mass flow evidence

• if a ring of bark (which includes the phloem but not the xylem) is removed from a woody stem, a bulge forms above the ring. The fluid from the bulge has a higher conc. of sugars than the fluid from below the ring. This is bc the sugars can't move past the area where the bark has been removed - this is evidence that there can be a downward flow of sugars
• sugars are made in the leaves - why there is a downwards flow - source to sink
• the build up of sugars above the ring causes a decrease in water potential, so water moves into the cells - adding to the bulge
• pressure in the phloem can be investigated using aphids (they pierce the phloem than their bodies are removed leaving the mouthparts behind, which allows the sap to flow out). The sap flows out quicker nearer the leaves than further down the stem - evidence that there is a pressure gradient
• radioactive tracer such as radioactive carbon (14C) can be used to track the movement of organic substances in a plant
• if a metabolic inhibitor (which stops ATP production) is put into the phloem, translocation stops - this is evidence that active transport is involved
• sugar travels to many different sinks, not just the one with the highest water potential, as the model would suggest
• the sieve plates would create a barrier to mass flow. a lot of pressure would be needed for the solutes to get through at a reasonable rate
• photosynth produces glucose - this is converted to sucrose for transport around the plant
• translocation of solutes can be modelled in an experiment using radioactive tracers. This can be done by supplying part of a plant (often a leaf) with an organic substance that has a radioactive label, then tracking its movement
• • carbon dioxide containing radioactive isotope 14C is used as a radioactive tracer
  • this radioactively-labelled CO2 can be supplied to a single leaf by being pumped into a container which completely surrounds the leaf
  • the radioactive carbon will then be incorporated into organic substances produced by the leaf (e.g. sugars produced by photosynth)
  • these are then moved around the plant by translocation
  • movement of these substances can be tracked using technique called autoradiography
• • to reveal where the radioactive tracer has spread to in a plant, the plant is killed (e.g. by freezing it using liquid nitrogen)
  • and then the whole plant or sections of it is placed onto photographic film
  • wherever the film turns black - the radioactive substance is present
• the results demonstrate the translocation of substances from source to sink over time e.g. autoradiographs of plants killed at different times show an overall movement of solutes (e.g. products of photosynth) from the leaves towards the roots