Translocation

Question 1

What is translocation?

Answer 1

Transport of organic solutes around a plant (molecules made by plants itself - assimilates)

Question 2

Example of assimilates.

Answer 2

1- sugar - glucose made in the leaves by photosynthesis -> converted into sucrose to be transported around plant in phloem.
2- Amino acids - plant needs variety of amino acids to make proteins transported around plant via phloem.
3- Plant hormones - different plant parts need different hormones e.g. auxin needed for cell growth.

Question 3

What are phloem tissues made up of?

Answer 3

Sieve tube elements joined together to make a continuous column that carriers organic tube elements and companion cells.
Sieve tube elements - narrow, living cells, lack many organelles (e.g. no nucleus, vacuole, ribosomes and little cytoplasm).
Companion cells = normal plant cells -> but with larger number of mitochondria and ribosomes.

Question 4

What is mass flow and what type of process is it?

Answer 4

• continuous columns of sap flowing in one direction
  Active process -> requires ATP energy

Question 5

What is source and sink?

Answer 5

Source -> parts of plant that loads organic solutes into the phloem.
Sinks -> parts of plant that takes organic solutes out of phloem.

Question 6

How does sap move into phloem?

Answer 6

1- organic solutes e.g. sucrose, actively loaded into phloem at the source, the water potential decreases inside sieve tubes.
2- so, water molecules enter the sieve tubes via osmosis, from surrounding cells and xylem vessels.
3- this increases hydrostatic pressure inside the sieve tubes -> causing phloem sap to move from area of high hydrostatic pressure to an area of low hydrostatic pressure.
4- Pressure gradient is what makes sap move.
5- at the sink, the organic solutes are removed from phloem (diffuses into cells with low sucrose conc).
6- this lowers the water potential in those cells and increases water potential in the sieve tubes.
7-so water moves out into those cells.
8-hydrostatic pressure in sieve tubes drop again
9-Note - phloem sap can move up/ down.

Question 7

How sucrose is actively loaded into the phloem -> apoplast route

Answer 7

1- H+ ions actively pumped out of companion cells, using ATP energy - active transport.
2- this creates a high conc of H+ ions outside the companion cells so they will diffuse back in due to concentration gradient (done via facilitated diffusion - involving a membrane protein - carrier).
3- This protein carrier only works if sucrose moves in at the same time with it (co-transport), so sucrose accumulates inside companion cells.
4- The sucrose is then at high conc inside the companion cells than it is in the sieve tubes via osmosis elements so it diffuses across via the plasmodesmata that connects the cells.

Question 8

Unloading sucrose from the phloem

Answer 8

• occurs at sinks
• Sucrose moves by the facilitated diffusion from an area of higher conc (in phloem sap) to an area of low conc (cell that requires it)
  These cells use the sucrose for:
• hydrolysis : hydrolyse sucrose into fructose and glucose -> which can be used for respiration or converted into storage molecules e.g. starch.
  These processes decrease conc of sucrose inside sink cells, maintaining conc gradient for more to diffuse across from phloem.

Question 9

Scientific evidence for mechanisms involved in phloem transport.

Answer 9

How we know process in active (uses ATP)
-> companion cells have lots of mitochondria -> high respiration -> lots of ATP mad.
How we know it uses this method:
- pH of companion cells higher than surrounding cells
Conc of sucrose higher in source than sink.