Evaluating the Mass Flow Hypothesis (AQA A Level Biology): Revision Note

When the phloem sieve tube is punctured phloem sap oozes out

The contents of the phloem exert pressure on the phloem walls

Phloem sap extracted near a source has a higher sucrose concentration than sap extracted near a sink

Water would move into the phloem by osmosis near a source and out of the phloem by osmosis near a sink

Metabolic inhibitors stop translocation

ATP is required for mass flow to occur; it is an active process

Removal of a ring of bark from trees results in a bulge above the ring, and fluid from the bulging region has a higher sugar concentration than fluid from below the ring

Removal of the phloem tissue (which is just below the bark) prevents the passage of sugars

Amino acids appear to travel more slowly than sucrose in the phloem

The mass flow hypothesis states should be flowing at the same rate

Experiments have detected different substances, within the same sieve tube, moving in opposite directions

The mass flow hypothesis states everything should be flowing in one direction

Sieve plates are present in the phloem

These create a barrier to mass flow, so there is no reason for them to have evolved