Question

Explain the mass flow hypothesis of transport in phloem.

Answer 1

Mass flow hypothesis

The Mass flow hypothesis is also known as the pressure flow hypothesis. It describes the movement of sap through the phloem, proposed by the German physiologist Ernst Munch in 1930

The following steps take place:-

*Active transport of sucrose from source to companion cells

*Phloem loading

*Movement of water from xylem to sieve tubes

*Transport in sieve tube

* Phloem unloading


Source to companion cells

Some amount of sucrose from the leaf (source) enters the companion cells through the plasmodesmata across the concentration gradient.




Phloem loading

The transport of sucrose from the companion cells to the sieve tube is called phloem loading. This happens through active transport as the sucrose moves against the concentration gradient.



Movement of water from xylem

As the water potential of the sieve tube has decreased compared to the water potential in the xylem, water from the xylem enters the sieve tube through osmosis


Transport in the sieve tube- pressure flow

The turgor pressure in the sieve tube of phloem increases due to the presence of water. As a result pressure flow begins, and the sap moves through the phloem sieve plates towards the sink.


Phloem unloading

The sucrose from the phloem sieve tube, through the process of active transport enters the sink ( storage organs like roots or actively metabolising organs like fruits). This is called phloem unloading.


As the sucrose has been unloaded the water potential of the sieve tube has now increased compared to the nearby xylem vessel. The water from the sieve tube now enters the xylem vessel.