mass flow hypothesis

• 1. 
     
  2. Active transport is used to actively load the solutes into the sieve tubes of the phloem at the source. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells. This creates a high pressure inside the sieve tubes at the source end of the phloem
• 1. Source

  Active transport is used to actively load the solutes into the sieve tubes of the phloem at the source. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells. This creates a high pressure inside the sieve tubes at the source end of the phloem
• 2. Sink

  At the sink end, solutes are removed from the phloem to be used up. This usually happens by diffusion because the solutes are at a higher concentration in the phloem than they are in the surrounding tissue at the sink. The removal of solutes increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis. This lowers the pressure inside the sieve tubes
• 3. Flow

  The result is a pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes towards the sink. When they reach the sink the solutes will be used or stored
• The higher the concentration of sucrose at the source, the higher the rate of translocation.
• how the mass flow hypothesis works

  A) low water potential, high pressure

  B) pressure gradient forces solutes down

  C) high water potential, low pressure