More plant bits.

Water/minerals go through the cell wall, can pass through fibres (apoplasric)

Endodermis controls what goes in.

Apoplastic Pathway:

• Water can move freely between fibres in the cellulose cell wall.
  
• There is very little resistance, and, as the water moves it pulls move water behind is due to the cohesive properties of water.
  

Symplastic Pathway:

• Takes place across the cytoplasm of cells of the cortex as a result of osmosis.
  
• Water passes from cell to cell along tiny passages/openings called Plasmodesmata. Each plasmodesmata is filled with a thin strand of cytoplasm.
  
• Water moves down a water potential gradient, since the cells near the centre have a more low water potential then those near the outside of the root.
  
• It is a slow route, because the membranes and cytoplasm restrict the rate at which water can move.
  

Passage of water into Xylem:

• Before water enters the xylem it has to pass through the endodermis.
• Water traveling the apoplastic way (along the cell wall) can't continue as there is the water proof barrier, the casparian strip.
  
• It has to pass through the membrane and cytoplasm and join the symplastic pathway.
  
• The ions must be actively transported through the cell, which builds up a high concentration, and the gradient goes backwards.
  
• This establishes a water potential gradient, from the root hairs, to the centre of the root, as the ions would lower the WP in the centre, and the water coming in makes it high at the tip. This is called root pressure.

Evidence for the existence of root pressure:

• The pressure increases with an increase in temperature.
  
• Metabolic inhibitors e.g. cyanide, cause root pressure to cease to exist. (It would stop the active transport, and therefore, no WP gradient is created.)
  
• A decrease in oxygen can cause a reduction in root pressures.
  

Transport into Xylem:

• ACTIVE SECRETION of mineral ions into the xylem.
  
• This lowers the water potential of the solution in the xylem
• This makes a water potenital gradient, and water passes into xylem via osmosis froma high water potential to low.
  

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