One-mark Questions and Answers:-
Q1: Define photosynthesis.
Ans: Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds.
Q2: Why are green plants called autotrophs? Ans: Green plants are called autotrophs because they make or synthesize the food they need through photosynthesis.
Q3: What conclusion was drawn from the variegated leaf experiment?
Ans: It showed that photosynthesis occurred only in the green parts of the leaves in the presence of light.
Q4: What is the function of KOH soaked cotton in the half-leaf experiment?
Ans: The KOH soaked cotton absorbs Carbon Dioxide (CO2) inside the test tube.
Q5: Who discovered oxygen in 1774?
Ans: Joseph Priestley discovered oxygen in 1774.
Q6: What role did Jan Ingenhousz demonstrate regarding sunlight and plants?
Ans: He showed that sunlight is essential to the plant process that purifies the air fouled by burning candles or breathing animals.
Q7: According to Julius von Sachs, in what form is glucose usually stored in plants?
Ans: Glucose is usually stored as starch.
Q8: Based on Cornelius van Niel’s studies, where does the oxygen evolved by green plants come from?
Ans: The oxygen comes from water (H2O), not from carbon dioxide.
Q9: Why are the biosynthetic reactions in the stroma called “dark reactions”?
Ans: They are called dark reactions because they are not directly light-driven but depend on the products of light reactions (ATP and NADPH).
Q10: How do chloroplasts usually align themselves in mesophyll cells to receive optimum light?
Ans: They align themselves along the walls of the mesophyll cells with their flat surfaces parallel to the walls.
Q11: Which pigment is considered the chief pigment associated with photosynthesis?
Ans: Chlorophyll a.
Q12: Name the four pigments responsible for the colour of leaves.
Ans: Chlorophyll a, Chlorophyll b, Xanthophylls, and Carotenoids.
Q13: What is the reaction centre chlorophyll a called in Photosystem I (PS I)?
Ans: It is called P700 because it has an absorption peak at 700 nm.
Q14: What is the “Z scheme”?
Ans: It is the scheme of electron transfer starting from PS II, uphill to the acceptor, down to PS I, and finally downhill to NADP+.
Q15: Where is the water splitting complex located within the chloroplast?
Ans: It is located on the inner side of the thylakoid membrane.
Q16: What are the products of cyclic photophosphorylation?
Ans: The synthesis of ATP only (without NADPH).
Q17: According to the chemiosmotic hypothesis, where do protons accumulate in the chloroplast? Ans: Protons accumulate in the lumen of the thylakoids.
Q18: Why is the proton gradient important in photosynthesis?
Ans: The breakdown of the proton gradient leads to the synthesis of ATP.
Q19: Which part of the ATP synthase enzyme forms a transmembrane channel?
Ans: The CF0 part forms the transmembrane channel.
Q20: Where is the NADP reductase enzyme located?
Ans: It is located on the stroma side of the membrane.
Q21: Who discovered that the first stable product of CO2 fixation is a 3-carbon organic acid?
Ans: Melvin Calvin.
Q22: What is the first stable product of the CO2 fixation in the C4 pathway?
Ans: Oxaloacetic acid (OAA), which is a 4-carbon organic acid.
Q23: What is the primary acceptor molecule of CO2 in the Calvin cycle?
Ans: A 5-carbon ketose sugar called ribulose bisphosphate (RuBP).
Q24: Which is the most crucial step of the Calvin cycle?
Ans: Carboxylation.
Q25: How many molecules of ATP and NADPH are required to make one molecule of glucose in the Calvin cycle?
Ans: 18 ATP and 12 NADPH molecules.
Q26: What is “Kranz” anatomy?
Ans: It refers to the special arrangement where bundle sheath cells form several layers around the vascular bundles in C4 plants.
Q27: Which enzyme is responsible for the primary fixation of CO2 in C4 plants?
Ans: PEP carboxylase or PEPcase.
Q28: In C4 plants, where does the Calvin pathway take place?
Ans: In the bundle sheath cells.
Q29: Why is photorespiration considered a wasteful process?
Ans: Because there is neither synthesis of sugars nor of ATP, but release of CO2 with the utilization of ATP.
Q30: Why does photorespiration not occur in C4 plants?
Ans: Because they have a mechanism to increase the concentration of CO2 at the enzyme site.
Q31: In C4 plants, which cell type lacks the RuBisCO enzyme? Ans: Mesophyll cells.
Q32: State Blackman’s Law of Limiting Factors.
Ans: If a chemical process is affected by more than one factor, its rate is determined by the factor which is nearest to its minimal value.
Q33: At what percentage of full sunlight does light saturation occur?
Ans: At 10 per cent of full sunlight.
Q34: What is the major limiting factor for photosynthesis in nature?
Ans: Carbon dioxide concentration.
Q35: Why are C4 plants able to tolerate higher temperatures than C3 plants?
Ans: C4 plants have a higher temperature optimum compared to C3 plants.
Q36: How does water stress affect the availability of CO2?
Ans: Water stress causes the stomata to close, thereby reducing the CO2 availability.
Q37: Where does the chemosynthetic pathway occur within the chloroplast?
Ans: It occurs in the stroma.
Q38: What is photolysis?
Ans: Splitting of water using light energy.
Q39: Name the enzyme responsible for CO₂ fixation in C₄ plants.
Ans: PEP carboxylase.
Q40: Which law explains limiting factors of photosynthesis?
Ans: Blackman’s law of limiting factors.
Two-mark Questions and Answers:-
Q1: Why is photosynthesis considered the basis of life on earth? Give two reasons.
Ans: Photosynthesis is considered the basis of life because:
- It is the primary source of all food on earth for all living forms.
- It is responsible for the release of oxygen into the atmosphere, which is essential for breathing.
Q2: Differentiate between autotrophs and heterotrophs.
Ans: Autotrophs: Organisms, like green plants, that synthesize the food they need through photosynthesis.
- Heterotrophs: All other organisms (including animals) that depend on green plants for their food, either directly or indirectly.
Q3: Describe the half-leaf experiment. What conclusion was drawn from it?
Ans: In the half-leaf experiment, a part of a leaf is enclosed in a test tube containing KOH soaked cotton (which absorbs CO2), while the other half is exposed to air. Conclusion: The exposed part tested positive for starch while the enclosed part tested negative, proving that Carbon Dioxide CO2) is required for photosynthesis.
