Translocation

Question 1

What does “sources” mean?

Answer 1

Areas that produce assimilates by photosynthesis or from storage molecules

Question 2

What does “sinks” mean?

Answer 2

Plant regions that require assimilates to supply their metabolic needs.

Question 3

What are “assimilates”?

Answer 3

Products of photosynthesis that are transported around the plant - such as sucrose

Question 4

Loading of the phloem - explain the process

Answer 4

Sucrose is loaded into the phloem from mesophyll cells by the symplast pathway which is passive or the apoplast pathway which is active

Question 5

Apoplast loading of sucrose

Answer 5

sucrose moves from source to the companion cells and phloem cells via cell walls and intracellular spaces and moved by diffusion.

To enter the companion cell or sieve tube element, it must cross the cell membrane which is an active process

Question 6

Loading of sucrose into the phloem

Answer 6

hydrogen ions actively pumped out of companion cells. requires ATP

H+ accumulates outside of companion cells
H+ returns to companion cells down a concentration gradient via a co-transporter protein

Sucrose is co transported with H+

sucrose accumulates in companion cells

sucrose travels via plasmodesmata to move along the phloem.

Question 7

Adaptions of companion cells to allow effective phloem loading

Answer 7

Many mitochondria to provide ATP to actively pump out H+ ions

Infoldings in cell membrane to give increased surface area for the active transport of sucrose into the cell cytoplasm

Question 8

What happens after sucrose is loaded?

Answer 8

Sucrose accumulates in sieve tube elements, reducing the water potential

Water moves into sieve tube elements down a water potential gradient - causing turgor pressure

Water moves up or down the plant by mass flow to areas of lower pressure

Question 9

Unloading of phloem

Answer 9

sucrose leaves phloem via diffusion

high to low conc

via channel proteins

loss of sucrose increases water potential, water moves out of cells via osmosis

Question 10

Evidence for phloem loading

Answer 10

Microscope observations of companion cell adaptions

Translocation stops if the mitochondria in companion cells are damaged

Flow of sugars in the phloem is a lot faster than it would be by diffusion alone

Aphid studies

Question 11

Aphid studies

Answer 11

If aphids are detached from their stylet, the sap is forced out of the stylet due to the pressure inside of the phloem - shows there’s positive pressure in the phloem

Question 12

Explain how assimilates are loaded and unloaded

Answer 12

1. Active transport of hydrogen ions out of companion cells
2. Created hydrogen ion gradient
3. This causes facilitated diffusion of H+ back into the companion cells
4. Sucrose moves in with the hydrogen ions through a co-transport protein
5. Sucrose diffuses through the plasmodesmata from the companion cell into the sieve tube element
6. the sucrose in the sieve tube element reduces the water potential
7. Water flows from the xylem & increases the hydrostatic pressure in the sieve tube element
8. Water moves down the sieve tube element from higher hydrostatic pressure at the source to lower hydrostatic pressure at the sink
9. Sucrose moves to surrounding cells
10. Increasing the water potential of the sieve tube elements so water molecules move into surrounding cells
11. reducing the hydrostatic pressure at the sink