Translocation

Question 1

What is translocation?

Answer 1

• The movement of organic compounds from where they are made at their source to where they are required at their sink.
• An active process which can be used to transport assimilates (products of photosynthesis) through the phloem, up/down the plant.
• Leaves are sources and roots are sinks.

Question 2

Sources and sinks

Answer 2

• Sources has a high conc. of assimilates as this is where they are produced whereas sink has low conc.
• So assimilates move from high to low conc.
• Some parts of plant can be both source and sink for example during growing season, sucrose is transported from root to leaves to provide energy.
• Enzymes maintain concentration gradient from the source to sink by ensuring there is always lower conc. at sink.

Question 3

Active loading at sink

Answer 3

• Active transport is used to actively load solutes into sieve tubes of phloem at source.
• This lowers water potential inside sieve tubes so water enters through osmosis from xylem and companion cells.
• This creates a high pressure inside the tubes at the source.

Question 4

Removal at sink

Answer 4

• At sink solutes are removed from phloem to be used up.
• Happens by diffusion which is passive as solutes are at high conc. in phloem than surrounding tissues at sink.
• The removal of solutes increases water pot. in sieve tubes so water leaves tubes via osmosis.
• This lowers the pressure in sieve tubes.

Question 5

Flows

Answer 5

• The result is a pressure gradient from source end to sink end.
• This gradient pushes solutes along the sieve tubes toward sink.
• When they reach sink, solutes will be used or stored.
• The higher the concentration of sucrose at the source, the higher the rate of translocation.

Question 6

Active loading

Answer 6

• Active loading is used at source to more substances into the companion cells from surrounding tissues and from companion cells into sieve tubes AGAINST conc. gradient.
• Conc. of sucrose is usually higher in companion cells than surrounding tissue cells and higher in the sieve tube than companion cells.
• Sucrose is moved through active transport and co-transport proteins.
• Co-transport proteins are a carrier protein that binds 2 molecules at a time and the conc. gradient of 1 molecule is used to move the other molecule AGAINST its own conc. gradient.
• H+ ions are used to move sucrose against its conc. gradient.

Question 7

Using co-transporters

Answer 7

1) In companion cell H+ are actively transported OUT and into surrounding cells so there is a conc. gradient and more H+ in surrounding than companion.
2) H+ bind to a co-transporter protein in companion cell MEMBRANE and re-enters the cell DOWN conc. gradient. A sucrose molecule binds to the co-transport protein at the same time. This moves sucrose AGAINST its own conc. gradient.
3) Sucrose molecules are then transported out of the companion cells and into sieve tubes by same process.