why does water move out of the phloem to enter the xylem in the sink?
The mechanism of translocation of sugar from source to sink is called as phloem loading and unloading.Source refers to the site of food synthesis i.e., leaves and sink refers to the storage sites
During the loading in phloem, sugar molecules synthesized at source actively transported into phloem sieve tube cells by passing through companion cells. These received solutes in phloem sieve tube cells create hypertonic condition and water is drawn from nearby xylem vessels. Water from xylem enters into phloem sieve tube cells by the process of osmosis.
During unloading the sugar molecules from phloem sieve tube cells enters into storage organs by active transport creating hypotonic condition, which raises water potential in phloem sieve tube cells. This increase in water potential allows water to move from phloem sieve tube cells into nearby xylem vessels by osmosis. So, water returns back from phloem into xylem.
During the loading in phloem, sugar molecules synthesized at source actively transported into phloem sieve tube cells by passing through companion cells. These received solutes in phloem sieve tube cells create hypertonic condition and water is drawn from nearby xylem vessels. Water from xylem enters into phloem sieve tube cells by the process of osmosis.
During unloading the sugar molecules from phloem sieve tube cells enters into storage organs by active transport creating hypotonic condition, which raises water potential in phloem sieve tube cells. This increase in water potential allows water to move from phloem sieve tube cells into nearby xylem vessels by osmosis. So, water returns back from phloem into xylem.