Monday, September 12, 2011

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

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