Phloem Loading Sequence for Mass Flow

Steps:

• Protons (H+) are actively pumped out of the companion cell (5), establishing an electrochemical gradient.
• Sucrose and H+ enter the companion cell via a co-transporter protein (2 then 1), loading sugars.
• Loaded sucrose moves into the sieve tube element, decreasing its water potential (4).
• Water enters the sieve tube by osmosis due to low water potential, producing high hydrostatic pressure (3) that drives mass flow.

Why D is correct:

• D (5→2→1→4→3) follows the pressure-flow hypothesis, where proton pumping creates a gradient for symport loading, solute accumulation lowers water potential, and osmosis builds turgor pressure for flow.

Why the others are wrong:

• A starts with sucrose entry (1→2) before proton pumping (5), violating gradient establishment.
• B begins with pressure production (3), which requires prior loading and osmosis.
• C opens with pressure (3) then pumps protons (5), reversing cause and effect.

Final answer: D