The Mass Flow Hypothesis

Question 1

The mass flow hypothesis was modelled by Ernst Münch in 1930. His simple model consisted of

Answer 1

Two partially permeable membranes containing solutions with different concentrations of ions (one dilute the other concentrated)

These two membranes were placed into two chambers containing water and were connected via a passageway

The two membranes were joined via a tube

As the membranes were surrounded by water, the water moved by osmosis across the membrane containing the more concentrated solution which forced the solution towards the membrane containing the more dilute solution (where water was being forced out of due to hydrostatic pressure)

Question 2

The Mass Flow Hypothesis was the model initially used to

Answer 2

explain the movement of assimilates in the phloem tissue

Question 3

Mass flow hypothesis step 1

Answer 3

Active transport is used to actively load the solutes (e.g. sucrose from photosynthesis) into the sieve tubes of the phloem at the source (e.g. the leaves)

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

Question 4

Mass flow step 2 t

Answer 4

At the sink end, solutes are removed from the phloem to be used up

This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis

This lowers the pressure inside the sieve tubes

Question 5

Mass flow step 3

Answer 5

The result is a pressure gradient from the source end to the sink end

This gradient pushes solutes along the sieve tubes to where they are needed