9.4 Translocation

Question 1

Define translocation

Answer 1

The transport of assimilates such as sucrose and amino acids from a source to a sink.

Energy requiring process in the phloem

Question 2

Define source and sink

Answer 2

Source : where a substance is made, so it’s at a high concentration there

Sink : where the substance is used up, so it’s at a lower concentration there

Question 3

What are the sources and sinks of sucrose?

Answer 3

Source = leaves

Sinks = other parts of the plant eg food storage organs and meristems

Question 4

Give an example of a parts of a plant that is both a sink and a source

Answer 4

Sucrose can be stored in the roots.

During growth season, sucrose is transported from roots to leaves.

Here roots are the source and leaves are the sink ( normally the other way around )

Question 5

How do enzymes create a concentration gradient between source and sink ?

Answer 5

By changing the dissolved substances at the sink, so it’s always at a lower concentration than the source.

eg in potatoes, sucrose is conveyed to starch at sink, lowering the concentration of sucrose. Therefore there is a constant supply of new sucrose reaching sink from phloem.

eg invertase breaks down sucrose into glucose ( and fructose)

Question 6

What is the mass flow hypothesis ?

Answer 6

Solutes are actively loaded into sieve tubes at the source by active transport

This lowers water potential inside sieve tubes, so water enters by osmosis from xylem and companion cells

This creates high pressure inside sieve tubes at source end of phloem

At sink end, solutes removed. This increases water potential so water leaves by osmosis. This lowers pressure.

This creates pressure gradient from source end to sink end.

Question 7

How do substances enter the phloem using active loading?

Answer 7

In companion cell, ATP is used to actively transport H+ ions out of cell into surrounding tissue cells

Creates concentration gradient ( more h+ ions in tissue than in cell)

H+ ion binds to co - transport protein in companion cell membrane and re-enters the cell (down gradient )

Sucrose molecule binds to co-transport protein at the same time. Movement of H+ ions move the sucrose into the cell ( against gradient )

Sucrose transported into sieve tubes by same process