Mass Flow Hypothesis

Question 1

What is Translocation?

Answer 1

process by which products of photosynthesis are transported from a source to a sink

Question 2

What is the structure of Phloem?

Answer 2

Sieve tubes
No nucleus or organelles just a think layer of cytoplasm around the edge of the cell, maximises space for mass flow
Sieve tube is supported by a companion cell, many mitochondria to produce ATP
End walls between cells but these contain holes to let solutes pass through- sieve plates

Question 3

What are the two concepts of Mass Flow Hypothesis?

Answer 3

creating hydrostatic pressure and creating a concentration gradient

Question 4

What is the first step of Mass Flow Hypothesis?

Answer 4

Source- releases sucrose into phloem
High sucrose concentration, sucrose moved into sieve tubes by active loading. decreasing water potential so that water moves in by Osmosis from xylem increase turgor pressure

Question 5

What is the second step of Mass Flow Hypothesis?

Answer 5

Sink- removes sucrose from Phloem
Low sucrose concentration as sucrose moves out of sieve tubes down a concentration gradient as cells use it or convert to glucose/starch
Increases water potential in sieve tubes so water moves back to xylem by osmosis, reduction in volume decreases turgor pressure

Question 6

What is the third step of Mass Flow Hypothesis?

Answer 6

Mass Flow of Solutes
Turgor pressure gradient created between source and sink causes solutes to be pushed in that direction. Concentration gradient of sucrose between source and sink created as sucrose constantly being used for respiration or converted for storage

Question 7

What is the Active Loading of sucrose into companion cells?

Answer 7

H= ions are actively pumped out of the companion cell( using ATP) into cells of the source tissue
Creates a hydrogen concentration gradient across the companion cell membrane
H+ ions can diffuse back down the gradient through special transport proteins which require a sucrose molecule to be co-transported
Increases the concentration of sucrose in the companion cells which can then diffuse into the sieve tube

Question 8

What are the examples of evidence that support the Mass Flow Hypothesis?

Answer 8

Sucrose concentration higher in leaves than roots
Increase in sucrose in leaf are followed by increases in sucrose in phloem