Question

How does the osmotically generated pressure-flow mechanism account for the movement of sugars from source to sink?

Answer 1

As glucose is prepared at source by photosynthesis, it is converted to sucrose (a disaccharide).

Sugar then moves in form of sucrose into companion cells and then into sieve tube cells by active transport.

This process of loading at the source produces a hypertonic condition in phloem.

Water from adjacent xylem moves into phloem by osmosis.

As osmotic pressure builds up, the phloem sap will move to areas of lower pressure i.e. sinks.

Again active transport is required to move sucrose out of phloem sap into cells (of sink) which will use sugar as source of energy or convert it into starch or cellulose.