phloem

Question 1

what do phloem tissue transport

Answer 1

organic substances like sugars

Question 2

translocation definition

Answer 2

the movement of solutes where they’re needed in a plant

Question 3

the mass flow hypothesis

Answer 3

1. active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source.
2. this lowers the water potential inside the sieve tubes, so water enters the tubes via osmosis from the xylem and companion cells.
3. this creates a high pressure inside the sieve tubes at the source end of the phloem.
4. at the sink end, solutes are removed from the phloem to be used up.
5. this increases the water potential inside the sieve tubes so water leaves the tubes via osmosis and lowers pressure inside the sieve tubes.
6. the result is a pressure gradient from the source end to the sink end, causing the solutes to be pushed along the sieve tubes towards the sink.

Question 4

evidence against the mass flow hypothesis

Answer 4

1. sugars travel to many different sinks, not just to the one with the highest water potential.
2. 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.

Question 5

evidence supporting the mass flow hypothesis

Answer 5

1. a radioactive tracer can be used to track the movement of organic substances.
2. if a metabolic inhibitor (which stops atp production) is put into the phloem, translocation stops, which is evidence that active transport is involved.
3. if a ring of bark (that includes the phloem not the xylem) is removed from a woody stem , a bulge forms above the ring. the fluid from the bulge has a higher concentration of sugars than the fluid from below the ring, this is evidence that there’s a downward flow of sugars.