Translocation

Question 1

What is translocation?

Answer 1

The movement of assimilates from where they are made (source) to where they are needed (sink)

Question 2

What happens during photosynthesis?

Answer 2

Plants make fructose as well as glucose which are combined to make sucrose at the source, prior to translocation

Question 3

What happens at the sink?

Answer 3

The sucrose is converted back into glucose and fructose by invertase

Question 4

What happens at the meristems?

Answer 4

Glucose is used for respiration or combined with nitrates to make amino acids, needed for growth

Question 5

What happens at the storage organs?

Answer 5

The glucose is converted starch for storage

Question 6

Why is there a need for a concentration gradient for sucrose?

Answer 6

To keep sucrose moving from source to sink

Question 7

What is the sucrose concentration gradient like?

Answer 7

High at source and low at sink

Question 8

What is the best supported theory as to how translocation happens?

Answer 8

Mass flow hypothesis

Question 9

What happens in Loading which is step 1 of mass flow?

Answer 9

-Active Transport is used to load assimilates into the sieve tubes at source
-Lowers the water potential inside sieve tubes causing water from nearby tissues to enter via osmosis

Question 10

What happens in Unloading which is step 2 of mass flow?

Answer 10

-At sink, assimilates diffuse from the phloem into surrounding tissues via the plasmodesmata
-Ag same time, increases the water potential inside sieve tubes so water moves back into surrounding tissues via osmosis

Question 11

What happens in Mass Flow which is step 3 of mass flow?

Answer 11

-Mean hydrostatic pressure inside the sieve tube at source is higher than inside the sieve tube at sink
-This pressure gradient pushes assimilates from source to sink in one direction which is called mass flow

Question 12

How does sucrose enter the companion cells?

Answer 12

By active loading because of the companion cells is higher than those at the source

Question 13

What is the first step in active loading?

Answer 13

ATP is used to actively pump H+ ions from the companion cells to the cells at the source so there are more H+ ions outside companion cells than inside

Question 14

What is the second step in active loading?

Answer 14

-H+ ions diffuse back into the companion cells using co-transporter proteins that only allow H+ ions through if accompanied by sucrose
-As H+ ions diffuse down their conc gradient, sucrose is moved against conc gradient into companion cells
-Sucrose then diffuses into the sieve tubes down a conc gradient called CO TRANSPORT