Q4: What was Joseph Priestley’s hypothesis regarding the relationship between plants and air? Ans: Based on his experiments with a bell jar, candle, and mouse, Priestley hypothesized that plants restore to the air whatever breathing animals and burning candles remove.
Q5: How did Cornelius van Niel demonstrate that oxygen evolved during photosynthesis comes from water?
Ans: Van Niel studied purple and green sulphur bacteria, which use $H_2S$ as a hydrogen donor and release sulphur instead of oxygen. He inferred that in green plants, $H_2O$ is the hydrogen donor and is oxidised to $O_2$. This was later proved using radioisotopic techniques.
Q6: What is an action spectrum? Who described the first action spectrum of photosynthesis?
Ans: An action spectrum is a graph plotting the biological effectiveness of different wavelengths of light (e.g., oxygen evolution). T.W. Engelmann described the first action spectrum using the green alga Cladophora and aerobic bacteria.
Q7: Differentiate between Light Reactions and Dark Reactions in terms of their nature and dependency.
Ans:
- Light Reactions: These are photochemical reactions that trap light energy to synthesize ATP and NADPH.
- Dark Reactions: These are enzymatic reactions (carbon reactions) that synthesize sugar. They are not directly light-driven but depend on the products of light reactions (ATP and NADPH).
Q8: Explain the “division of labour” within a chloroplast.
Ans: The membrane system (grana and stroma lamellae) is responsible for trapping light energy and synthesizing ATP and NADPH. The stroma is responsible for enzymatic reactions that synthesize sugar, which forms starch.
Q9: What are accessory pigments? State their two functions.
Ans: Accessory pigments include chlorophyll b, xanthophylls, and carotenoids. Functions:
- They absorb light at wider ranges of wavelengths and transfer the energy to chlorophyll a.
- They protect chlorophyll a from photo-oxidation.
Q10: Differentiate between Photosystem I (PS I) and Photosystem II (PS II) based on their reaction centres.
Ans:
- PS I: The reaction centre chlorophyll a has an absorption peak at 700 nm, hence called P700.
- PS II: The reaction centre chlorophyll a has an absorption peak at 680 nm, hence called P680.
Q11: Explain the splitting of water in photosynthesis. Where does it occur?
Ans: Water splitting is the process where water is split into 2H+, [O], and electrons to replace electrons moved from PS II. It occurs on the inner side of the thylakoid membrane.
Q12: Under what conditions does cyclic photophosphorylation occur? What is the product?
Ans: It occurs when only PS I is functional or when only light of wavelengths beyond 680 nm is available. The product is ATP (no NADPH is produced).
Q13: Why is the proton gradient important in the chemiosmotic hypothesis?
Ans: The proton gradient is crucial because the breakdown of this gradient releases energy. This energy causes a conformational change in the CF1 particle of ATP synthase, leading to the synthesis of ATP.
Q14: List the four requirements for chemiosmosis to occur. Ans: Chemiosmosis requires:
- A membrane
- A proton pump
- A proton gradient
- ATP synthase
Q15: How are C3 and C4 plants classified based on their first stable product of $CO_2$ fixation? Ans:
C3 Plants: The first stable product of CO2 fixation is a 3-carbon acid, PGA (3-phosphoglyceric acid).
C4 Plants: The first stable product of CO2 fixation is a 4-carbon acid, OAA (oxaloacetic acid).
Q16: Explain the step of Carboxylation in the Calvin Cycle.
Ans: Carboxylation is the fixation of CO2 into a stable organic intermediate. CO2 is utilised for the carboxylation of RuBP, catalysed by the enzyme RuBisCO, to form two molecules of 3-PGA.
Q17: Describe the special features of “bundle sheath cells” in C4 plants.
Ans: Bundle sheath cells form layers around vascular bundles and are characterised by:
- Having a large number of chloroplasts.
- Thick walls impervious to gaseous exchange and no intercellular spaces.
Q18: Calculate the total ATP and NADPH required to synthesize one molecule of glucose in the Calvin Cycle.
Ans: Six turns of the cycle are required for one glucose molecule. Since one turn uses 3 ATP and 2 NADPH, the total requirement is 18 ATP and 12 NADPH.
Q19: Name the primary CO2 acceptor in C4 plants. In which cells is it found?
Ans: The primary CO2 acceptor is Phosphoenol pyruvate (PEP), which is a 3-carbon molecule. It is found in the mesophyll cells.
Q20: Why does photorespiration occur in C3 plants but not in C4 plants?
Ans: In C3 plants, O2 binds to RuBisCO when O2 levels are high, decreasing O2 fixation. C4 plants avoid this by increasing intracellular O2 concentration at the enzyme site, ensuring RuBisCO acts as a carboxylase.
Q21: Compare C3 and C4 plants based on the site of the Calvin Cycle.
Ans:
- C3 Plants: The Calvin cycle takes place in the mesophyll cells.
- C4 Plants: The Calvin cycle takes place in the bundle sheath cells.
Q22: State Blackman’s Law of Limiting Factors.
Ans: It states that if a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value. This factor directly affects the process if its quantity is changed.
Q23: How does Carbon Dioxide concentration affect C3 and C4 plants differently?
Ans: C4 plants show saturation at about 360 muL-1, while C3 plants respond to increased CO2 concentration and saturation is seen only beyond 450 muL-1. Thus, current atmospheric CO2 levels are limiting for C3 plants.
Q24: Why are “greenhouse crops” like tomatoes grown in CO2 enriched atmospheres?
Ans: Tomatoes are C3 plants. They respond to higher CO2 concentration by showing increased rates of photosynthesis, which leads to higher yields.
Q25: Explain two ways in which water stress affects photosynthesis. Ans:
- Water stress causes stomata to close, reducing CO2 availability.
- It makes leaves wilt, reducing the surface area of the leaves and their metabolic activity.
Q3. Name any two requirements for photosynthesis.
Answer:
Two essential requirements for photosynthesis are light and chlorophyll. Carbon dioxide and water are also required.
Q30. What was proved by the experiment using KOH and a leaf?
Answer:
The experiment proved that carbon dioxide is essential for photosynthesis, as the part of the leaf deprived of CO₂ did not form starch.
