Transport of Water: Mechanism

1.      Two theories to explain water + minerals transport in plants

-   root pressure theory

-   cohesion-tension theory.

2.      Root-pressure theory:

-   accumulation of mineral ions in the xylem

-   enhances water molecules to move into root hairs (by osmosis).

-   water pressure ↑ builds up in the root

-   pressure pushes up water +  dissolved minerals

-   through the xlem

-   toward the top of the plant.

-   but not strong enough to push up water to the top of tall trees.

Root pressure



3.      In small plants:

-   root pressure can build high enough

-   to force water and minerals completely out of the tips of the leaves

-   the process = guttation. 

4.      Cohesion-tension theory suggests that:

-   water inside the xylem is pulled upward

-   by the -ve pressure (or tension)

-   that extends all the way from leaves to roots.

5.      In the leaf xylem:

-   -ve pressure (tension) builds

-   as water evaporates during transpiration.

-   evaporated water is continually replaced

-   thus cohesive bond pull the string of water molecules up

-   to create a transpiration pull.

-   transpiration pull is relayed

-   molecule by molecule

-   down the entire column of water in the xylem.


Transpirational pull in the leaf



6.      In the stem, water molecules:

-   exist as a long unbroken chain in the xylem.

-   are pulled upwards by tensions produced (during transpiration).

-   are held by cohesion + adhesion forces

Cohesion and adhesion forces in the xylem

7.      Transpiration pull:

-   can extend down to the roots

-   only through an unbroken chain of water molecules.

8.      At the cellular level:

-   the gradients of water potential

-   drive the osmostic movement of water

-   from cell to cell

-   within the roots up to the leaves.

Water potential in leaf, stem and root