3.3.7 translocation

Question 1

assimilates

Answer 1

substances which have become part of the plant

Question 2

sink

Answer 2

part of plant where those materials (assimilates) are removed from transport system
eg. roots receive sugars & store as starch (sink)

Question 3

how are the roots able to be a source & sink

Answer 3

sink = receive sugars & store as starch
source = (another time of year) starch may be converted back to sugars & transported to growing stem

Question 4

source

Answer 4

part of plant which loads materials into transport system
eg. leaves photosynthesise & sugars moved to other parts of plant

Question 5

translocation

Answer 5

transport of assimilates throughout a plant

Question 6

describe steps of active loading

Answer 6

• sucrose loaded into sieve tube
  1. involves use of energy from ATP in companion cells
  2. energy actively transports hydrogen ions (H+) out of companion cells = increases conc. outside of cell & decreases conc. inside
  3. creates conc. gradient
  4. hydrogen ions diffuse back into companion cells via special cotransporter proteins (w/ sucrose molecules) = cotransport
  5. as conc. of sucrose in companion cell increases, it can diffuse through plasmodesmata into sieve tube

Question 7

what do cotransporter proteins only allow

Answer 7

movement of hydrogen ions into cell if accompanied by sucrose molecules

Question 8

another name for cotransport

Answer 8

secondary active transport
(as results from active transport of hydrogen ions out of cell & moves sucrose against conc. gradient)

Question 9

movement of sucrose in the phloem

Answer 9

= mass flow
- solution of sucrose, amino acids & other assimilates = sap
- move up/down phloem
- caused by difference in hydrostatic pressure between 2 ends of phloem, producing pressure gradient
–> water enters at source (increasing pressure) & leaves tube at sink (reducing pressure) = sap flows source to sink

Question 10

how is the hydrostatic pressure increased in the sieve tube elements at the source

Answer 10

• sucrose enters sieve tube element = water potential more negative
• water molecules move into sieve tube element by osmosis

Question 11

how is the hydrostatic pressure decreased in the sieve tube elements at the sink

Answer 11

• sucrose removed from phloem sieve tubes eg. respiration/growth in meristem
• where sucrose’s being used in cells, it can diffuse out of sieve tube via plasmodesmata
• sucrose can be removed by active transport
• removal of sucrose from sap = water potential less negative (higher)
  –> water moves out of sieve tube (into surrounding cells)

Question 12

describe the movement of sap along the phloem

Answer 12

• water enters sieve tube at source = increases hydrostatic pressure
• water leaves sieve tube at sink = decreases hydrostatic pressure
  = creates pressure gradient
• sap flows high to low pressure in either direction –> depends where sucrose is being produced/is needed
• possible for sap to flow in diff. directions in diff. sieve tubes at same time
• sap all moving same direction in single sieve tube = mass flow