9.2 - Water Transport in Multicellular Plants

Question 1

What are the roles of water in plants?

Answer 1

• Turgor pressure (hydrostatic) as a result of osmosis in plant cells, gives a hydrostatic skeleton - supports stem and leaves.
• Loss of water by transpiration keeps plants cool
• Water is a transport medium for mineral ions and assimilates
• Water is a raw material for photosynthesis.

Question 2

What is the role and what are the adaptations of a root hair cell?

Answer 2

• Exchange surface where water is taken into plant
• Long thin extension from root hair cell is root hair

Adaptations

• Small size - can penetrate soil particles
• Each hair has a large SA:V ratio
• Each hair has a thin surface layer to reduce diffusion and osmosis distance to quicken the process
• Solute concentration in root hair cell cytoplasm maintains water potential gradient between soil and cell.

Question 3

What is the symplast pathway?

Answer 3

• Symplast - continous cytoplasm of living plant cells that are connected through plasmodesmata
• Water moves by osmosis down the water potential gradient from cell to cell, starting at the root hair cell until the xylem

Question 4

What is the apoplast pathway?

Answer 4

Apoplast - cell wall and extracellular spaces of the cell

• Carries water through cell wall and between cells
• Water does not enter the cytoplasm or pass the plasma membranes

Question 5

Describe the movement of water into the xylem.

Answer 5

• After water move by symplast and apoplast it reaches the endodermis (layer of cells surrounding xylem and phloem
• Casparian Strip
  
  • Band of waxy material (suberin) that surrounds endodermal cells forming a water proof layer.
  • It stops water from entering via the apoplast pathway, it is forced into the symplast pathway
• For water to enter the symplast pathway from apoplast, it must pass the selectively permeable membrane which stops toxic solutes from entering from the soil as there are no carrier proteins to admit them

Question 6

After entering the xylem does the water stay in the symplast pathway?

Answer 6

No it re-uses the apoplast pathway

Question 7

What is root pressure?

Answer 7

• Root endodermis uses metabolic energy to pump mineral ions into root medulla
  
  • Reducing water potential in medulla and xylem to lower than the cortex, so water moves across the endodermis into the xylem
• Water cannot move back to the cortex as it is blocked by the Casparian Strip, creating pressure in the cortex pushing water up the xylem