Mass Transport in plants

Question 1

How is sucrose transported from source to the sieve tube element by translocation?

Answer 1

• Photosynthesis occuring in chloroplasts of the leaves creates sucrose
• This creates a high concentration of sucrose at the source, therefore sucrose moves down its concentration gradient into the companion cells via facilitated diffusion
• Active transport of H+ occurs from the companion cell into spaces within the cell walls using energy
• This creates a concentration gradient and therefore the H+ move down their concentration gradient via carrier proteins into the sieve tube elements
• Co-transport of sucrose with the H+ ions occurs via protein co-transporters to transport the sucrose into sieve tube element

Question 2

How is sucrose transported along the sieve tube elements?

Answer 2

• the increase of sucrose lowers the water potential in the seive tube element
• Water enters the sieve tube from the surrounding xylem vessels via osmosis
• This increases water volumes in the sieve tube element increases hydrostatic pressure causing the liquid to be forced towards the sink

Question 3

How is sucrose transported into
Sink?

Answer 3

• sucrose is used in respiration at the sink or stored as strch
• more sucrose is actively transported into the sink cell, which causes the water potential to decrease
• this results in osmosis of water from the sieve tube elements into the sink cell
• the removal of water decreases the volume in the sieve tube element and therefore the hydrostatic pressure
• movement of sucrose is due to the difference in hydrostatic pressure between source and sink

Question 4

Explain how sucrose-transport proteins in leaf cells enable the production of a pressure gradient?

Answer 4

1.. Sucrose co-tranported into phloem
2. Causes decrease in water potential (inside
phloem);
3. Water enters by osmosis (into phloem);