Translocation

Question 1

what is translocation

Answer 1

movement of assimilates through phloem from source to sink
it is bidirectional

Question 2

how does phloem transport phloem sap

Answer 2

sucrose made in leaves and diffuses to phloem companion cells where it is actively transported in

Question 3

what type of active transport is it

Answer 3

active cotransport

Question 4

how does active cotransport work

Answer 4

ATP pump H+ out cytoplasm of companion to surrounding tissue against conc gradient

creates diffusion gradient for h+ to diffuse back in through cotransported proteins

bring sucrose at same time

sucrose builds in companion and diffuses through plasmodesmata to sieve tube elements

Question 5

how does sucrose move in the phloem

Answer 5

pressure flow theory

sucrose enters phloem lowering water potential

water enters by osmosis from xylem creating high hydrostatic pressure in phloem

pressure builds up and sucrose is pushed to area of low hydrostatic pressure

Question 6

what happens at sink

Answer 6

sucrose diffuses out of phloem into sink cell and is converted to starch or glucose to be used or stored

maintains sucrose concentration

water potential in phloem now higher so moves back to xylem by osmosis

water is taken up transpiration stream in xylem creating low hydrostatic pressure in phloem maintaining pressure gradient

Question 7

how does sugars move generally

Answer 7

high to low pressure by mass flow

Question 8

evidence of transport of sucrose

Answer 8

radioactive tracers - label carbon-12 as it is in sucrose

ringing trees - if ring loose phloem so bulges

aphids - they extract sap and then we analyse it from them