Water transport in plant

A) Mechanism of water transpot along xylem:

1) Cohesion - tension developed from transpiration lead to massflow of water in xylem

1. Water is lost from lead by transpiration, developed water potential gradient, which constantly draws water from leaf xylem to leaf cell and from xylem to leaf xylem.
2. A negative pressure tension is created and pull water along the xylem vessel, water potential in root is lowered.
3. water draws in from soil to root by suction.

*cohesive forve between water molecuke enables water inside xylem unbreakable like a chain.

2) Root pressure:

1. Active transport of mineral ion from surrounding parenchyma endodermis by root into root xylem, which lower the water potential in xylem.
2. water is absorbed from soil to xylem by osmosis.
3. a positive pressure is created to push water upward along the xylem.

 

* only significant at night or soil moisture is high, which the transpiration pull is weak.

 

B) Movement of water and mineral salts across the root

From soil to root : (Active)

Root hair actively transpot mineral ion from soil to root hair, which lower the water potential in root hair vacuole. water is drawn across the cell well and the selectively permeable protoplasm by osmosis into vacuole.

From root hair to inner parenchyma : (Passive)

Water is than pass from root hair to parenchyma by the following mechanism:

1. vacuolar :(~0.1%) water drawn from vacuole to vacuole by osmosis
2. symplast :(~ 10%) water flows through the cytoplasm , diffusing from cell to cell via plasmodesmata.
3. apoplast :(~ 90%) water diffuses through the cellulose of adjascent cells and through the small intercellular spaces.

* driving force: transpiration of xylem , xylem develop low water potential , which countious remove water from inner parenchyma to xylem , producing necessary osmotic potential gradient for (1) and water potential gradient for (2) and (3).

From inner parenchyma to xylem : (Active)

Active transpot of mineral salt into stele, creating water potential gradient across root. Water flow from inner parenchyma to xylem, through the cytoplasm of the endodermis because the Casparian Strip around the cell wall is impermeable to water.