Page 11 - 11-Bio-14 Transport

14. Transport eLearn.Punjab

By cohesion-tension of water molecules, and the transpiration pull providing the necessary energy,
the sap (water and minerals) in xylem tissue is pulled upwards to the leaves. Large quantities of
water are carried at relatively high speed, upto 8mh-1 being recorded in tall trees, and commonly
in other plants at 1mh-1.

The total water pulled up in the leaves is transpired, except about 1% which 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. As a leaf transpires the water potential of its mesophyll cells
drops. This drop, causes water to move by osmosis from the xylem cells of 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 the xylem, the process continues all the way to the root - where water
is pulled from the xylem cells (tracheids and vessels). This pull also causes water to move down its
concentration gradient transversely from the root epidermis (root hairs) to cortex by endosmosis
and to pericycle. 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 from its higher water potential to
lower water potential of epidermis of root by osmosis.

Animation 14.3: Transpiration pull V: 1.1
Source and Credit: gifsoup

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