Data on the Mass Flow Hypothesis (AQA A Level Biology)

• The mechanism of transport that occurs within the phloem is still debated by scientists
• The mass flow hypothesis suggests that there is a translocation of sucrose and other sugars within the phloem occur via a continuous unidirectional flow of water and dissolved nutrients
  • The direction of flow is from the source (where sugars are produced) to the sink (where sugars are respired)

• Many experiments have been conducted to try and determine whether the mass flow hypothesis is an accurate representation of what occurs within the phloem
• If the mass flow hypothesis is correct then in any one sieve tube there should be a bulk flow of phloem sap in one direction, travelling at the same rate. This should also be occurring at the same time in most sieve tube elements.

Evidence supporting the mass flow hypothesis

• There are several observations that support the hypothesis
• When the phloem sieve tube is punctured phloem sap oozes out
  • This suggests that it is under pressure

• Phloem sap taken from near a source has a higher sucrose concentration than sap taken from near a sink
  • This suggests that different water potentials would result in osmosis into/out of the sieve tubes at those two locations

• When a plant virus is applied to leaves that are well-lit the virus can be observed moving down the phloem towards the roots
  • This demonstrates the bulk flow of substances in one direction

• When the virus is applied in the dark it is not transported
  • This suggests that for translocation to occur photosynthesis and the production of sucrose is required in the source tissue

Evidence contradicting the mass flow hypothesis

• The rate of translocation of different organic substances was measured and the results showed that amino acids appeared to travel more slowly than sucrose
  • The mass flow hypothesis states should be flowing at the same rate

• Some scientists have conducted experiments that detected different substances (within the same sieve element) moving in opposite directions
  • The mass flow hypothesis states everything should be flowing in one direction

• It has been suggested that some sieve tubes translocate at different times
  • The mass flow hypothesis states that nearly all sieve tubes should be involved in translocation at the same time as they are all connected to the same leaves