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

Transport of Water: Concept

1.      Dissolved substances (inside a plant cell) = contribute to solute potential (ψs).
2.      More solute molecules present --> the lower is the water potential (ψ).
3.      When water pontential is lower than> external solution:
-   water molecules move into the cell .
-   pressure inside the cell increases
-   sell contents press against the cell wall
-   create a pressure potentials).
4.      Water potential (of a plant cell) = solute potential + pressure potential.
5.      Water potential = free energy of water.
6.      By convention, water potential of pure water = 0 megapascal (MPa).
7.      Water will move:
-   from a region of higher (less -ve) water potential
-   to a region of lower (more -ve) water potential.
8.      In plasmolysed cell:
-   pressure potential = zero
-   water potential = solute potential.
9.      As more water molecules enter a cell
-   pressure potential ↑ increases
-   so is its water potential
-   cell becomes turgid.
-   less and less water molecules enter the cell.

10.  Most minerals
-   are actively transported into the root.
-   there is a gradient of successfully
-   lower water potentials  from root hair to the xylem vessels
-   result in water uptake by osmosis is enhanced.
11.  Water moves by osmosis in the roots follows three pathways:
-   apoplast
-   symplast
-   vacuole.
12.  Apoplastic pathway:
-   water travels along the cell wall
-   and extracelular spaces.

Apoplast pathway

13.  Symplastic pathway:
-   water moves across the cytoplasm of one cell to the next
-   across the plasma membrane
-   through the plasmodesmata.

Symplast pathway

14.  Vacuole pathway:
-   water moves from vacuole to vacuole of one cell to the next
-   across the plasma membrane
-   through the plasmodesmata.
Vacuole pathway