Transport of oragnic substance in phloem

Question 1

What are the phases or mass flow theory

Answer 1

Transfer of sucrose into sieve elements from photosynthesising tissue

Mass flow of sucrose through sieve tube elements

Transfer of sucrose from sieve tube elements into storage or other sink cells

Question 2

What is the general site where substance are transported from

Answer 2

Sources

Question 3

What is the general site where substance is transported to

Answer 3

Sinks

Question 4

Evidence for mass flow

Answer 4

Pressure in sieve tubes - sap released when cut
Concentration of sucrose higher in leaves than roots
Downward flow of phloem only occurs in the light
Increase in sucrose in leaf followed by similar increase in phloem

Question 5

Evidence against mass flow

Answer 5

Function of sieve plates unclear - should hinder mass flow
Not all solutes move at same speed
Sucrose delivered same rate to all regions - not dependant on sucrose conc of sink as mass flow would suggest

Question 6

Translocation

Answer 6

The transport of organic materials in the plant

Question 7

Mass Flow process (big answer)

Answer 7

Photosynthesising tissue at the source produce glucose, bonded with fructose to make sucrose
H+ ions actively transported through H+ ion pump from companion cell to palisade cell against conc gradient
Sucrose and H+ ion diffuse out of cell via co transport protein to companion cell
Sucrose molecules diffuse from companion cell into sieve tube elements via plasmodesmata
Sieve tube element water potential decreases
Water potential in xylem is relatively higher, move into sieve tube via osmosis
Increase in hydrostatic pressure, move down pressure gradient
Sucrose is actively transported into a companion cell
Water potential in sieve tube elements higher than xylem
Water moves from Sieve tube to xylem via osmosis
Hydrostatic pressure decreases, mass flow stops

Question 8

What can be transported in the phloem

Answer 8

Sucrose
Amino Acids
Potassium Ions
Chloride Ions