Q.34 Principal form of carbohydrate that is translocated through phloem is
- Glucose
- Sucrose
- Fructose
- Starch
Sucrose is the principal form of carbohydrate translocated through phloem (Option B).
Phloem sap primarily transports sucrose, a non-reducing disaccharide, from photosynthetic source tissues to sink organs via pressure-flow mechanism, as it’s osmotically active and chemically stable for long-distance travel.
Option Breakdown
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Glucose: Monosaccharide produced in photosynthesis; quickly converted to sucrose in mesophyll cells—too reactive (forms Schiff bases) and osmotically disruptive for phloem loading.
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Sucrose: Correct. Main photoassimilate (5-15% of sap); actively loaded via SUT transporters into sieve tubes, driving turgor pressure up to 2 MPa for bulk flow.
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Fructose: Monosaccharide component of sucrose; not translocated independently, as plants prioritize disaccharide form to minimize osmotic stress.
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Starch: Insoluble polysaccharide storage form in chloroplasts/amyloplasts; hydrolyzed to sucrose for export—too large/non-mobile for phloem sap.
The principal form of carbohydrate that is translocated through phloem is sucrose, enabling efficient distribution of photosynthetic products across the plant. This disaccharide dominates sap (up to 1 M concentration), powering growth in sinks like roots and fruits.
Phloem Transport Mechanism
Sucrose exits mesophyll via SWEET efflux transporters, enters apoplast, then loads into companion cell-sieve tube complexes via proton-sucrose symporters (e.g., SUC2). Osmotic water influx creates pressure gradients for mass flow (1 m/hour).
Carbohydrate Role Comparison
Carbohydrate Translocated in Phloem? Reason/Role Glucose No Converted to sucrose Sucrose Yes (principal) Stable transport form Fructose No Sucrose component Starch No Storage, not mobile Essential for plant physiology exam prep on translocation.
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