9.4 Translocation

Question 1

What is a source?

Answer 1

region of plant PRODUCING ASSIMILATES by PHOTOSYNTHESIS

/STORAGE

Question 2

What is a sink?

Answer 2

region of plant REQUIRING ASSIMILATES UNLOADED

to supply METABOLIC needs

Question 3

What are examples of sources?

Answer 3

mature leaf/stem, storage organs, seeds

Question 4

What are examples of sinks?

Answer 4

fruits, meristem, roots

Question 5

What is translocation?

Answer 5

movement of ORGANIC SOLUTES around a plant

in PHLOEM vessels from source to sink

Question 6

What occurs to transport sucrose from source to sink?

Answer 6

1. H+ ions ACTIVELY TRANSPORTED from COMPANION cells ⟶ surrounding tissue
   
   • proton gradient
2. Sucrose + H+ ⟶ companion cell by FACILITATED diffusion using COTRANSPORT protein
3. Sucrose diffuses into SIEVE TUBE ELEMENT via PLASMODESMATA.
   
   • decreases Ψ of sieve tube element.
4. Water enters sieve tube element by OSMOSIS
   
   • increases HYDROSTATIC pressure
5. Assimilates transported by MASS FLOW ⟶ low pressure.
6. Sucrose diffuses/actively transported out of sieve tube element into surrounding tissue via plasmodesmata.
   
   • increases Ψ of sieve tube element.
7. Water moves out of sieve tube element by OSMOSIS.
   
   • decreases hydrostatic pressure
   • area of low pressure @ sink.

Question 7

Is a growing leaf a sink or a source?

Answer 7

sink

still growing&raquo_space; REQUIRES assimilates unloaded to supply metabolic processes

Question 8

If a mature leaf a sink or a source?

Answer 8

source

photosynthesises to PRODUCE assimilates

Question 9

What is the evidence for active transport in translocation?

Answer 9

advances in MICROSCOPY - can see adaptation (￪ mitochondria) of companion cells

POISONED MITOCHONDRIA = no translocation - indicates active process

FLOW is 1000x faster than DIFFUSION - active process drives mass flow