Monday, September 12, 2011

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

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