Which one of the following reactions takes place during the reduction phase of the Calvin - Benson cycle?
1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate
Calvin Cycle Reduction Phase Explained
The Calvin-Benson cycle is a crucial part of photosynthesis, taking place in the stroma of chloroplasts. It uses the ATP and NADPH produced during the light-dependent reactions to convert carbon dioxide ($\text{CO}_2$) into glucose. The cycle has three main phases:
- Carboxylation (Carbon Fixation)
- Reduction
- Regeneration
Reduction Phase in Detail
The reduction phase is where the energy captured from sunlight (stored in ATP and NADPH) is used to convert the carbon compound from an oxidized state to a more reduced state, eventually forming sugars. The main event in this phase is the conversion of 3-phosphoglycerate (which is formed in the carboxylation phase) into glyceraldehyde-3-phosphate (G3P).
This conversion happens in two main steps, involving ATP and NADPH:
- First, 3-phosphoglycerate is phosphorylated by ATP to form 1,3-bisphosphoglycerate.
- Second, 1,3-bisphosphoglycerate is reduced by NADPH, and also dephosphorylated, to form glyceraldehyde-3-phosphate (G3P). This specific reaction is the core reduction step of the cycle.
The reaction is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase and requires NADPH as the reducing agent and involves the removal of a phosphate group.
Let's look at the options provided:
- Ribulose 1,5-bisphosphate to 3-phosphoglycerate: This is the initial step of the Calvin cycle, where $\text{CO}_2$ is fixed to ribulose 1,5-bisphosphate. This occurs in the Carboxylation phase, catalyzed by RuBisCO.
- 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate: As explained above, this reaction involves the reduction of 1,3-bisphosphoglycerate using NADPH (and ATP for the preceding step) to produce glyceraldehyde-3-phosphate. This is the defining reaction of the Reduction phase.
- Dihydroxyacetone phosphate to fructose 1,6-bisphosphate: Dihydroxyacetone phosphate (DHAP) is an isomer of glyceraldehyde-3-phosphate. This reaction is part of synthesizing sugars (like fructose) from G3P and DHAP and occurs after the reduction phase, sometimes considered part of regeneration or sugar synthesis pathways branching off the cycle. It does not represent the core reduction step within the cycle that uses NADPH.
- Ribulose 5-phosphate to ribulose 1,5-bisphosphate: This reaction is part of the Regeneration phase, where ribulose 5-phosphate is phosphorylated by ATP to regenerate the initial $\text{CO}_2$ acceptor, ribulose 1,5-bisphosphate. This is an energy-consuming step, but not a reduction step in the sense of using NADPH to reduce a carbon compound.
Therefore, the reaction that specifically takes place during the reduction phase of the Calvin-Benson cycle is the conversion of 1,3-bisphosphoglycerate to glyceraldehyde-3-phosphate.
Important Questions from Photosynthesis
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?
Calvin-Benson cycle is divided into three phases, namely carboxylation, reduction and regeneration. The following statements are related to the three phases of Calvin-Benson cycle:
A. The product of light reaction, ATP and NADPH is utilized in the carboxylation phase.
B. Six molecules of 3-phosphoglycerate is converted into six molecules of glyceraldehyde 3-phosphate in the reduction phase.
C. The action of aldolase enzyme for the production of fructose 1, 6-bisphosphate takes place in reduction phase.
D. Formation of seven carbon compound, sedoheptulose-7- phosphate takes place in the regeneration phase.
Which one of the following combinations is correct?
Which one of the following metabolites formed during Calvin-Benson cycle in chloroplast is involved in starch biosynthesis and can also be transported to cytosol?
Following are certain statements regarding C2, C3, C4 and CAM carbon metabolism in plants.
A. In C3 cycle, one molecule of 3-phosphoglycerate formed during carboxylation phase is utilized for the biosynthesis of sugars, fatty acid and amino acids.
B. During C2 cycle glycine is transported from peroxisome to mitochondria and glycerate is transported from peroxisome to chloroplast.
C. The concentration of CO2 in bundle sheath of C4 plants is several fold lower than the external atmosphere.
D. The stomata of CAM plants open at night.
Which one of the following combination of statements is correct?
