Q.36 The site of sucrose synthesis in plants during the process of photosynthesis is
- Chloroplast
- Cytosol
- Mitochondria
- Golgi apparatus
Cytosol is the site of sucrose synthesis in plants during photosynthesis (Option B).
Triose phosphates (G3P, DHAP) from the Calvin cycle exit chloroplasts via the triose phosphate/Pi antiporter and assemble into sucrose in the cytosol by sucrose phosphate synthase (SPS) and sucrose phosphate phosphatase (SPP). This enables phloem export while starch stays chloroplastic.
Option Breakdown
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Chloroplast: Produces triose phosphates but synthesizes starch (via ADP-glucose), not sucrose; lacks SPS/SPP enzymes—trioses export for cytosolic sucrose formation.
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Cytosol: Correct. Mesophyll cytosol hosts SPS (UDP-glucose + F6P → sucrose-6P) and SPP (dephosphorylation); regulated by light via phosphorylation.
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Mitochondria: Site of respiration/ATP production; no role in sucrose biosynthesis from photosynthetic intermediates.
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Golgi apparatus: Processes glycoproteins/lipids; minor sucrose modification possible but not primary photosynthetic synthesis site.
The site of sucrose synthesis in plants during the process of photosynthesis is the cytosol, where exported triose phosphates from chloroplasts form the transport sugar. Sucrose (principal phloem carbohydrate) arises via SPS/SPP, partitioning carbon between local starch storage and systemic distribution.
Sucrose Biosynthesis Pathway
Chloroplast-exported DHAP/G3P → F6P + G1P → UDP-glucose; SPS condenses to sucrose-6P, SPP yields sucrose irreversibly. Cytosolic Pi recycling via antiporter balances stromal levels for sustained Calvin cycle.
Organelle Role Comparison
Site Sucrose Synthesis? Primary Function in C Metabolism Chloroplast No Triose-P production, starch Cytosol Yes (principal) Sucrose assembly/export Mitochondria No Respiration Golgi apparatus No Protein glycosylation Essential for plant physiology exams on photosynthetic carbon partitioning.
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