Q31. Name the scientist who discovered oxygen and contributed to photosynthesis experiments.
Answer:
Joseph Priestley discovered oxygen and demonstrated the role of plants in restoring air.
Q32. What is meant by absorption spectrum?
Answer:
The absorption spectrum shows the wavelengths of light absorbed by a pigment, such as chlorophyll, at different wavelengths.
Q33. Name the chief photosynthetic pigment in plants.
Answer:
Chlorophyll a is the chief photosynthetic pigment in green plants.
Q34. What are accessory pigments? Give one example.
Answer:
Accessory pigments are pigments that absorb light and transfer energy to chlorophyll a.
Example: Chlorophyll b.
Q35. Name the two photosystems present in the thylakoid membrane.
Answer:
The two photosystems are Photosystem I (PS I) and Photosystem II (PS II).
Q36. What is photolysis of water?
Answer:
Photolysis of water is the splitting of water molecules into protons, electrons, and oxygen during light reactions.
Q37. Where does the Calvin cycle occur?
Answer:
The Calvin cycle occurs in the stroma of the chloroplast.
Q38. Name the primary CO₂ acceptor in the Calvin cycle.
Answer:
Ribulose-1,5-bisphosphate (RuBP) is the primary CO₂ acceptor.
Q39. What is photorespiration?
Answer:
Photorespiration is a process in which RuBisCO binds oxygen instead of carbon dioxide, leading to the release of CO₂ without sugar formation.
Q40. Name the first stable product of CO₂ fixation in C₃ plants.
Answer:
3-phosphoglyceric acid (PGA) is the first stable product in C₃ plants.
Q41. What is Kranz anatomy?
Answer:
Kranz anatomy is a special leaf anatomy in C₄ plants where bundle sheath cells surround the vascular bundles.
Q42. Name the enzyme responsible for CO₂ fixation in C₄ plants.
Answer:
PEP carboxylase is the enzyme responsible for CO₂ fixation in C₄ plants.
Q44. State Blackman’s law of limiting factors.
Answer:
Blackman’s law states that the rate of a physiological process is limited by the factor that is present in the least amount.
Q45. Why does water stress reduce photosynthesis?
Answer:
Water stress causes stomatal closure, reducing CO₂ entry into the leaf and thereby lowering the rate of photosynthesis.
Three-mark Questions and Answers:-
Q1: Define photosynthesis and explain its significance for life on earth.
Ans:
- Definition: Photosynthesis is a physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds (food).
- Significance:
- Primary Food Source: It is the primary source of all food on earth, as all animals (heterotrophs) depend directly or indirectly on plants.
- Oxygen Release: It is responsible for the release of oxygen into the atmosphere, which is essential for the survival of breathing organisms.
Q2: Describe the “Half-Leaf Experiment”. What was the setup and the conclusion?
Ans:
- Setup: A part of a leaf is enclosed in a test tube containing KOH soaked cotton (which absorbs CO2), while the other half of the leaf is exposed to air. The setup is placed in light.
- Observation: On testing for starch, the exposed part tests positive, while the portion inside the tube tests negative.
- Conclusion: This experiment demonstrates that Carbon Dioxide CO2) is essential for photosynthesis to occur.
Q3: Explain Joseph Priestley’s experiment with the bell jar, candle, and mouse. What hypothesis did he formulate?
Ans:
- Experiment: Priestley observed that a burning candle or a mouse placed in a closed bell jar would soon extinguish or suffocate. However, when he placed a mint plant in the jar, the mouse stayed alive and the candle continued to burn.
- Hypothesis: He hypothesized that plants restore to the air whatever breathing animals and burning candles remove (i.e., they purify the air).
Q4: How did Cornelius van Niel contribute to the understanding of photosynthesis? Explain with the help of the general equation he proposed.
Ans:
- Contribution: Based on studies of purple and green sulphur bacteria, van Niel demonstrated that photosynthesis is a light-dependent reaction where hydrogen from a suitable oxidisable compound reduces carbon dioxide to carbohydrates.
- Inference: He inferred that in green plants, HO2 is the hydrogen donor and is oxidised to O2 (unlike bacteria where H2S is the donor and sulphur is released).
- Equation: 2H2A + CO2 →{Light} 2A + CH2O + H2O
Q5: Describe the division of labour within the chloroplast. Differentiate between the functions of the membrane system and the stroma.
Ans: There is a clear division of labour within the chloroplast:
- Membrane System (Grana and Stroma Lamellae): It is responsible for trapping light energy and synthesizing high-energy chemical intermediates, ATP and NADPH (Light Reactions).
- Stroma (Matrix): It contains enzymes responsible for the reduction of CO2 to synthesize sugars, which eventually form starch (Dark Reactions).
Q6: Name the four pigments involved in photosynthesis. What is the specific role of accessory pigments?
Ans:
- Pigments: Chlorophyll a (bright/blue green), Chlorophyll b (yellow green), Xanthophylls (yellow), and Carotenoids (yellow to yellow-orange).
- Role of Accessory Pigments:
- They absorb light at a wider range of wavelengths and transfer the energy to chlorophyll a, making photosynthesis more efficient.
- They protect chlorophyll a from photo-oxidation.
Q7: Explain the “Z Scheme” of electron transport. Why is it named so?
Ans:
- Process: The Z Scheme describes the transfer of electrons starting from Photosystem II (PS II), uphill to an acceptor, downhill to Photosystem I (PS I), excitation to another acceptor, and finally downhill to NADP+ to form NADPH.
- Naming: It is called the Z scheme due to its characteristic shape when all the electron carriers are placed in sequence on a redox potential scale.
- Result: This process results in the formation of both ATP and NADPH + H+.
Q8: What is the splitting of water? Where does it occur and what are the products? Ans:
- Process: The splitting of water is the process by which water molecules are split into protons (H+), oxygen ([O]), and electrons to replace the electrons removed from PS II.
- Location: It occurs on the inner side of the thylakoid membrane.
- Products: The process releases Oxygen O2 as a byproduct and protons accumulate in the lumen.
- Equation: 2H2O → 4H+ + O2 + 4e–
Q9: Explain the Chemiosmotic Hypothesis for ATP synthesis. What causes the proton gradient to break down?
