mass flow hypothesis was first proposed by Ernst munch (1930). it explains the transport of solute or sugars, i.e., sucrose in the phloem tissue.
it involves the following three steps
(a) Phloem loading
(b) Translocation of solutes
(c) Phloem unloading
Loading of sieve tubes takes place in leaves. Photosynthetic cells make sugars, particularly sucrose, and other organic solutes.
Companion cells use energy to collect solutes by active transport. As solute concentration increases in the companion cells, water enters by osmosis.
A pressure is created, which pushes the solutes through plasmodesmata into the sieve
tubes. It is an active process.
Translocation of sucrose takes place through phloem column. Increase in hydrostatic pressure inside sieve tubes is greatest at the source and lowestat the sink. It pushe ssucrose etc., from source to sink.
Unloading of the sieve tubes takes place at the sink sites. It results in active transport of sugars out of the pholem converting them into complex sugars. Loss of solutes from the
phloem create a high water potential, and water moves out of phloem, returning back to
xylem.
Sinks are the regions where solutes, sugars or food are being used, eg., roots, fruits,
storage organs and regions of growth.
