Question

Describe the mass flow hypothesis for translocation of organic solutes (food) in plants.

Answer 1

The mass flow Hypothesis for translocation of organic solutes (food) in plants.

• Hypothesis was put forward by Munch.
• According to this, mass flow of organic solutes takes place from site of higher concentration (source) to the site of lower concentration (sink)
• As glucose is prepared at the source (by photosynthesis) it is converted to sucrose (a dissacharide).The sugar is then moved in the form of sucrose into the companion cells and then into the living phloem sieve tube cells by active transport.
• This process of loading at the source produces a hypertonic condition in the phloem. Water in the adjacent xylem moves into the phloem by osmosis.
• As osmotic pressure builds up the phloem sap will move to areas of lower pressure.
• At the sink osmotic pressure must be reduced. Again active transport is necessary to move the sucrose out of the phloem sap and into the cells which will use the sugar – converting it into energy, starch, or cellulose.
• As sugars are removed, the osmotic pressure decreases and water moves out of the phloem.
• The movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a sieve tube. Loading of the phloem sets up a water potential  gradient that facilitates the mass movement in the phloem.