Name: Facilitated Diffusion: Transport in Plants
Uploaded: 2019-07-19
Duration: 18 min 2 s
Description: Facilitated diffusion occurs when important molecules pass through the membranes via special holes called channels. Watch the video to learn more.

Question

Write about the mass flow hypothesis.

Accepted Answer

Hartig (1860) postulated the Mass or Pressure flow hypothesis. Munch (1930) formulated this hypothesis as a scientific theory. This hypothesis is the most accepted mechanism used for explaining the translocation of sugars from source to sink. Originally the living plant cells were thought to be responsible for such translocation, but now the mechanism is thought to be restricted to the sieve tubes. Photosynthesis results in an increase in osmotic concentration in the leaf due to the synthesis of sugar. As a result, turgor pressure increases in the mesophyll cells causing some amount of cell solution to move into the sieve tubes and a fraction to combine which later on enters xylem, from there, it moves to the leaf cells. Most of the solute enters into the root where they are utilised for metabolic processes or converted to insoluble storage products, thereby lowering osmotic concentration.

According to this hypothesis, the process of translocation is as follows:

Photosynthesis results in the formation of carbohydrate (glucose, a monosaccharide) which is later converted to a translocable form i.e., sucrose (a disaccharide).

Sucrose enters companion cells and then into sieve tube cells by active transport produces a hypertonic condition in the phloem.

Leaf cells with high sucrose concentration result in absorption of water from xylem vessels of the stem.

Water enters from the xylem to the phloem by osmosis. This causes an increase in hydrostatic pressure or turgor pressure in these cells.

Active transport causes a movement of sucrose out of phloem into cells where it is utilised for growth, storage, and respiration, resulting in a decrease of sucrose concentration which lowers osmotic concentration and causes a decrease of hydrostatic pressure.

Water moves out of phloem. The green leaves serving as a region of synthesis is called 'source' and the region of utilisation is called·'sink'.

Hydrostatic pressure or turgor pressure gradient thus created causes mass flow of solution and dissolved solutes from source to sink via the phloem.

Movement of solute takes place from high to low turgor pressure. Increased hydrostatic pressure in the sieve tube causes a pressure flow resulting in the movement of sap through phloem.

Loss of solute produces high water potential in phloem resulting in passing out of water from phloem to xylem.

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