Ans:
- Hypothesis: ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane (protons accumulate in the lumen).
- Breakdown: The gradient is broken down due to the movement of protons across the membrane to the stroma through the transmembrane channel of the CF0 of the ATP synthase enzyme.
- ATP Synthesis: This breakdown releases energy that causes a conformational change in the CF1 particle, catalyzing the synthesis of ATP.
Q10: Why is the biosynthetic phase called the “dark reaction”? Is this term accurate? Explain.
Ans:
- Reason: It is called the dark reaction because it is not directly driven by light energy but depends on the products of the light reaction (ATP and NADPH).
- Accuracy: The term is considered a misnomer because the process is not independent of light; it stops shortly after light becomes unavailable because the supply of ATP and NADPH runs out. It does not necessarily occur in darkness.
Q11: Describe the three main stages of the Calvin Cycle. Ans:
- Carboxylation: Fixation of CO2 into a stable organic intermediate where CO2 combines with RuBP (catalyzed by RuBisCO) to form two molecules of 3-PGA.
- Reduction: A series of reactions using ATP and NADPH to form glucose.
- Regeneration: The CO2 acceptor molecule, RuBP, is regenerated to continue the cycle (requires ATP).
Q12: What is “Kranz Anatomy”? List two characteristics of the cells involved.
Ans:
- Definition: Kranz anatomy is a special leaf anatomy found in C4 plants (like maize) where bundle sheath cells form a ring (wreath) around the vascular bundles.
- Characteristics of Bundle Sheath Cells:
- They have thick walls impervious to gaseous exchange.
- They possess a large number of chloroplasts and lack intercellular spaces.
Q13: Describe the initial steps of the C4 pathway (Hatch and Slack Pathway) that occur in the mesophyll cells. Ans:
- Acceptance: The primary CO2 acceptor is Phosphoenol pyruvate (PEP), a 3-carbon molecule, located in the mesophyll cells.
- Fixation: The reaction is catalyzed by PEP carboxylase (PEPcase). Mesophyll cells lack RuBisCO.
- Product: The C4 acid OAA (Oxaloacetic acid) is formed, which is then converted to other 4-carbon acids like malic acid before transport to bundle sheath cells.
Q14: Explain the process of Photorespiration. Why is it considered a wasteful process?
Ans:
- Process: In C3 plants, when O2 concentration is high, RuBisCO binds to O2 instead of CO2. RuBP is converted into phosphoglycerate and phosphoglycolate.
- Wasteful Nature: It is wasteful because:
- It does not synthesize sugars or ATP.
- It results in the release of CO2 while actually consuming ATP (energy).
Q15: Differentiate between C3 and C4 plants with respect to: (a) The primary CO2 acceptor (b) The enzyme responsible for initial carboxylation (c) The presence of photorespiration Ans:
- (a) Primary Acceptor: In C3 plants, it is RuBP (5-carbon). In C4 plants, it is PEP (3-carbon).
- (b) Initial Enzyme: In C3 plants, it is RuBisCO. In C4 plants, it is PEPcase.
- (c) Photorespiration: It is present (high) in C3 plants but negligible/absent in C4 plants.
Q16: State Blackman’s Law of Limiting Factors. Give an example to explain it. Ans:
- Law: If a chemical process is affected by more than one factor, its rate will be determined by the factor which is nearest to its minimal value. This factor directly affects the process if its quantity is changed.
- Example: A plant with a green leaf and optimal light and CO2 may not photosynthesize if the temperature is very low. Photosynthesis will start only if the temperature (the limiting factor) is increased.
Q17: How does Carbon Dioxide concentration act as a limiting factor for C3 and C4 plants? Ans:
- Limiting Factor: CO2 is a major limiting factor because atmospheric concentration (0.03-0.04%) is low.
- Response:
- C4 Plants: Show saturation at about 360 muL–.
- C3 Plants: Respond to higher concentrations and show saturation only beyond 450 360 muL–.
- Conclusion: Current atmospheric levels are limiting mainly for C3 plants, which is why they benefit from CO2 enriched greenhouse conditions.
Q18: Summarize the two stages of photosynthesis (Light and Carbon Fixing reactions). Ans:
- Light Reaction: Occurs in the membrane system (grana). Light energy is absorbed by pigments to generate high-energy intermediates ATP and NADPH. O2 is evolved from water splitting.
- Carbon Fixing (Dark) Reaction: Occurs in the stroma. CO2 is added to RuBP (by RuBisCO) and converted into sugars (glucose/starch) via the Calvin cycle, utilizing the ATP and NADPH produced in the light reaction.
Q19. Why are green plants called autotrophs? (3 marks)
Answer:
Green plants are called autotrophs because they synthesise their own food by the process of photosynthesis. They use carbon dioxide, water, and light energy in the presence of chlorophyll to produce carbohydrates. Thus, they do not depend on other organisms for their nutrition.
Q20. State the role of chlorophyll in photosynthesis. (3 marks)
Answer:
Chlorophyll is the green pigment present in chloroplasts. It absorbs light energy mainly from blue and red regions of the visible spectrum. The absorbed light energy is converted into chemical energy during the light reactions of photosynthesis.
Q21. What conclusions were drawn from the experiment using KOH and a leaf? (3 marks)
Answer:
The part of the leaf exposed to air tested positive for starch, while the part enclosed with KOH did not. KOH absorbed carbon dioxide inside the test tube. This showed that carbon dioxide is essential for photosynthesis.
Q22. State two observations and one conclusion from Ingenhousz’s experiment. (3 marks)
Answer:
In bright sunlight, bubbles were released from green parts of aquatic plants, while no bubbles were observed in darkness. The bubbles were identified as oxygen. This proved that light and green parts of plants are necessary for oxygen evolution during photosynthesis.
Q23. What is meant by action spectrum of photosynthesis? (3 marks)
Answer:
The action spectrum of photosynthesis shows the rate of photosynthesis at different wavelengths of light. It indicates that photosynthesis is maximum in blue and red regions of the spectrum. It closely resembles the absorption spectrum of chlorophyll.
Q24. Define light reaction and mention two of its products. (3 marks)
Answer:
Light reaction is the photochemical phase of photosynthesis that occurs in the thylakoid membranes. It involves absorption of light energy and splitting of water. The main products are ATP, NADPH, and oxygen.
