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About This Video ........................
In This Video Lecture You Will Learn Basic Concept Of Cohesion Tension Theory
Cohesion tension theoryis one of the most important theories proposed by Dixon. This
(i) Cohesion: It is the attraction among water molecules which hold water together, forming a solid
chain-like column within the xylem tubes. The water molecules form hydrogen bonds between
them.
to leaves of plants, in bulk low or mass low (Fig. 14.5). This depends on the following:
theory provides a reasonable explanation of low of water and minerals upwards from the roots
(ii) Tension: It is provided when this water chain is pulled up in the xylem. Transpiration provides the necessary energy or force. Tension is between the molecules of water by hydrogen
This xylem water tension is strong enough to pull water up to 200 metres (more than 600 feet) in
plants.
(iii) Adhesion: It may be added that the water molecules also adhere to the cell walls of xylem cells,
so that the column of water in xylem tissue does not break. The composition of cell wall provides
bonds.
necessary adhesion to water molecules that helps water creep up. The cellulose component of cell
wall especially has great afinity for water. It can imbibe water.
v) Strong xylem walls: It is essential that the xylem walls should have high tensile strength if they
are not to buckle inwards. The lignin and cellulose provides strength to cell wall of xylem vessels
By cohesion-tension of water molecules, and the transpiration pull providing the necessary energy,
Mechanism of transpiration pull in cohesion tension theory
drops. This drop, causes water to move by osmosis from the xylem cells of leaf into dehydrating
potential of root epidermal cells. Then water in the soil moves from its higher water potential to
concentration gradient transversely from the root epidermis (root hairs) to cortex by endosmosis
the sap (water and minerals) in xylem tissue is pulled upwards to the leaves. Large quantities of
is a process called transpiration. As a leaf transpires the water potential of its mesophyll cells
in the same xylem tube by hydrogen bonds (cohesion of water molecules). Therefore, when one
various activities including photosynthesis.
and to pericycle. This pulling force or transpiration pull is so strong that it also reduces the water
The evaporation of water from the aerial parts of the plant especially through stomata of leaves
The total water pulled up in the leaves is transpired, except about 1% which is used by plant in
water are carried at relatively high speed, upto 8mh-1 being recorded in tall trees, and commonly
.
water molecule moves up the xylem, the process continues all the way to the root - where water
mesophyll cells. The water molecules leaving the xylem are attached to other water molecules
is pulled from the xylem cells (tracheids and vessels). This pull also causes water to move down its
in other plants at 1mh-1
lower water potential of epidermis of root by osmosis.
About This Channel.....................
I make these videos cause I love to draw and connect the complexity of science and medicine into art. I'm not saying. I'm 100% correct in all my videos, but I do try to obtain the information from credible sources.
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Visit My Other Channel Playlist .....................
NMDCAT PART 1: • New 1st Year Biology
NMDCAT PART 2: • New 2nd Year Biology
Class 11th Lectures: • New 1st Year Biology
Class 12th Lectures: • New 2nd Year Biology
Class 10th Lectures:
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