9.4 Flashcards
what is translocation?
movement of assimilates from source to sink
what is the main assimilate transported around plants?
sucrose
why is sucrose transported instead of glucose?
sucrose cannot be used up by metabolic processes
what are the main sources in plants?
leaves
storage organs
depleting food stores
what are the main sinks in plants?
roots
meristems
forming food stores
how is sucrose moved into companion cells from source cells?
H+ ions are actively pumped out of the companion cells using a proton pump
sucrose moves back into the companion cells with the H+ ions by cotransportation
how is sucrose moved into the sieve tube elements from the companion cell?
sucrose moving into the companion cell increases the concentration of sucrose
this is greater in the companion cell than in the sieve tube elements
therefore sucrose diffuses through the plasmodesmata down the concentration gradient and into the sieve tube element
how does water move into the sieve tube elements?
the high concentration of sucrose in the sieve tube elements means a low water potential
water moves through the pits in the xylem, down the water potential gradient and into the sieve tube elements
what happens to the turgor pressure when water moves into the sieve tube elements?
turgor pressure increases
how do the solute (assimilates+water) move to the sink cells?
it moves down the pressure gradient from high pressure to low pressure
how is sucrose unloaded into sink cells?
by diffusion down the concentration gradient
what happens to the water when sucrose is unloaded?
sucrose leaving creates a high water potential so water moves into the surrounding cells by osmosis
what evidence is there for translocation?
microscopes can see the adaptations of companion cells for active transport
poisoning the mitochondria causes translocation to stop
