7.9 Mass Flow Hypothesis

Question 1

What is translocation?

Answer 1

Mass flow of sucrose and amino acids from source to sink

Question 2

What are features of translocation?

5 features

Answer 2

• It requires energy.
• It transports substances from sources (where they are made, like the leaves) to sinks (where they are used, like the roots).
• The substances are transported through the phloem.
• Water provides the medium in which these substance dissolve for transport in the phloem.
• It maintains a concentration gradient using enzymes.

Question 3

What is the mass flow hypothesis?

Answer 3

Translocation occurs due to pressure gradients

Question 4

How does mass flow hypothesis propose translocation occurs?

8 steps

Answer 4

• At the source, solutes like sucrose are actively loaded into sieve tube elements from companion cells.
• This decreases the water potential in sieve tube elements.
• Water enters the sieve tube elements from the xylem and companion cells by osmosis.
• This increases hydrostatic pressure in the sieve tube elements at the source.
• At the sink, solutes are actively removed from the sieve tube elements.
• This increases the water potential in sieve tube elements at the sink.
• Water leaves the phloem by osmosis, decreasing the hydrostatic pressure at the sink.
• This creates a pressure gradient, pushing solutes from the source to areas of lower pressure at the sink.

Question 5

What happens at the sink in mass flow hypothesis?

Answer 5

Solutes actively unloaded

Question 6

What evidence supports mass flow hypothesis?

2 reasons

Answer 6

• Ringing experiments
• radioactive tracers
• aphids
• metabolic inhibitors

Question 7

What evidence is against mass flow hypothesis?

Answer 7

• Sugars don’t only move to highest water potential
• The pressure gradient wouldn’t be high enough for a sufficient rate of flow through sieve plates, especially through large trees.