: Oxygen is released from the photolysis of water during the light reaction.
7. What is the role of ATP in photosynthesis?
Answer: ATP provides energy for CO₂ fixation in the dark reaction.
8. What is photorespiration?
Answer: Photorespiration is a wasteful process in C3 plants where RuBisCO uses O₂ instead of CO₂, releasing CO₂.
9. What is the first stable product in C4 plants?
Answer: Oxaloacetic acid (4C) is the first stable product in C4 plants.
10. What is Kranz anatomy in C4 plants?
Answer: Kranz anatomy is the arrangement of bundle sheath and mesophyll cells in C4 plant leaves.
11. What is the significance of CAM plants?
Answer: CAM plants fix CO₂ at night to conserve water in arid conditions.
12. What is the role of RuBisCO in photosynthesis?
Answer: RuBisCO catalyzes CO₂ fixation with RuBP in the Calvin cycle.
13. What is cyclic photophosphorylation?
Answer: Cyclic photophosphorylation involves PS-I, producing ATP without NADPH or O₂.
14. How does light intensity affect photosynthesis?
Answer: Photosynthesis increases with light intensity up to an optimum, then slows in very high or low intensity.
15. What is Blackman’s law of limiting factors?
Answer: The rate of photosynthesis is limited by the factor present in the minimum amount.
Long Questions
1. Explain the structure of a chloroplast and its role in photosynthesis.
Answer: A chloroplast is a double-membrane organelle containing stroma and thylakoids organized into grana. Thylakoid membranes house chlorophyll for the light reaction, capturing solar energy to produce ATP and NADPH. The stroma contains enzymes for the dark reaction, where CO₂ is fixed into glucose using ATP and NADPH.
2. Describe the role of chlorophyll and carotenoids in light absorption.
Answer: Chlorophyll a and b absorb blue, violet, and red light, converting solar energy into chemical energy during the light reaction. Carotenoids absorb violet to blue light, transferring energy to chlorophyll and protecting it from photo-oxidation. Together, they enhance light absorption across the visible spectrum, optimizing photosynthesis.
3. What is the significance of the Hill reaction in understanding photosynthesis?
Answer: The Hill reaction demonstrated that isolated chloroplasts produce O₂ in light without CO₂, proving water as the O₂ source. It showed that light-driven photolysis of water releases electrons for CO₂ reduction. This clarified the mechanism of oxygen evolution in photosynthesis.
4. Differentiate between cyclic and non-cyclic photophosphorylation.
Answer: Cyclic photophosphorylation involves only PS-I, recycling electrons to produce ATP without NADPH or O₂. Non-cyclic photophosphorylation involves both PS-I and PS-II, producing ATP, NADPH, and O₂ via water photolysis. Non-cyclic is the primary pathway, while cyclic supplements ATP when needed.
5. How does the Calvin cycle contribute to glucose synthesis?
Answer: The Calvin cycle, occurring in the stroma, fixes CO₂ into RuBP to form 3-PGA, which is reduced to 3-PGAL using ATP and NADPH. Some 3-PGAL forms glucose, while others regenerate RuBP for continuous CO₂ fixation. It requires 18 ATP and 12 NADPH per glucose molecule.
6. Why is photorespiration considered a wasteful process in C3 plants?
Answer: Photorespiration occurs in C3 plants when RuBisCO uses O₂ instead of CO₂, forming phosphoglycolate, which leads to CO₂ release. It consumes energy and loses ~25% of fixed carbon without producing glucose. This reduces photosynthetic efficiency under high O₂ and temperature conditions.
7. Explain the C4 pathway and its advantages over the C3 pathway.
Answer: In the C4 pathway, CO₂ is fixed into oxaloacetic acid in mesophyll cells, then transferred as malic acid to bundle sheath cells for the Calvin cycle. This minimizes photorespiration by concentrating CO₂, enhancing efficiency. C4 plants are more productive in high light and temperature environments.
8. How do CAM plants adapt to desert conditions?
Answer: CAM plants open stomata at night to fix CO₂ into malic acid, storing it in vacuoles to reduce water loss. During the day, malic acid releases CO₂ for the Calvin cycle while stomata remain closed. This adaptation allows efficient photosynthesis in arid environments.
9. Discuss the impact of external factors like light and CO₂ on photosynthesis.
Answer: Light intensity and quality (red and blue wavelengths) drive photosynthesis, with optimal rates in bright, diffused sunlight. CO₂ concentration, typically 0.03% in the atmosphere, limits photosynthesis; higher levels (up to 1%) increase the rate. Both factors interact, with one becoming limiting if the other is insufficient, per Blackman’s law.
10. Why are C4 plants more productive than C3 plants despite higher ATP requirements?
Answer: C4 plants require 30 ATP per glucose compared to 18 ATP in C3 plants due to additional steps in CO₂ fixation. However, their Kranz anatomy and CO₂ concentration mechanism eliminate photorespiration, enhancing efficiency. This makes them highly productive in tropical regions with high light and temperature.
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