Phloem

Transfer of Sucrose into the Phloem

Photosynthetic (Source) cells

• Producing sucrose (increasing conc of sugars)

• Diffuse (facilitated diffusion) into companion cells

Companion cells

• Hydrogen ions are actively transported (using ATP) into the cell walls

• H+ ions diffuse (carrier proteins) into the sieve tube elements

Sieve tube elements

• Sucrose is transported with H+ ions through carrier proteins (co-transport proteins)

Mass flow of sucrose through sieve tube element

Sucrose transported into Sieve tube element

• High sucrose concentration, lowers water potential

• Water moves from neighbouring xylem into the phloem

• Increases hydrostatic pressure

The sink cells (respiring/storage)

• Sucrose is used up (respiration or starch production)

• Sucrose is co-transported into the sink cells (down the concentration gradient)

• Lowers the water potential in the sink cells