Translocation (Sugar Movement)

Question 1

What is translocation?

Answer 1

transport of assimilates throughout the plant in phloem tissue

Question 2

What is a source?

Answer 2

Releases sucrose into the phloem

Question 3

What is a sink?

Answer 3

Removes sucrose from the phloem

Question 4

How does sucrose enter the phloem?

Answer 4

ATP is used by companion cells to actively transport H ions out of cytoplasm into the surrounding tissue, makes diffusion gradient and H ions diffuse back in through cotransporter proteins which enable sucrose mols to be brought in, gradient builds up until sucrose diffuses into sieve tube element through plasmodesmata

Question 5

What happens to sucrose at the source?

Answer 5

Lowers the water potential inside sieve tube so water mols move in from surrounding tissue, increases the hydrostatic pressure at the source

Question 6

How is sucrose used at the sink?

Answer 6

used in cells surrounding phloem, converted into starch for storage or in metabolic processes like respiration - reduces sucrose concentration in cells

Question 7

What happens when sucrose concentration is reduced in cells surrounding the phloem?

Answer 7

Sucrose mols move by diffusion or active transport from sieve tube element to surrounding cells, increase water potential in sieve so water mols move into surrounding cells: reduces hydrostatic pressure at sink

Question 8

What is mass flow?

Answer 8

water entering at the source and leaving the phloem at the sink makes water flow along phloem, can occur in either direction depending where sugars needed

Question 9

What are 2 sources in plants? How do these parts also act as sinks?

Answer 9

Leaf (sugars made during photosynthesis) but during spring leaves growing so need energy: sink, roots where stored carbohydrates move into phloem, during summer/autumn store sugars as starch: sink

Question 10

How do we know that phloem is used in translocation?

Answer 10

Radioactively labelled CO2 (used in photosyn.) is present in phloem, ringing a tree: phloem blocked sugars accumulate, aphid feeding on stem shows mouthparts are taking sugars from phloem

Question 11

How do we know translocation needs metabolic energy (ATP)?

Answer 11

companion cells many mitochondria, translocation stopped by poison that inhibits ATP. rate of sugar flow in phloem is so high that energy must be needed