Question

why does water move out of the phloem to enter the xylem in the sink?

Answer 1

The mechanism of translocation of sugar from source ( site of food synthesis-levaes) to sink (storage sites) is called as phloem loading and unloading.

During phloem loading , sugar molecules synthesized at source actively transported into phloem sieve tube cells by passing through companion cells. This gaining of solutes in phloem sieve tube cells creates hypertonic condition in them that draws water from nearby xylem vessels. Water from xylem enters into phloem sieve tube cells by the process of osmosis.

Similarly, during phloem unloading sugar molecules from phloem sieve tube cells enters into storage organs by active transport. This creates hypotonic condition, raises water potential in phloem sieve tube cells that allows water to move from phloem sieve tube cells into nearby xylem vessels by osmosis. Thus, water returns back from phloem into xylem.