water transport

• What is meant by the apoplastic pathway?

  a method of osmosis through the root hair cells, where water moves through the cell walls and intercellular spaces

  this pathway can only be used until water reaches the Casparian strip
• What is the symplastic pathway?

  a method of osmosis through the root hair cells, where water moves through the cytoplasm via plasmodesmata

  to begin this pathway, water must be actively transported into cells
• What is the cohesion-tension theory?

  water molecules form hydrogen bonds with each other, causing them to 'stick' together (cohesion)

  the surface tension of the water also creates this sticking effect

  thus as water is lost through transpiration, more can be drawn up the stem from the roots
• How does the casparian strip prevent dissolved ions from reaching the xylem by the apoplast pathway?

  the strip doesn't allow solutions/water to pass through, instead it forces the solutions/water through the cell-surface membrane

  the phospholipid repels charged ions
• Why do water molecules form hydrogen bonds with nitrate ions?

  as water is a polar molecule

  and nitrate ions are charged

  hydrogen bonds can form between the H on water and O on nitrate
• how does the casparian strip prevent ions such as Cl- from reaching the xylem of the plant by the apoplast pathway?

  the strip is impervious to water and solutions

  thus forces water/solutions to pass through cell-surface membrane

  phospholipid bilayer repels charged ions
• what is the symplastic pathway?


  a method of osmosis through the continuous cytoplasm of the living plant cells that are connected via plasmodesmata

  the root hair cell has a higher water potential than the next cell along (due to water diffusing in from the soil), thus water moves by osmosis from the root hair cell into the next cell along
• how is a steep water potential gradient maintained?


  as water leaves the root hair cell by osmosis, the water potential of the cytoplasm falls again

  ensuring as much water as possible continues to move into the cell from the soik
• what is the apoplast pathway?


  movement of water through the cell wall and intercellular spaces
• what happens in the apoplast pathway?


  water fills the spaces between the loose, open network of fibres in the cellulose cell wall
• how does water move into the xylem?


  water moves across the root in the apoplast and symplast pathways until it reaches the endodermis of the root
• what is the endodermis of the root?


  the layer surrounding the vascular tissue (xylem and phloem)
• what is present at the endodermis of the root?


  casparian strip, which is a band of waxy material that runs around each of the endodermal cells, forming a waterproof layer
• what happens at the endodermis due to the Casparian strip?


  water in the apoplast pathway is forced into the cytoplasm of the cell, joining the water in the symplast pathway
• what is significant about the diversion of water from the apoplast pathway into the cytoplasm?


  the water must pass through the selectively permeable cell-surface membranes, thus this excludes any potentially toxic solutes from the soil from reaching living tissues

  (as the membranes have no carrier proteins to admit them)
• once the water is inside the vascular bundle what happens?


  water returns to the apoplast pathway to enter the xylem itself?
• how does the Casparian strip prevent Na+ and Cl- from reaching the xylem of the plant by the apoplast pathway?


  the Casparian strip is impervious to water

  thus it forces water to pass through the cell-surface membrane and join the water in the symplast pathway

  the phospholipid bilayer repels ions/charged particles
• how does water enter the endodermis?


  the root cells?? actively transport mineral ions against the concentration gradient into the endodermis, lowering the water potential of the endodermal cells

  this creates a water potential gradient between the root cortex and the endodermis

  thus water moves into the endodermal cells by osmosis down the water potential gradient
• what is the third pathway?

  vacuolar pathway, passing through the vacuoles