Q25. What is photolysis of water? Where does it occur? (3 marks)
Answer:
Photolysis of water is the splitting of water molecules into protons, electrons, and oxygen using light energy. It occurs in Photosystem II. Oxygen is released as a by-product during this process.
Q26. Give two differences between cyclic and non-cyclic photophosphorylation. (3 marks)
Answer:
In cyclic photophosphorylation, only Photosystem I is involved and only ATP is formed. In non-cyclic photophosphorylation, both Photosystem I and II are involved and ATP, NADPH, and oxygen are produced.
Q27. Name the primary CO₂ acceptor in the Calvin cycle and the first stable product formed. (3 marks)
Answer:
The primary CO₂ acceptor in the Calvin cycle is ribulose-1,5-bisphosphate (RuBP). The first stable product formed is 3-phosphoglyceric acid (PGA), which is a three-carbon compound.
Q28. Why is RuBisCO called a dual-function enzyme? (3 marks)
Answer:
RuBisCO is called a dual-function enzyme because it can act both as a carboxylase and an oxygenase. It binds carbon dioxide during photosynthesis and oxygen during photorespiration. Its activity depends on the relative concentration of CO₂ and O₂.
Q29. What is Kranz anatomy? Mention one significance. (3 marks)
Answer:
Kranz anatomy is a special leaf anatomy found in C₄ plants where bundle sheath cells surround the vascular bundles. It helps in concentrating carbon dioxide around RuBisCO, thereby reducing photorespiration.
Q30. State Blackman’s law of limiting factors. (3 marks)
Answer:
Blackman’s law states that when a physiological process is affected by several factors, the rate of the process is controlled by the factor that is present in the least amount. Only the limiting factor directly affects the rate of the process.
Five-mark Questions and Answers:-
Q1: Describe the early experiments that led to the understanding of the essential requirements for photosynthesis.
Ans: Several simple experiments established the requirements for photosynthesis:
- Variegated Leaf Experiment: Experiments using a variegated leaf (or a leaf partially covered with black paper) exposed to light showed that starch formation occurs only in the green parts of the leaves in the presence of light.
- Half-Leaf Experiment: A part of a leaf was enclosed in a test tube containing KOH soaked cotton (which absorbs CO2), while the other half was exposed to air. When exposed to light, the exposed part tested positive for starch, while the enclosed part tested negative. This proved that Carbon Dioxide (CO2) is required for photosynthesis,.
- Priestley’s Bell Jar Experiment (1770): Joseph Priestley observed that a burning candle or a mouse in a closed bell jar would extinguish or suffocate. However, placing a mint plant in the jar allowed the candle to burn and the mouse to live. He hypothesized that plants restore the air damaged by breathing animals and burning candles,.
- Ingenhousz’s Experiment: Jan Ingenhousz showed that sunlight is essential for this purification process. Using an aquatic plant, he showed that small bubbles (identified as oxygen) formed around green parts in bright sunlight, but not in the dark,.
- Von Sachs’ Contribution (1854): He provided evidence that plants produce glucose when they grow, which is usually stored as starch within chlorophyll-containing structures (chloroplasts).
Q2: Explain the “Z Scheme” of electron transport during the light reaction. Include the role of Photosystems and water splitting.
Ans: The “Z Scheme” describes the flow of electrons during the light reaction:
- Excitation of PS II: The reaction centre of Photosystem II (PS II), called P680, absorbs red light at 680 nm. This excites electrons, causing them to jump to an orbit farther from the atomic nucleus.
- Electron Transport: These electrons are picked up by an electron acceptor and passed downhill (in terms of redox potential) to an electron transport system consisting of cytochromes,.
- Excitation of PS I: The electrons are passed to pigments of Photosystem I (PS I). Simultaneously, the reaction centre of PS I (P700) is excited by red light at 700 nm and transfers electrons to another acceptor with greater redox potential.
- Reduction of NADP+: These electrons move downhill again to a molecule of $NADP^+$. The addition of electrons reduces NADP+ to NADPH + H+.
- Splitting of Water: To ensure a continuous supply of electrons, water is split associated with PS II. Water splits into 2H+, [O], and electrons. This releases oxygen as a net product and provides electrons to replace those removed from PS I,.
- Equation: 2H2O → 4H+ + O2 + 4e–.
Q3: Describe the Chemiosmotic Hypothesis for ATP synthesis in chloroplasts. What are the requisites for this process?
Ans: Mechanism:
- Proton Gradient: ATP synthesis is linked to the development of a proton gradient across the thylakoid membrane. Protons accumulate inside the lumen (inner side),.
- Causes of Gradient:
- Splitting of water on the inner side releases protons into the lumen.
- As electrons move through photosystems, protons are transported from the stroma across the membrane.
- NADP reductase enzyme on the stroma side removes protons from the stroma to reduce NADP+.
- Result: High proton concentration in the lumen and low in the stroma (decrease in pH in lumen).
- ATP Synthesis: The breakdown of this gradient provides energy. Protons move back to the stroma through the transmembrane channel of the CF0 of ATP synthase. This causes a conformational change in the CF1 particle, catalyzing the synthesis of ATP,.
Requisites: Chemiosmosis requires a membrane, a proton pump, a proton gradient, and ATP synthase.
Q4: Explain the Calvin Cycle (C3 Pathway). Describe its three main stages. Ans: The Calvin Cycle occurs in the stroma and is the biosynthetic phase where sugar is synthesized. It consists of three stages:
- Carboxylation: This is the fixation of CO2 into a stable organic intermediate. CO2 combines with a 5-carbon acceptor, Ribulose bisphosphate (RuBP), catalyzed by the enzyme RuBisCO. This results in the formation of two molecules of 3-PGA (3-phosphoglyceric acid).
- Reduction: This series of reactions leads to the formation of glucose. It involves the utilization of 2 molecules of ATP (for phosphorylation) and 2 molecules of NADPH (for reduction) for every CO2 molecule fixed. To form one molecule of glucose, six turns of the cycle are required (fixing 6 CO2).
- Regeneration: For the cycle to continue uninterrupted, the CO2 acceptor RuBP must be regenerated. This step requires 1 ATP for phosphorylation to form RuBP.
