mass flow hypothesis Flashcards
what is translocation
mass flow of sucrose and amino acids from one part of a plant, the source, to another part of the plant, the sink
give features of translocation (5)
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.
outline mass flow hypothesis?
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.
how does the ringing experiment provide evidence for the mass flow hypothesis?
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.
how does radioactive tracers provide evidence for the mass flow hypothesis?
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.
How do Aphids provide evidence for Mass flow hypothesis?
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.
how do metabolic inhibitions provide evidence for mass flow hypotheis?
Translocation stops when inhibitors that prevent ATP synthesis are added into the phloem.
This shows that some aspect of translocation requires energy.
what is the evidence against mass flow hypothesis?
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.
