💡 Questions and Answers
Q1. Describe the process of photosynthesis and explain why it is considered a physico-chemical process.
Answer: Photosynthesis is the process by which green plants synthesise carbohydrates from carbon dioxide and water using light energy in the presence of chlorophyll, releasing oxygen as a by-product. It is considered a physico-chemical process because it involves:
- Physical reactions such as absorption of light energy by pigments
- Chemical reactions including oxidation-reduction reactions, splitting of water (photolysis), ATP synthesis, and reduction of CO₂ to carbohydrates
Q2. Explain the structure of chloroplast with the help of a labelled description and mention the division of labour within it.
Answer: A chloroplast is a double-membrane bound organelle found in green plant cells.
- Outer and Inner Membranes: Enclose the organelle
- Stroma: Fluid-filled matrix containing enzymes for the Calvin cycle, circular DNA, and 70S ribosomes
- Thylakoids: Flattened membranous sacs
- Grana: Stacks of thylakoids where light reactions occur
- Stroma lamellae: Connect different grana
Division of labour:
- Light reactions occur in thylakoid membranes
- Dark reactions (Calvin cycle) occur in the stroma
Q3. Describe the light-dependent reactions of photosynthesis.
Answer: Light reactions occur in the thylakoid membranes and involve:
- Absorption of light by photosystems
- Excitation of electrons and electron transport through Z-scheme
- Photolysis of water producing O₂, protons, and electrons
- Formation of ATP by photophosphorylation
- Reduction of NADP⁺ to NADPH
Q4. Explain the Calvin cycle with suitable steps.
Answer: The Calvin cycle occurs in the stroma and consists of three stages:
- Carboxylation: CO₂ combines with RuBP forming 3-PGA, catalysed by RuBisCO
- Reduction: 3-PGA is reduced to G3P using ATP and NADPH
- Regeneration: RuBP is regenerated using ATP
For fixation of 6 CO₂ molecules, 18 ATP and 12 NADPH are required.
Q5. Describe the C₄ pathway and explain how it reduces photorespiration.
Answer: In C₄ plants, CO₂ is first fixed in mesophyll cells by PEP carboxylase forming OAA. OAA is converted to malate/aspartate and transported to bundle sheath cells, where CO₂ is released for the Calvin cycle. This increases CO₂ concentration around RuBisCO, suppressing oxygenase activity and reducing photorespiration.
Q6. What is the role of accessory pigments in photosynthesis?
Answer: Accessory pigments (like chlorophyll b, carotenoids, xanthophylls) have two main functions:
- Light Harvesting: They absorb light at wavelengths that chlorophyll a does not absorb well and transfer this energy to chlorophyll a.
- Protection: They protect the chlorophyll a reaction center from photo-oxidation (damage by excessive light).
Q7. Differentiate between an absorption spectrum and an action spectrum.
Answer:
- Absorption Spectrum: A graph showing the amount of light absorbed by a specific pigment (e.g., chlorophyll a) at different wavelengths.
- Action Spectrum: A graph showing the actual rate of photosynthesis (e.g., measured by O₂ release) at different wavelengths of light. The action spectrum closely matches the absorption spectrum of chlorophylls, proving they are the pigments driving photosynthesis.
Q8. What is the 'Z-scheme' of the light reaction?
Answer: The Z-scheme is the pathway of non-cyclic photophosphorylation. It involves both PS II and PS I. Light excites PS II (P680), which loses an electron. This electron is replaced by one from the splitting of water (releasing O₂). The electron passes down an electron transport chain (ETC) to PS I, generating ATP. PS I (P700) is also excited by light, and its electron is passed to NADP⁺ reductase, which reduces NADP⁺ to NADPH. The 'hole' in PS I is filled by the electron from PS II. It is called the 'Z-scheme' because of the shape it makes on a redox potential diagram.
Q9. Explain the chemiosmotic hypothesis for ATP synthesis.
Answer: Chemiosmosis explains how ATP is made. A proton gradient (high H⁺ concentration) is built up inside the thylakoid lumen. This happens from (1) protons (H⁺) released by the splitting of water inside the lumen and (2) protons pumped into the lumen from the stroma by the ETC. This gradient is a form of potential energy. The protons flow back down their gradient (from lumen to stroma) through the ATP synthase enzyme, and the energy of this flow is used by the enzyme to synthesize ATP from ADP and Pi.
Q10. What are the three stages of the Calvin Cycle?
Answer:
- Carboxylation: The fixation of CO₂ into a stable 3-carbon acid (3-PGA). This is catalysed by the enzyme RuBisCO, which combines CO₂ with the acceptor molecule, RuBP.
- Reduction: The 3-PGA is converted into glyceraldehyde-3-phosphate (G3P, a sugar) using the energy from ATP and the reducing power of NADPH (both from the light reactions).
- Regeneration: The CO₂ acceptor, RuBP, is regenerated from the G3P molecules, a process that consumes ATP.