Ch#14 |Transportation| Lec #14 | Translocation Of Organic Solutes |Pressure Flow Theory @riffatjahan

Mechanism of Phloem Translocation/Transport:
There were two types of theories about the movement of sap in the phloem
(i) Active Theories:
These involve the use of energy for the movement of materials in the phloem.
These rejected because there is no evidence to support these theories.
(ii) Passive Theories:
These do not involve in the use of energy for the movement of materials in the phloem. These include:

(a) Diffusion (b) Pressure Flow Theory
(a) Diffusion:
Diffusion is very slow process.
The rate of diffusion is one (01) metre per eight (08) years.
However the average.
Velocity of sugar movement in the phloem is 1 metre per hour.
Therefore this theory is rejected.
(b) Pressure Flow Theory:
This is the most acceptable theory for the transport in the phloem of angiosperms.
In 1930 it was proposed by Ernst Munch as a hypothesis.
It states that the flow of solution in the sieve elements is driven by an osmotically generated pressure gradient between source and sink.
Now this hypothesis is called theory.
Explanation:
1. The glucose formed in the photo synthesizing cells, is used within
the cell (for respiration etc.)
And the rest is converted into sucrose (non- reducing sugar).
2. Sucrose is actively transported through the bundle sheath cells to
the companion cell of the smallest vein in the leaf
It is a short distance transport (involving 2-3 cells).
From companion cell the sucrose is actively transported through palsmodesmata to the sieve tube cell.
The concentration of sucrose in the sieve tube cell rises..
The pathway taken by sucrose is symplast, however sometimes apoplastic movement can take place.
3. The water moves by osmosis from the nearby xylem in the leaf vein.
This increases the hydrostatic pressure of the sieve tube cell.
Hydrostatic pressure helps move to the sucrose and other substances in the sieve tube cells to sinks (c.g fruits).
4. In the storage sinks (such as sugar beet root and sugarcane stem) sucrose is moved into apoplast before entering symplast of the sink.
In symplastic pathway sucrose (or sugars) moves through plasmodesmata to the receiver cell.
5. Water moves out of the sieve tube cell by osmosis.
The hydrostatic pressure become low.
Thus according to pressure flow theory, When sucrose enter into sieve tube, it moves from source to sink pressure gradient.
Energy is also involves in this process.
6. The sieve plates increases the resistance along the pathway.
Therefore there is the generation and maintenance of pressure gradient in the sieve elements between source and sink.
The materials in sieve element are physically pushed much like water flowing through a garden pipe.
The carbohydrates move from the mesophyll cells to phloem tissue by diffusion and active transport (carrier mediated transport).
In the phlocm tissue (sieve tubes) the movement is according to pressure flow theory.
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