Instant Notes: Plant Biology

J4 – Respiration and carbohydrate metabolism 153

H 2 O

H 2 O

H 2 O H 2 O

H 2 O

H 2 O

H 2 O

H 2 O

Xylem Phloem Companioncell Source

cell

Sink cell

Companion cell

Sucrose

Sucrose

Sucrose

TRANSPIRATION STREAM

Fig. 4. Transport in the phloem. Sucrose is actively loaded into the companion cells and

phloem tubules by sucrose/proton co-transporters. This increases the solute concentration

resulting in an influx of water, generating a high turgor pressure at the site of loading. Sucrose

is removed at the sink tissue giving a low turgor pressure.