During active phloem loading, sucrose enters companion cells through a symporter coupled to proton movement. What provides the immediate energy source that drives sucrose uptake against its concentration gradient?

• A. Water potential differences between xylem and phloem
• B. Kinetic energy from bulk flow in sieve tubes
• C. Energy stored in the proton gradient across the membrane
• D. Passive transport of sucrose through membrane channels

Answer: (C)