7. Phloem

Question 1

mass flow hypothesis

establishing high pressure at source end

Answer 1

1. active transport - solutes (sucrose from photosynthesis) from companion cells into sieve tubes at source (leaves)
2. water potential in sieve tubs lowered. water enters by osmosis from xylem and companion cells
3. high hydrostatic pressure in sieve tubes at source end

Question 2

mass flow hypothesis

establishing low pressure at sink end

Answer 2

1. solutes removed to be used up

2. increase water potential inside sieve tubes. water leaves by osmosis = lower pressure

Question 3

mass flow hypothesis

over all result

Answer 3

1. pressure gradient from source end to sink end
2. gradient pushes solutes towards sink
   3 when reach sink solutes used e.g. in repiration or stored e.g. as starch

Question 4

supporting evidence for mass flow

bark?

Answer 4

ring of bark removed (includes phloem but not xylem) bulge forms above ring which has a higher conc of sugars than fluid below ring
(downward flow of sugar!)

Question 5

supporting evidence of mass flow

radioactive tracer

Answer 5

e.g. C14

tracks movement of organic substances in plant

Question 6

supporting evidence of mass flow

aphids

Answer 6

sap flows out quicker near leaves than further down stem - evidence of pressure gradient

Question 7

supporting evident of mass flow 
metabolic inhibitor (stops ATP production)

Answer 7

translocation stops

evidence that Active transport involved