Topic 3- Mass flow hypothesis

Question 1

How does phloem sap move?

Answer 1

By mass flow

Question 2

Why are carbohydrates usually transported as sucrose?

Answer 2

-Allows for efficient energy transfer + increased energy storage as it is a disaccharide (contains more energy)
-Less reactive as it is a non-reducing so reactions dont occur while it is being transported

Question 3

What is the advnatge of mass flow?

Answer 3

Faster than diffussion

Question 4

Why does it require energy?

Answer 4

to create pressure differences for the mass flow of the organic solutes

Question 5

What is the 1st step in mass flow?

Answer 5

-Pressure differences created by activley loading sucrose into the seive elements at the source

Question 5

What is the 2nd step of mass flow?

Answer 5

-Water potential of the sap is lowered
-Results in water moving into the sieve elements as it travels down the water potential gradient by osmosis

Answer 6

• solutes (eg. sucrose) are removed / unloaded from the sieve elements causing water to follow by osmosis at the sink (creating a low hydrostatic pressure), a hydrostatic pressure gradient occurs

Answer 6

-presence of water within the sieve elements increases the hydrostatic or turgor pressure at the source