Loading of Phloem Sieve Tubes at the Source

• Companion cells use ATP to actively transport H+ ions out of their cytoplasm into the surrounding tissue; sets up a proton diffusion gradient (electrochemical gradient)
• So hydrogen ions diffuse back into the companion cells, down the gradient, diffusion occurs through hydrogen ion-sucrose co-transporter protein carriers, in the companion cell plasma membranes
• Sucrose concentration increases in companion cells, then diffuses into the adjacent sieve tube element, via plasmodesmata, down a diffusion gradient