Net Input/Output for 1 Glucose: 6 CO2, 18 ATP, and 12 NADPH are used to produce 1 Glucose.
Q5: What are C4 plants? Describe the Hatch and Slack Pathway found in these plants.
Ans: C4 Plants: These are plants adapted to dry tropical regions (e.g., Maize, Sorghum). They have a special “Kranz anatomy” where bundle sheath cells surround vascular bundles. Hatch and Slack Pathway:
- Initial Fixation (Mesophyll): The primary CO2 acceptor is PEP (phosphoenol pyruvate), a 3-carbon molecule present in mesophyll cells. The fixation is catalyzed by PEPcase. Mesophyll cells lack RuBisCO. This forms a 4-carbon acid, OAA (Oxaloacetic acid).
- Transport: OAA is converted to other 4-carbon acids (like malic acid) and transported to bundle sheath cells.
- Decarboxylation (Bundle Sheath): In the bundle sheath cells, the C4 acids serve to release CO2 and a 3-carbon molecule.
- Calvin Cycle: The released CO2 enters the Calvin pathway (common to all plants) in the bundle sheath cells, which are rich in RuBisCO.
- Regeneration: The 3-carbon molecule is transported back to the mesophyll to regenerate PEP, completing the cycle.
Q6: Compare C3 and C4 plants on the basis of leaf anatomy, primary CO2 acceptor, and photorespiration.
Ans:
| Characteristic | C3 Plants | C4 Plants |
| 1. Leaf Anatomy | Standard anatomy. Mesophyll cells perform photosynthesis. | Kranz Anatomy. Bundle sheath cells form layers around vascular bundles; photosynthesis is divided between mesophyll and bundle sheath. |
| 2. Primary CO2 Acceptor | RuBP (Ribulose bisphosphate), a 5-carbon compound. | PEP (Phosphoenol pyruvate), a 3-carbon compound. |
| 3. Primary Fixation Product | PGA (3-carbon acid). | OAA (4-carbon acid). |
| 4. Site of Calvin Cycle | Mesophyll cells. | Bundle Sheath cells. |
| 5. Photorespiration | Occurs. High O2 leads to binding with RuBisCO, releasing CO2 and wasting ATP. | Does not occur. C4 plants concentrate CO2 at the enzyme site, minimizing oxygenase activity. |
Q7: Explain the effect of Light and Carbon Dioxide concentration on the rate of photosynthesis. Ans: 1. Light:
- Low Intensity: There is a linear relationship between incident light and CO2 fixation rates.
- Saturation: Light saturation occurs at 10% of full sunlight. Beyond this point, the rate does not increase as other factors become limiting.
- High Intensity: Increase beyond a point causes the breakdown of chlorophyll and a decrease in photosynthesis.
2. Carbon Dioxide (CO2):
- It is the major limiting factor in nature because atmospheric concentration is very low (0.03-0.04%).
- Response: Increase up to 0.05% can increase fixation rates.
- C3 vs C4: C4 plants saturate at about 360 muL–, while C3 plants show saturation only beyond 450 muL. Thus, current levels are limiting for C3 plants.
- Application: Greenhouse crops (C3 plants like tomatoes) are grown in CO2 enriched atmospheres to obtain higher yields.
Q8. Explain the significance of photosynthesis in sustaining life on Earth. (5 marks)
Answer:
Photosynthesis is the most important biological process that sustains life on Earth. Green plants synthesise their own food using light energy, carbon dioxide, and water, thereby forming the base of all food chains. All heterotrophic organisms, including animals and humans, depend directly or indirectly on plants for food. During photosynthesis, oxygen is released into the atmosphere as a by-product, which is essential for aerobic respiration. Photosynthesis also converts solar energy into chemical energy stored in carbohydrates, making sunlight available in a usable form for living organisms. Without photosynthesis, atmospheric oxygen would decline and life on Earth would not be possible.
Q9. Describe Priestley’s bell jar experiment and explain how it contributed to the understanding of photosynthesis. (5 marks)
Answer:
Joseph Priestley performed a series of experiments in 1770 using a bell jar to study the role of air in plant life. He observed that a burning candle placed inside a closed bell jar was extinguished after some time and a mouse placed inside the jar suffocated due to lack of air. However, when a green mint plant was placed along with the mouse or candle inside the bell jar, the mouse survived and the candle continued to burn. From this, Priestley concluded that plants restore something to the air that is removed by burning candles and breathing animals. Although he did not know about oxygen at that time, his experiment demonstrated that green plants play an essential role in maintaining the quality of air, laying the foundation for understanding photosynthesis.
Q10. Explain Engelmann’s experiment and state its importance in photosynthesis research. (5 marks)
Answer:
T.W. Engelmann conducted an experiment using a filamentous green alga, Cladophora, and aerobic bacteria. He split white light into its different wavelengths using a prism and allowed the spectrum to fall on the alga. Aerobic bacteria were added to detect regions of oxygen evolution. Engelmann observed that bacteria accumulated mainly in the blue and red regions of the spectrum. This indicated maximum oxygen production in these regions. The experiment provided the first action spectrum of photosynthesis and showed that blue and red light are most effective for photosynthesis. It also established a close relationship between the absorption spectrum of chlorophyll and the rate of photosynthesis.
Q11. Describe the structure of chloroplast and explain how it is suited for photosynthesis. (5 marks)
Answer:
Chloroplasts are double-membrane bound organelles found in the mesophyll cells of leaves. Inside the chloroplast is a membranous system consisting of flattened sacs called thylakoids, which are arranged into stacks known as grana. The grana are connected by stroma lamellae. The inner fluid-filled space is called the stroma. Light reactions occur on the thylakoid membranes where pigments are present, leading to the synthesis of ATP and NADPH. The stroma contains enzymes required for the Calvin cycle, where carbon dioxide is fixed into carbohydrates. This structural division allows efficient trapping of light energy and synthesis of sugars, making the chloroplast ideally suited for photosynthesis.
Q12. Explain the role of different photosynthetic pigments in plants. (5 marks)
Answer:
Photosynthetic pigments are substances that absorb light energy at specific wavelengths. In green plants, photosynthesis involves four major pigments: chlorophyll a, chlorophyll b, carotenoids, and xanthophylls. Chlorophyll a is the chief pigment directly involved in the conversion of light energy into chemical energy. Accessory pigments such as chlorophyll b and carotenoids absorb light at wavelengths not absorbed by chlorophyll a and transfer the energy to it. These pigments increase the range of light that can be used for photosynthesis. Additionally, carotenoids protect chlorophyll from photo-oxidation by dissipating excess light energy.
