Mass flow in the phloem is explained by the pressure-flow hypothesis. What is the first step in loading at the source?
Answer and explanation
Correct answer: C. Sucrose is actively loaded into the sieve tubes
- Step 1: Active loading lowers water potential in the sieve tube. Step 2. Water enters by osmosis, raising hydrostatic pressure, which drives mass flow to the sink. Answer: Sucrose is actively loaded into the sieve tubes. Why not the others: water moves by osmosis not by pumps; sucrose moves into the sieve tube, not out at the source; starch hydrolysis occurs in sinks.
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