Transport in plants

Question 1

Describe the transport of carbohydrates in plants

Answer 1

Translocation
At the source sucrose is actively transported into the phloem
By Companion cells!!!
This reduces the water potential and causes water tome move in from the xylem BY OSMOSIS
This increases the hydrostatic pressure
This creates Mass flow to respiring cells
At the sink sucrose is actively transported out of the xylem

Question 2

Describe the steps in cohesion tension theory

Answer 2

1. Solar heat energy causes the evaporation or transpiration of water from stomata in leaves
   
   2. Water moves from cell to cell across the leaf by osmosis down a water potential gradient
   3. Water is drawn from the xylem creating a tension (‘negative pressure’) in the xylem vessels effectively “pulling up” the water and dissolved ions
   4. The water column is maintained in the xylem by cohesive forces and adhesive forces.
      ○ Cohesion - the attraction of the water molecules to each other by hydrogen bonding
      ○ Adhesion - the attraction of the water molecules to the xylem walls
   5. The upward movement of the water from the xylem in the roots maintains the water potential gradient across the root cells so water enter form soil via osmosis

Question 3

Evidence for cohesion tension theory

Answer 3

• As the rate of transpiration increases the diameter of the tree trunk and branches decreases
  
  • Evaporation of water from the leaves draws water from the xylem by osmosis, water is pulled up the xylem creating a tension
  • The tension pulls in the walls of the xylem vessels so the overall diameter of the trunk & branches decreases