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?
Triose phosphate
Calvin-Benson Cycle Metabolite Transport
The Calvin-Benson cycle is a crucial part of photosynthesis that takes place in the stroma of chloroplasts. It converts carbon dioxide into sugars using energy from the light-dependent reactions. During this cycle, various metabolites are produced and transformed.
Triose Phosphate in the Calvin-Benson Cycle
One of the key metabolites formed during the Calvin-Benson cycle is triose phosphate. Triose phosphate is a 3-carbon sugar phosphate molecule. It is produced when 3-phosphoglycerate (3-PGA) is reduced using ATP and NADPH from the light reactions. Specifically, glyceraldehyde-3 phosphate (G3P) is a type of triose phosphate produced in the cycle.
Dual Role of Triose Phosphate
Triose phosphate plays a vital dual role:
- Starch Biosynthesis: A significant portion of the triose phosphate produced remains within the chloroplast. It is used as the building block for the synthesis of starch, which is stored within the chloroplast as a temporary energy reserve.
- Transport to Cytosol: Triose phosphate can also be exported from the chloroplast to the cytosol. This transport occurs via a specific phosphate translocator protein in the chloroplast envelope membrane. In exchange for triose phosphate moving out, inorganic phosphate (Pi) moves into the chloroplast.
Once in the cytosol, triose phosphate is used to synthesize sucrose, which is the primary sugar transported throughout the plant to provide energy for growth and other metabolic processes.
Why Other Options Are Not Primarily Involved in Both Processes
- Glyceraldehyde-3 phosphate: While G3P is a form of triose phosphate and is involved in the same pathways, the term "Triose phosphate" is a broader category that encompasses G3P and sometimes its isomer, dihydroxyacetone phosphate (DHAP), which are interconverted. Both are transported and used for starch/sucrose synthesis. The question uses the broader term.
- Fructose-6-phosphate: Fructose-6-phosphate is also an intermediate in carbohydrate metabolism, formed from triose phosphates both in the chloroplast (as part of starch synthesis) and the cytosol (as part of sucrose synthesis). However, triose phosphate is the molecule primarily transported out of the chloroplast to initiate cytosolic sucrose synthesis, and it is the direct precursor for building hexose phosphates like fructose-6-phosphate inside the chloroplast for starch synthesis.
- Ribulose 1, 5-bisphosphate: Ribulose 1, 5-bisphosphate (RuBP) is the CO2 acceptor molecule in the Calvin-Benson cycle. It is regenerated within the chloroplast stroma and is not typically transported to the cytosol or directly used for starch biosynthesis without being converted to 3-PGA and then triose phosphate first.
Therefore, triose phosphate is the key metabolite that serves as a direct precursor for starch synthesis within the chloroplast and is also the primary form of carbon exported to the cytosol for sucrose synthesis.
Important Questions from Photosynthesis
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?
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?
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?
