Mass Flow Hypothesis

This lesson covers: 

1. What translocation is
2. The mass flow hypothesis
3. Experiments that investigate translocation

What is translocation?

Translocation is mass flow of sucrose and amino acids from one part of a plant, the source, to another part of the plant, the sink.

Features of translocation:

• 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.

The mass flow hypothesis

The mass flow hypothesis proposes that translocation occurs due to pressure gradients.

It is hypothesised to occur as follows:

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

At the sink, solutes are actively unloaded from the sieve tube element into companion cells. They can then move into sink cells where the solutes can be used, for example in respiration, or stored.

Evidence for and against the mass flow hypothesis

You need to be able to evaluate the evidence for and against the mass flow hypothesis.

Evidence for the mass flow hypothesis

Ringing experiments:

• The stem bulges with fluid containing more sugar above sections where a ring of bark has been removed.
• This shows that removal of phloem prevents the downward flow of sugars.

Radioactive tracers:

• Radioactive isotopes of carbon in the form of CO₂ that is supplied to a leaf is incorporated into organic substances and can be traced around the plant.
• This shows that organic substances move from source to sink.

Aphids:

• Sap fed on by aphids flows out of the stem more quickly at the top of the stem.
• This shows that there is a pressure gradient in the phloem.

Metabolic inhibitors:

• Translocation stops when inhibitors that prevent ATP synthesis are added into the phloem.
• This shows that some aspect of translocation requires energy.

Evidence against the mass flow hypothesis

The main pieces of evidence against the mass flow hypothesis are:

• Sugars do not just move to sinks with the highest water potential.
• The pressure gradient wouldn’t be high enough for a sufficient rate of flow through sieve plates, especially through large trees.