9.4 - Translocation

Question 1

Define translocation.

Answer 1

Movement of organic solutes in phloem from sources to sinks.

- Active process (requires energy), substances transported up and down plants

Question 2

Define assimilates.

Answer 2

Products of photosynthesis that are transported around plant
- Glucose is made during photosynthesis but the main assimilate is sucrose
Cell sap sucrose level - 0.5%
Phloem sap sucrose level: 20-30%

Question 3

What are sources of assimilates?

Answer 3

• Green leaves and green stem
• Storage organs (in tubers and tap roots) - they unload stores at beginning of growth period
• Food stores in seeds when they germinate

Question 4

What are sinks of a plant?

Answer 4

• Roots that are growing and/or actively absorbing mineral ions
• Actively dividing meristems
• Any part of plants that are laying down food stores such as developing seeds, fruit stores in organs

Question 5

What are the 2 ways for phloem loading?

Answer 5

Passive route

Active route (uses apoplast route): Sucrose travels from source, through cell walls and inter-cell spaces to companion cells & sieve-tube-elements
- By diffusion down conc. gradient - gradient maintained by removal of sucrose from phloem vessels.

Question 6

What is a proton pump?

Answer 6

Intrinsic protein that creates a proton (H+) gradient across a biological membrane

Question 7

What is a co-transporter?

Answer 7

Intrinsic protein that transports 2 substances simultaneously across a biological membrane

Question 8

Describe the process of active phloem loading.

Answer 8

1. H+ ions actively pumped (uses ATP, ATP + H2O > ADP + Pi + energy) out of companion cell into surrounding cell (creating conc. gradient)
2. Co-transporter transports H+ ions and sucrose molecules into the companion cells down the concentration gradient into the sieve-tube-elements.
   (Sucrose moves through plasmodesmata into STE)
3. Since solute conc. increases, water potential decreases, so water moves into the STE by osmosis increasing turgor pressure
   - Turgor pressure moves water carrying assimilates up and down plant.

Question 9

What are the features of companion cells?

Answer 9

• Many infoldings in cell membrane to increase SA for active transport of sucrose
• Contains many mitochondria to provide ATP for transport pump

Question 10

What is the effect of an increase in turgor pressure?

Answer 10

Creates pressure difference between phloem and sink, allowing rapid transportation of solutes and water up and down plant.

Question 11

Define mass flow

Answer 11

Assimilates flowing from source to sink down pressure gradient.

Question 12

Describe the process of phloem unloading.

Answer 12

Sucrose unloaded to sink via diffusion down conc. gradient

• Sucrose moves to other cells to maintain the conc. gradient between cells and phloem
• Increases water potential in STE, H2O then flows into sink, or into xylem and subsequently the transpiration stream.

Question 13

Give evidence for translocation.

Answer 13

• Microscopy allows us to see adaptations of companion cells
• If mitochondria in companion cells are poisoned, translocation is stopped
• Flow of sugars in phloem is 10,000x faster than diffusion, suggesting it is an active process
• Aphids feed off of plant tissue, they insert their stylet (mouth), if aphids are anaesthetised and removed, sap continues to move out of phloem.