Q13. Describe the organisation of pigments into photosystems in the thylakoid membrane. (5 marks)
Answer:
Photosynthetic pigments are organised into two functional units called Photosystem I (PS I) and Photosystem II (PS II), which are embedded in the thylakoid membrane. Each photosystem consists of a light-harvesting complex containing hundreds of pigment molecules attached to proteins. These pigments absorb light energy and transfer it to a single chlorophyll a molecule known as the reaction centre. In PS I, the reaction centre chlorophyll absorbs light at 700 nm and is called P700, while in PS II it absorbs light at 680 nm and is called P680. This organisation ensures efficient absorption and transfer of light energy during photosynthesis.
Q14. Explain the Z-scheme of electron transport during light reactions. (5 marks)
Answer:
The Z-scheme represents the pathway of electron flow during the light reactions of photosynthesis. In Photosystem II, the reaction centre chlorophyll P680 absorbs light energy, causing electrons to become excited and transferred to an electron acceptor. These electrons move downhill through an electron transport chain and reach Photosystem I. In Photosystem I, electrons are again excited by light absorbed by P700 and transferred to another acceptor with a higher redox potential. Finally, the electrons reduce NADP⁺ to NADPH. The overall pattern of energy changes during electron transfer resembles the letter ‘Z’ when plotted on a redox potential scale.
Q15. Compare cyclic and non-cyclic photophosphorylation. (5 marks)
Answer:
In non-cyclic photophosphorylation, both Photosystem I and Photosystem II are involved, and electrons flow from water to NADP⁺. This process results in the formation of ATP, NADPH, and oxygen. Water splitting occurs in PS II to replace lost electrons. In cyclic photophosphorylation, only Photosystem I operates, and the excited electrons return back to PS I through the electron transport chain. This process produces only ATP and does not involve the formation of NADPH or oxygen. Cyclic photophosphorylation helps meet the extra ATP requirement of the Calvin cycle.
Q16. Explain the chemiosmotic hypothesis for ATP synthesis in chloroplasts. (5 marks)
The chemiosmotic hypothesis explains ATP synthesis based on the development of a proton gradient across the thylakoid membrane. During light reactions, splitting of water releases protons into the thylakoid lumen. As electrons pass through the electron transport chain, additional protons are pumped from the stroma into the lumen. This creates a high concentration of protons inside the lumen and a low concentration in the stroma. Protons move back to the stroma through ATP synthase, and the energy released during this movement is used to synthesise ATP from ADP and inorganic phosphate.
Q17. Describe the structure and function of ATP synthase enzyme. (5 marks)
Answer:
ATP synthase is a multi-subunit enzyme located in the thylakoid membrane of chloroplasts. It consists of two main components: CF₀ and CF₁. The CF₀ part is embedded in the membrane and forms a channel through which protons pass from the lumen to the stroma. The CF₁ portion projects into the stroma and contains the catalytic sites for ATP synthesis. The flow of protons through CF₀ causes a conformational change in CF₁, leading to the formation of ATP. Thus, ATP synthase converts proton gradient energy into chemical energy.
Q18. Describe the Calvin cycle and explain its three major phases. (5 marks)
Answer:
The Calvin cycle is the biosynthetic pathway of photosynthesis that occurs in the stroma of chloroplasts. It involves three major phases: carboxylation, reduction, and regeneration. In the carboxylation phase, carbon dioxide combines with ribulose-1,5-bisphosphate (RuBP) in the presence of the enzyme RuBisCO to form two molecules of 3-phosphoglyceric acid (PGA). During the reduction phase, PGA is reduced to triose phosphate using ATP and NADPH produced during light reactions. In the regeneration phase, RuBP is regenerated using ATP so that the cycle can continue. Six turns of the cycle are required to produce one molecule of glucose.
Q19. Explain the C₄ pathway with reference to Kranz anatomy. (5 marks)
Answer:
The C₄ pathway occurs in plants adapted to hot and dry environments and is associated with Kranz anatomy. In these plants, the leaves have two types of photosynthetic cells: mesophyll cells and bundle sheath cells. Carbon dioxide is initially fixed in the mesophyll cells by the enzyme PEP carboxylase to form a four-carbon compound, oxaloacetic acid. This compound is converted into malate or aspartate and transported to the bundle sheath cells. There, carbon dioxide is released and enters the Calvin cycle. This mechanism increases the concentration of CO₂ around RuBisCO and reduces photorespiration.
Q20. Explain photorespiration and state why it does not occur in C₄ plants. (5 marks)
Answer:
Photorespiration is a process in which the enzyme RuBisCO binds oxygen instead of carbon dioxide, leading to the formation of phosphoglycolate. This pathway results in the release of carbon dioxide and consumption of ATP without producing sugars, thereby reducing photosynthetic efficiency. Photorespiration occurs in C₃ plants, especially under high oxygen and low carbon dioxide conditions. In C₄ plants, photorespiration is absent because carbon dioxide concentration is increased in bundle sheath cells. This ensures that RuBisCO functions mainly as a carboxylase, preventing oxygenation.
Q21. Discuss the factors affecting the rate of photosynthesis. (5 marks)
Answer:
The rate of photosynthesis is influenced by both internal and external factors. Internal factors include the number and size of leaves, chlorophyll content, and internal carbon dioxide concentration. External factors include light intensity, carbon dioxide concentration, temperature, and water availability. According to Blackman’s law of limiting factors, when multiple factors affect photosynthesis, the rate is determined by the factor present in the least amount. For example, even if light and CO₂ are sufficient, low temperature can limit photosynthesis. Thus, the rate depends on the most limiting factor at any given time.
NCERT TEXTBOOK SOLVED QUESTIONS WITH ANSWERS
1. By looking at a plant externally, can you tell whether a plant is C₃ or C₄? Why and how?
It is not possible to distinguish between C₃ and C₄ plants by looking at their external morphology alone. Both types of plants may appear similar externally, as differences between them are mainly anatomical and biochemical. However, most C₄ plants are found in tropical and subtropical regions and are adapted to high temperature and intense light, whereas many C₃ plants grow in temperate regions. This ecological difference is only indicative and not a reliable method of identification.
