translocation

Question 1

sink

Answer 1

areas of the plant where the products of photosynthesis are used gro growth or storage

Question 2

the mass flow theory was originally thought to be passive, true or false

Answer 2

true

Question 3

movement of organic substances in the phloem is now thought to be passive, true or false

Answer 3

false

Question 4

source

Answer 4

the part of the plant where the assimilates are produced

Question 5

describe the process for the translocation of sugars

Answer 5

hydrogen ions are actively transported out of the companion cells

hydrogen ions diffuse back into the companion cells, coupled with sucrose, down the diffusion gradient through co-transporter proteins. this is facilitated diffusion

sucrose diffuses into sieve tube elements through the plasmodesmata

water potential inside the sieve tube decreases; so water moves into the sieve tube element by osmosis

hydrostatic pressure in the sieve tube at the source increases. Fluid containing the sucrose from the source moves down the sieve tube down it’s pressure gradient, from a higher hydrostatic pressure to a lower hydrostatic pressure (into the sink)

Sucrose molecules move move from the sieve tube into the surrounding cells by facilitated diffusion or active transport.

Sucrose enters the sink to be used in respiration or to be converted into starch for storage

water moves out of the sieve tube by osmosis, so the hydrostatic pressure at the sink falls

Question 6

what are the two experiments used to demonstrate translocation

Answer 6

the ringing/girdling experiment
the use of radioactive tracers in aphids or autoradiography

Question 7

describe the ringing experiment

Answer 7

a ring of bark is scraped away from the stem of the plant
this also removes the phloem at that part of the plant

after a while, sugar trying to be transported down the stem will be stopped at the ring

a bulge of sugar is formed at the ring

this suggests that sugar moves down the stem in the phloem

Question 8

what is the name of the specialised mouth parts of aphids
it’s function?

Answer 8

stylets

to penetrate phloem tubes

Question 9

how are phloem saps collected for analysis using the stylets of aphids

Answer 9

the aphids are anaesthetised with carbon dioxide and the stylet is cut off, so that it remains in the phloem

pure phloem sap can be collected through the stylet for analysis

Question 10

how are radioactive tracers used in aphids to investigate translocation

Answer 10

radioactive labelled carbon dioxide is placed in a bag surrounding an illuminated individual leaf

the carbon dioxide is incorporated into sugars and transported in the phloem

aphids feeding on the sucrose(sugars) in the phloem can be used to trace the movement of sucrose in the plant from the source to the link

Question 11

the use of autoradiography to investigate translocation

Answer 11

radioactive labelled CO2 placed in a bag surrounding an illuminated individual leaf

the source and sink leaves are placed firmly on a photographic film in the dark for 24 hours

when the film is developed, the presence of radioactivity shows up as “fogging” of the negatives

this indicates that sucrose travels both and down the stem

Question 12

mass flow

Answer 12

this is when the assimilates flow from the source to the sink down the pressure gradient

Question 13

is phloem unloading an active or passive process

Answer 13

passive

Question 14

is phloem loading an active or passive process

Answer 14

active