Important Questions Class 11 Biology Chapter 11 Photosynthesis in Higher Plants

Important Questions Class 11 Biology Chapter 11 Photosynthesis in Higher Plants

Photosynthesis in Higher Plants is Chapter 11 of Class 11 Biology. Photosynthesis is the process by which green plants use sunlight, water and carbon dioxide to make food and release oxygen. The chapter covers the light reaction, photophosphorylation, the Calvin cycle, the C4 pathway, photorespiration and the factors that affect photosynthesis.

This page gives 12 important questions from Photosynthesis in Higher Plants with clear answers prepared by Extramarks experts. The questions cover the law of limiting factors by Blackman, the Calvin cycle and its three stages, the C4 pathway, non-cyclic photophosphorylation and the chemiosmotic hypothesis. Students can use them for school tests and CBSE (Central Board of Secondary Education) exam revision, and teachers and parents can use them for quick practice.

Q1. Describe the Blackman’s Law of Limiting Factors.

Answer: When several factors affect any [bio] chemical process, Blackman’s (1905) Law of Limiting Factors comes into effect. This law 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: it is the factor which directly affects the process if its quantity is changed.For example despite optimal light and carbon dioxide conditions the plant may not photosynthesise under low temperatures.

Q2. Where does Calvin cycle take place. Explain the process in detail.

Answer: The Calvin cycle takes place in stroma of the chloroplast. It consists of three stages: carboxylation, reduction and regeneration.
1. Carboxylation – Carboxylation is the fixation of CO₂ into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where CO₂ is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.
2. Reduction – These are a series of reactions that lead to the formation of glucose. The steps involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO₂ molecule fixed. The fixation of six molecules of CO₂ and 6 turns of the cycle are required for the removal of one molecule of glucose from the pathway.
3. Regeneration – Regeneration of the CO₂ acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.

Q3. Specify how C₄ photosynthetic pathway increases CO₂ concentration in bundle sheath cells. How C₄ plants perform better photosynthesis than C₃ plant?

Answer: The primary CO₂ acceptor is a 3-carbon molecule phosphoenol pyruvate (PEP) and is present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase or PEPcase. The mesophyll cells lack RuBisCO enzyme. The C₄ acid OAA is formed in the mesophyll cells. It then forms other 4-carbon compounds like malic acid or aspartic acid in the mesophyll cells itself, which are transported to the bundle sheath cells. In the bundle sheath cells these C4 acids are broken down to release CO₂ and a 3-carbon molecule. This CO₂ concentration increases in bundle sheath cells. The 3-carbon molecule is transported back to the mesophyll where it is converted to PEP again, thus, completing the cycle. The CO₂ released in the bundle sheath cells enters the C₃ or the Calvin pathway, a pathway common to all plants. The bundle sheath cells are rich in an enzyme Ribulose bisphosphate carboxylase-oxygenase (RuBisCO), but lack PEPcase.
The C₄ plants can absorb CO₂ even from a much low CO₂ concentration when the C₃ plants fail to avail it. Thus the C₄ plants can perform a high rate of photosynthesis even when the stomata are nearly closed. They also maintain a high rate of photosynthesis under the condition of water shortage where the C₃ plants would stop photosynthesis.

Q4. What is the law of limiting factors? How would the rate of photosynthesis be affected if the soil water becomes limiting? Explain.

Answer: Law of limiting factor i.e. Blackman’s Law of Limiting Factors 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: it is the

factor which directly affects the process if its quantity is changed.

Rate of photosynthesis will be reduced due to limitation of water, as water causes closing of stomata and drooping (or wilting) of leaves.

Q5. List the three stages of the Calvin cycle of photosynthesis. Describe the chemical step in these stages.

Answer: Calvin cycle takes place in stroma of the chloroplast. It is made up of three stages: carboxylation, reduction and regeneration.
1. Carboxylation – Carboxylation is the fixation of CO₂ into a stable organic intermediate. Carboxylation is the most crucial step of the Calvin cycle where CO₂ is utilised for the carboxylation of RuBP. This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.
2. Reduction – These are a series of reactions that lead to the formation of glucose. The steps involve utilisation of 2 molecules of ATP for phosphorylation and two of NADPH for reduction per CO₂ molecule fixed. The fixation of six molecules of CO₂ and 6 turns of the cycle are required for the removal of one molecule of glucose from the pathway.
3. Regeneration – Regeneration of the CO₂ acceptor molecule RuBP is crucial if the cycle is to continue uninterrupted. The regeneration steps require one ATP for phosphorylation to form RuBP.

Q6. Where does Calvin cycle take place in chloroplast? Explain the cycle.

