Ch#14 |Transportation| Lec #06| ASCENT OF SAP(a) Cohesion Tension Theory(FSc Bio part 1)@riffatjahan

ASCENT OF SAP:
We know that water and dissolved minerals pass through the cortex and endodermis and reach the xylem of roots.
Then the water and dissolved minerals are carried or pulled upwards towards the leaves through xylem tissue.
This is called ascent of sap.
This may involve the following:
(a) Cohesion Tension Theory
(b) Root Pressure
(c) Imbibition
(a) Cohesion Tension Theory:
Cohesion tension theory was proposed by Dixon
This theory explains the flow of water and minerals upward iron, the roots to leaves, in bulk flow or mass flow, This depends or, the following:
(i) Cohesion:
It is the attraction among the water molecules which hold water together.
As a result a solid chain-like column is formed within the xylem tubes.
Hydrogen bonds are formed between the water molecules.
(ii) Tension
It is provided when the water chain is pulled up in the xylem.
Transpiration provides the energy or force.
Tension is between the molecules of water by hydrogen bonds.
The xylem water tension is very strong and can pull water up to 200 meters (more than 600 feet) in plants.
(iii) Adhesion:
The cellulose component of cell wall has great affinity with water
It can imbibe water.
Therefore-the cell walls of xylem helps to adhere and creep up water molecules.
As a result the column of water in xylem tissue does not break.
(iv) Strong Xylem Walls:
The lignin and cellulose provide strength to cell wall of xylem vessels.
Therefore the xylem walls have high tensile strength.
By cohesion-tension of water molecules, and the transpiration pull (providing energy), the sap (water and minerals) in xylem is pulled upwards to the leaves.
Large quantities of water are carried at relatively high speed, which is
(i) Up to 8mh-1 in tall trees
(ii) Commonly At 1 mh-1 in other plants.
Water Used by the Leaves:
Of the total water pulled up in the leaves, 99% is transpired.
Only about 1% is used by plant in various activities including photosynthesis.
Mechanism Of Transpiration Pull In Cohesion Tension Theory:
The evaporation of water from the aerial parts of the plant especially through stomata of leaves is a process called transpiration.
When a leaf transpires, the water potential of its mesophyll cells drops.
The water moves by osmosis from the xylem cells of the leaf into" dehydrating mesophyll cells.
The water molecules leaving the xylem are attached to other water molecules in the same xylem tube by hydrogen bonds (cohesion of water molecules).
Therefore when one water molecule moves up in the xylem, the process continues all the way to the root.
In the root also water is pulled up in the xylem (trachieds and vessels).
Due to this pull, water moves transversely from the root epidermis (root hairs), cortex, endosmosis and pericycle and enters xylem.
This of water is down the concentration gradient (high to low.
This pulling force (or transpiration pull) is so strong that it also reduces the water potential of root epidermal cells.
Then water in the soil moves form its higher water potential to lower water potential of epidermis of root hy osmosis.
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