2. By looking at which internal structure of a plant can you tell whether a plant is C₃ or C₄? Explain.
C₃ and C₄ plants can be distinguished by studying the internal structure of their leaves. C₄ plants show a characteristic Kranz anatomy. In this anatomy, vascular bundles are surrounded by a ring of large, thick-walled bundle sheath cells arranged in a wreath-like manner. The mesophyll cells are arranged concentrically around the bundle sheath cells and are connected to them by plasmodesmata. Mesophyll cells contain chloroplasts with well-developed grana and carry out the initial fixation of CO₂ using the enzyme PEP carboxylase. Bundle sheath cells contain large agranal chloroplasts where the Calvin cycle occurs. In contrast, C₃ plants lack Kranz anatomy and perform the Calvin cycle in mesophyll cells.
3. Even though very few cells in a C₄ plant carry out the biosynthetic (Calvin) pathway, yet they are highly productive. Explain why.
C₄ plants are highly productive because they possess an efficient mechanism to concentrate CO₂ around the enzyme RuBisCO. Initial fixation of CO₂ occurs in the mesophyll cells by PEP carboxylase, forming a four-carbon compound that is transported to the bundle sheath cells. This increases the concentration of CO₂ in bundle sheath cells and prevents photorespiration. As a result, RuBisCO functions efficiently as a carboxylase. C₄ plants can photosynthesise even at low CO₂ concentration and partially closed stomata, which reduces water loss. Absence of photorespiration and efficient utilisation of light energy make C₄ plants more productive than C₃ plants, especially in hot and dry environments.
4. RuBisCO is an enzyme that acts both as a carboxylase and an oxygenase. Why does RuBisCO carry out more carboxylation in C₄ plants?
In C₄ plants, RuBisCO carries out more carboxylation because of the high concentration of CO₂ in bundle sheath cells. CO₂ is initially fixed in mesophyll cells by PEP carboxylase to form oxaloacetic acid, which is converted into malate or aspartate. These compounds are transported to bundle sheath cells, where they are decarboxylated to release CO₂. This creates a high CO₂ and low O₂ environment around RuBisCO, favouring its carboxylase activity and suppressing oxygenase activity. As a result, photorespiration is minimised and carboxylation predominates.
5. Suppose there were plants that had a high concentration of chlorophyll b but lacked chlorophyll a. Would they carry out photosynthesis? Why do plants have chlorophyll b and other accessory pigments?
Plants lacking chlorophyll a would not be able to carry out photosynthesis because chlorophyll a is the primary pigment and forms the reaction centre of photosystems. It directly converts light energy into chemical energy. Chlorophyll b and other accessory pigments cannot perform this primary role. However, plants possess chlorophyll b and other accessory pigments because they absorb light of different wavelengths and transfer the absorbed energy to chlorophyll a. This broadens the spectrum of light available for photosynthesis and also protects chlorophyll a from photo-oxidation.
6. Give comparison between the following:
(a) Comparison between C₃ and C₄ pathways
| Feature | C₃ Pathway | C₄ Pathway |
| First stable product of CO₂ fixation | 3-phosphoglyceric acid (PGA) | Oxaloacetic acid (OAA) |
| Number of carbon atoms in first product | Three | Four |
| Primary CO₂ acceptor | RuBP | Phosphoenol pyruvate (PEP) |
| Enzyme for initial CO₂ fixation | RuBisCO | PEP carboxylase |
| Site of Calvin cycle | Mesophyll cells | Bundle sheath cells |
| Photorespiration | Present | Absent |
| Efficiency of photosynthesis | Lower | Higher |
| Adaptation | Moderate climate | Hot and dry climate |
(b) Comparison between cyclic and non-cyclic photophosphorylation
| Feature | Cyclic photophosphorylation | Non-cyclic photophosphorylation |
| Photosystems involved | Only Photosystem I | Photosystem I and II |
| Electron flow | Cyclic (returns to PS I) | Non-cyclic (from water to NADP⁺) |
| Photolysis of water | Absent | Present |
| Oxygen evolution | Absent | Present |
| Products formed | ATP only | ATP, NADPH, and O₂ |
| Final electron acceptor | Same photosystem | NADP⁺ |
(c) Comparison between leaf anatomy of C₃ and C₄ plants
| Feature | C₃ plants | C₄ plants |
| Leaf anatomy | Normal anatomy | Kranz anatomy |
| Bundle sheath cells | Inconspicuous | Large and prominent |
| Mesophyll cells | Loosely arranged | Arranged around bundle sheath |
| Chloroplasts | Only in mesophyll | In mesophyll and bundle sheath |
| Grana in chloroplasts | Present | Absent in bundle sheath |
| Site of Calvin cycle | Mesophyll cells | Bundle sheath cells |
| Photorespiration | Present | Absent |
7. Look at leaves of the same plant on the shady side and the sunny side. Which are darker green and why?
Leaves growing on the shady side are darker green than those growing in sunlight. This is because shaded leaves contain a higher concentration of chlorophyll to absorb maximum available light. In high light intensity, chloroplasts align themselves along the lateral walls of mesophyll cells to avoid excess light, whereas in moderate or low light they align along the tangential walls to capture more light. This results in darker green colour in shaded leaves.
8. The given figure shows the effect of light on the rate of photosynthesis. Answer the following:
(a) At which points is light the limiting factor?
Light is the limiting factor in regions A and B, where an increase in light intensity increases the rate of photosynthesis.
(b) What could be the limiting factor in region A?
In region A, light intensity is very low and acts as the primary limiting factor for photosynthesis.
(c) What do regions C and D represent on the curve?
Region C represents light saturation where further increase in light does not increase photosynthesis.
Point D represents the stage where other factors such as CO₂ concentration or temperature become limiting.
9. Why does the colour of a leaf kept in the dark frequently become yellow or pale green? Which pigment is more stable?
When a leaf is kept in darkness, chlorophyll breaks down due to lack of light, while carotenoid pigments do not degrade easily. As a result, the yellow or pale green colour of carotenoids becomes visible due to unmasking. Therefore, carotenoids are more stable than chlorophyll.
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