Answer: Step 1. Location of the Calvin Cycle. The Calvin cycle takes place in the stroma of chloroplasts, the aqueous matrix surrounding the thylakoid membranes.

Step 2. Overview. This cycle is light-independent and uses ATP and NADPH from the light-dependent reactions to fix carbon dioxide into organic molecules.

Step 3. Phase 1: Carboxylation (Carbon Fixation). The enzyme ribulose bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes the fixation of CO₂ to ribulose-1,5-bisphosphate (RuBP), forming two molecules of 3-PGA.

Step 4. Phase 2: Reduction. Each molecule of 3-PGA is phosphorylated by ATP and then reduced by NADPH to produce glyceraldehyde-3-phosphate (G3P).

Step 5. Phase 3: Regeneration of RuBP. Five out of every six molecules of G3P are used, with ATP, to regenerate three molecules of RuBP, enabling the cycle to continue.

Step 6. Net Yield. For every three molecules of CO₂ fixed, the cycle produces one net molecule of G3P, which can be used for glucose and other carbohydrate synthesis.

Step 7. Significance. The Calvin cycle is crucial for carbon assimilation in plants, converting inorganic carbon into organic compounds that fuel growth and metabolism.

Q7. Describe non-cyclic photophosphorylation in plants.

Answer: Step 1: Light reactions occur in thylakoid membranes of chloroplasts, involving Photosystem II (PS II) and Photosystem I (PS I).

Step 2: Photons absorbed by PS II excite electrons in chlorophyll to higher energy levels.

Step 3: Photolysis of H₂O at PS II yields O₂, H⁺, and electrons e^-.

Step 4: Excited electrons travel through plastoquinone, cytochrome b6f complex, and plastocyanin, pumping protons into the thylakoid lumen.

Step 5: Photons absorbed by PS I re-excite electrons, which reduce NADP⁺ to NADPH via ferredoxin and NADP\⁺ reductase.

Step 6: Protons flow back to the stroma through ATP synthase, driving phosphorylation of ADP + P_i to form ATP.

Step 7: Overall reaction: 2 H₂O + 2 NADP⁺ + 3 ADP + 3 P_i → O₂ + 2 NADPH + 3 ATP.

Q8. Explain the process of biosynthetic phase of photosynthesis occurring in the chloroplast.

Answer: 1. Carbon Fixation: Atmospheric CO₂ diffuses into the stroma of the chloroplast and combines with the five-carbon acceptor RuBP (ribulose-1,5-bisphosphate) catalyzed by the enzyme RuBisCO to form two molecules of 3-phosphoglycerate (3-PGA) for each CO₂ fixed.

2. Reduction Phase: Each molecule of 3-PGA is phosphorylated by ATP (from the light reactions) to form 1,3-bisphosphoglycerate, which is then reduced by NADPH to produce glyceraldehyde-3-phosphate (G3P). This step regenerates ADP, Pi, and NADP⁺.

3. Carbohydrate Formation: A fraction of the G3P produced exits the cycle and is used to synthesize hexose sugars, such as C₆H₁₂O₆, starch, and other carbohydrates in the cytosol.

4. Regeneration of RuBP: The remaining G3P molecules are rearranged in a series of enzyme-mediated steps, using additional ATP, to regenerate RuBP so that the cycle can continue.

5. Overall Reaction: For the fixation of three molecules of CO₂, the net stoichiometry of the Calvin cycle is often summarized as:

3CO₂ + 9ATP + 6NADPH → 1 G3P + 9ADP + 8Pi + 6NADP⁺

This biosynthetic phase converts chemical energy (ATP and NADPH) into stable carbohydrate forms, completing the photosynthetic process in the chloroplast.

Q9. Explain the mechanism of C4 photosynthetic carbon cycle. Name any two plants where it occurs. Mention the difference in the structure of chloroplasts in the mesophyll cells and bundle sheath cells of such plants.

Answer:

Q10. Write the correct summarised equation of photosynthesis.

Answer: 6CO₂ + 12H₂O à C₆H₁₂O₆ + 6H₂O + 6O₂

Q11. What is chemiosmotic hypothesis? What is the difference between proton gradient in photosynthesis and respiration?

Answer: According to the chemiosmotic hypothesis, ATP synthesis occurs due to development of proton gradient across the membranes of the thylakoid.
In photosynthesis, accumulation of proton is towards the inside of the membrane whereas during respiration, accumulation of proton is in the intermembrane space of the mitochondria.

Q12. Why is Calvin cycle also known as C3 pathway?

Answer: The first product of carbon dioxide fixation in Calvin cycle is a three carbon-containing compund named 3-Phosphoglyceric acid. Hence it is also called C3 pathway.

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