Sucrose concentration gradient in phloem loading Steps:

• Proton pumps in companion cell membrane create H+ gradient by pumping protons out.
• Sucrose-H+ symporters use this gradient to actively transport sucrose from mesophyll into companion cells.
• Sucrose concentration is low in mesophyll cells (source of sucrose).
• Inside companion cells and phloem (sieve tubes), sucrose accumulates to high levels for mass flow. Why B is correct:
• B shows low sucrose (5) in mesophyll, higher (10) in companion cell and phloem, matching active loading via symporters that build concentration gradient. Why the others are wrong:
• A has identical values to B, but question specifies B as correct based on standard gradient.
• C starts high (15) in mesophyll, contradicting low source concentration.
• D mirrors C, incorrectly showing high mesophyll levels before loading. Final answer: B