Active loading of sucrose into sieve tube elements

DONE

1. Sucrose is

Actively loaded into companion cell via a transport protein

1a H+ moved

Moved across a cell surface membrane via proton pump, resulting in excess H+ in cell wall

1b H+ moves

Passively down concentration gradient back into cell through transport protein

1c Sucrose molecules

Carried against their concentration gradient through co-transporter with H+

2. Sucrose molecules then

Diffuse into sieve tube elements through plasmodesmata

3. This lowers

The water potential in sieve tube element, so water moves in from xylem, beginning the mass flow process