Answer:
Mass flow hypothesis
was first proposed by Ernst Munch (1930). It explains the transportof 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 soluteconcentration increases in the
cell 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 lowest at the sink.
It pushes sucrose 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 phloem 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 i.e., sugars or food are being used, e.g., roots, fruits, storage
organs and regions of growth.
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