Sucrose loading sequence at plant source

Steps:

• Identify key events at source: sucrose synthesis, active loading into phloem, osmotic water influx, pressure buildup for flow.
• Match sequence to source processes: start with synthesis (3), then loading (7), concentration increase (5), water entry (2).
• Eliminate sink-related steps like unloading or diffusion out.
• Verify against pressure-flow hypothesis for phloem transport.

Why A is correct:

• Sequence 3-7-5-2 follows pressure-flow hypothesis, where sucrose synthesis leads to active loading, high solute concentration draws water, building turgor for export.

Why the others are wrong:

• B: Includes 8 (likely sink unloading) and 1 (diffusion), irrelevant to source loading.
• C: Starts with 4 (possibly water entry first), violating osmotic sequence.
• D: Begins with 2 (water entry) and includes 8/6 (sink events), misplaces source initiation.

Final answer: A