translocation

Question 1

what is translocation?

Answer 1

translocation is the process where the products of photosynthesis are transported from the source to the sink. process occurs in the phloem.

Question 2

what is first step of translocation?

Answer 2

sucrose enters the phloem through active loading.
active loading is where companion cells use ATP to transport hydrogen ions to surrounding tissue which creates a diffusion gradient. after this has been created hydrogen ions diffuse back into companion cells.

Question 3

what is the second step of translocation?

Answer 3

through facilitated diffusion sucrose is able to enter companion cells with hydrogen ions. the concentration of sucrose in the companion cells increases.

Question 4

what is the third step of translocation?

Answer 4

sucrose diffuses out of companion cells down concentration gradient and into sieve tube element

Question 5

what is the fourth step of translocation?

Answer 5

sucrose enters the sieve tube causing water potential to reduce forcing water to enter via osmosis from xylem. this increases the hydrostatic pressure

Question 6

what is the fifth step of translocation?

Answer 6

water moves down sieve tube from an area of high hydrostatic pressure to an area of low hydrostatic pressure

Question 7

what is the final step of translocation?

Answer 7

sucrose removed from sieve tubes via diffusion or active transport into surrounding cells. water potential increases so water leaves sieve tube via osmosis into the xylem. pressure in phloem at sink reduced.

Question 8

evidence for mass flow?

Answer 8

pressure in sieve tube shown when sap released when plant stem cut.
concentration of sucrose higher in leaves than the roots.
high sucrose levels in leaves followed by similar high concentration of sucrose in phloem
lack of oxygen inhibits translocation of sucrose in phloem

Question 9

evidence against mass flow?

Answer 9

function of sieve plants unclear as they appear to hinder mass flow
nto all solutes move at same speed but should if it is mass flow
sucrose delivered at the same rate to all regions not going more quicklt to area with low sucrose concentration

Question 10

what are the 2 types of experiment to investigate mass flow?

Answer 10

tracers and ringing

Question 11

Ringing?

Answer 11

to investigate if phloem is responsible for mass flow
bark and phloem of a tree leaving just xylem in the middle
tissues above missing so ring swell with sucrose solution and tissue below dies.

Question 12

Tracer?

Answer 12

investigate transport of sucrose
plants grown in environtment containing 14CO2 incorporated into sugar produced by photosynthesis.
movement of these sugars can be traced using autoradiography.
areas exposed to radiation appear black.