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The **pressure-flow hypothesis** (Münch, for phloem transport) proposes that:
ASucrose is actively pumped up by xylem
BLoading of sugar at source raises osmotic concentration → water enters → high turgor → mass flow toward sink, where unloading reverses the gradient
CSugars move by simple gas-phase diffusion
DPhloem actively contracts to pump
Answer & Solution
Correct answer: B. Loading of sugar at source raises osmotic concentration → water enters → high turgor → mass flow toward sink, where unloading reverses the gradient
Münch hypothesis (1930): source cells actively load sucrose into sieve tubes → water follows osmotically → high turgor at source → low turgor at sink (where sugar is unloaded) → bulk flow of phloem sap from source to sink driven by pressure gradient.
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