A student listed following combinations of enzymes and their involvement in different phases of Calvin‐Benson cycle: A. Phosphoglycerate kinase – Reduction phase B. Glyceraldehyde‐3‐phosphate dehydrogenase – Regeneration phase C. Triose‐phosphate isomerase – Reduction phase D. Phospho-ribulokinase – Regeneration phase Which one of the following combinations is correct?
A and D only
The Calvin-Benson cycle, also known as the C3 cycle, is the metabolic pathway that takes place in the stroma of chloroplasts in photosynthetic organisms. It converts carbon dioxide into glucose. The cycle consists of three main phases: Carbon Fixation, Reduction, and Regeneration.
Calvin Cycle Phases and Enzymes
Let's examine the role of each enzyme listed and determine which phase of the Calvin-Benson cycle it is involved in:
- Phosphoglycerate kinase: This enzyme catalyzes the transfer of a phosphate group from ATP to 3-phosphoglycerate (3-PGA), forming 1,3-bisphosphoglycerate (1,3-BPGA). This step consumes ATP and is considered part of the Reduction phase because it prepares 3-PGA for reduction by glyceraldehyde-3-phosphate dehydrogenase. Thus, statement A is correct.
- Glyceraldehyde-3-phosphate dehydrogenase: This enzyme catalyzes the reduction of 1,3-bisphosphoglycerate (1,3-BPGA) to glyceraldehyde-3-phosphate (G3P), using NADPH and releasing an inorganic phosphate. This is the core reaction of the Reduction phase. The statement claims it's involved in the Regeneration phase, which is incorrect. Thus, statement B is incorrect.
- Triose-phosphate isomerase: This enzyme catalyzes the interconversion of glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). These two molecules are triose phosphates produced during the Reduction phase. While this interconversion is essential for the flow of carbon into the Regeneration phase (as DHAP is needed for subsequent steps) and for the export of sugars, the enzyme acts on the products of reduction. Assigning it strictly to the Reduction phase might be debated, but it's not typically listed as a primary enzyme *of* the Reduction phase itself or the core Regeneration phase enzymes involved in rearranging molecules back to RuBP. The statement places it in the Reduction phase. Compared to other enzymes, its role is more about interconverting the products of reduction and preparing them for subsequent steps (including regeneration). Given the correct option, it's likely considered incorrectly placed in the Reduction phase in this context. Thus, statement C is incorrect.
- Phospho-ribulokinase (Ribulose-5-phosphate kinase): This enzyme catalyzes the phosphorylation of ribulose-5-phosphate (Ru5P) using ATP to form ribulose-1,5-bisphosphate (RuBP). This is the final step in the Regeneration phase, regenerating the CO2 acceptor (RuBP). Thus, statement D is correct.
Based on this analysis:
- Statement A (Phosphoglycerate kinase – Reduction phase) is correct.
- Statement B (Glyceraldehyde‐3‐phosphate dehydrogenase – Regeneration phase) is incorrect; it is in the Reduction phase.
- Statement C (Triose‐phosphate isomerase – Reduction phase) is incorrect or at least less accurate than placing it after reduction, interconverting products.
- Statement D (Phospho-ribulokinase – Regeneration phase) is correct.
Therefore, the correct combination of statements is A and D only.
Here is a summary table:
| Enzyme | Stated Phase | Actual Phase | Correctness of Statement |
|---|---|---|---|
| Phosphoglycerate kinase | Reduction phase | Reduction phase | Correct (A) |
| Glyceraldehyde-3-phosphate dehydrogenase | Regeneration phase | Reduction phase | Incorrect (B) |
| Triose-phosphate isomerase | Reduction phase | Interconversion of reduction products (important for flow to regeneration) | Incorrect (C) |
| Phospho-ribulokinase | Regeneration phase | Regeneration phase | Correct (D) |
The statements that correctly pair the enzyme with its involvement in the Calvin-Benson cycle phase are A and D.
Important Questions from Photosynthesis
Kranz anatomy is found in _____ plant.
Which plant pigment is present in tomatoes?
Which one of the following reactions takes place during the reduction phase of the Calvin - Benson cycle?
The following statements were made with the assumption that the concentration of 3- phosphoglycerate is high inside chloroplasts of an actively photosynthesizing leaf.
A. There will be high concentration of triose phosphate in the chloroplast.
B. The activity of ADP-glucose pyrophosphorylase will be inhibited.
C. The carbon flow will be diverted from sucrose to starch.
D. Starch synthesis will be inhibited and carbon flow will be more towards sucrose synthesis.
Which one of the following combinations of above statements is correct?
The table below shows photosynthetic type, temperature and sunlight intensity levels.
Photosynthetic Type Temperature Sunlight Intensity A. C3 Plant i High P. High B. C4 plant ii Moderate Q. Moderate iii Low R Low Which of the following correctly matches the plant photosynthetic type with the temperature and sunlight conditions in which photosynthetic rate per unit leaf area is maximum for that plant?
