Miss lee - Translocation

Question 1

What is translocation

Answer 1

Translocation is the movement of assimilates (e.g. sucrose/amino acids etc) from where they are made (source) to where they are required (sink)

Question 2

What is needed for translocation

Answer 2

• Translocation occurs in phloem vessels.
• Requires ATP energy to create a pressure difference.
• Movement is bidirectional.
• Glucose is transported as sucrose

Question 3

Why is glucose turned to sucrose?

Answer 3

(as it is insoluble and inactive) – it doesn’t get used up in transport.

Question 4

What is a sink

Answer 4

The site where sucrose /assimilates are unloaded from the phloem for use or storage

E.g. Growing roots. Active processes in the roots and stem. Meristem cells activity dividing. Developing stores, e.g. seeds, fruits or storage organs

Question 5

What is a source

Answer 5

This is the site where sucrose/assimilates are made and loaded into the phloem. (high concentration)
E.g. Green leaves & stems. Storage organs. E.g. tubers and root taps. Food stores in seeds. Sucrose makes up 20-30% of phloem sap

Question 6

What is the process of translocation

Answer 6

There are 3 stages to the movement of sucrose and assimilates from source to sink.
1) Active loading at the source.
2) Mass flow of sucrose through the sieve tube
elements.
3) Unloading of sucrose at the sink

Question 7

In detail what happens during active loading at the source (phloem loading)

Answer 7

• Sucrose/assimilates are actively loaded from the source into the phloem sieve tubes by the companion cells.
• Firstly, hydrogen ions (H+) are actively pumped out of the companion cell via a proton pump.
• This increases the H+ ion concentration outside of the companion cell compared to inside, creating a concentration gradient.
• The hydrogen ions then re-enter the companion cells by facilitated diffusion.
• H+ ions bind to co-transporter proteins in the companion cell membrane. Sucrose
molecule also binds to the same co-transporter protein (at a different site).
• H+ ions can now move back into the companion cell (down its concentration gradient) bringing the sucrose molecule with it (via the co-transporter protein) by facilitated diffusion.
• Now there is a higher concentration of sucrose inside the companion cell (compared to the phloem).
• Sucrose can diffuse (from high – low concentration) into the phloem sieve tubes via the plasmodesmata.

Question 8

In detail what happens at mass loading

Answer 8

• Sugars/sucrose/assimilates enter the sieve tube element (at the source), this lowers the water potential in the sieve tube.
• Water enters the sieve tube by osmosis from the xylem.
• This raises the hydrostatic pressure at the source.
• When assimilates leave the sieve tube at the sink, this increases the water potential inside the sieve tube.
• Water leaves the sieve tube by osmosis, down a water potential gradient and lowers the hydrostatic pressure.
• Water moves down the hydrostatic pressure gradient (from high to low) towards the sink, also moving sucrose (and other assimilates) along the phloem.

Question 9

In detail what happens at unloading at the sink

Answer 9

• Sucrose needs to be unloaded into the sink where it is needed.
• Because of the higher concentration of sucrose in the phloem,
sucrose can diffuse rapidly into the sink (surrounding cells).
• Sucrose may be used for metabolic processes (e.g. respiration) or
stored as starch.
• When sucrose diffuses out of the sieve tubes, this increases the
water potential of the tube.
• Water therefore moves out of the sieve tube by osmosis, this creates a low hydrostatic pressure at the sink, compared to the higher hydrostatic pressure at the source.

Question 10

What are the two roots that assimilates can take

Answer 10

The symplast route and apoplast route

Question 11

What is the symplast route

Answer 11

• Assimilates are stored in the vacuoles of cells.
• The assimilates are moved through the cytoplasm of mesophyll cells into the sieve tubes across connecting plasmodesmata
• This is a largely passive process
• The assimilates are moved by changes in water potential of cells

Question 12

What is the apoplast route

Answer 12

• Assimilates diffuse through the cell wall and intermembrane spaces the apoplast)
• When they reach the companion cells they are actively transported across the membrane into sieve cells cytoplasm
• Hydrogen ions act as co- transporters and actively move assimilates across the membrane

Question 13

What is the evidence that suggests mass flow

Answer 13

1. The advances in microscopy means we are able to see the physical adaptations of a companion cell for active transport
2. If the mitochondria is poisoned it means that translocation will stop
3. The rate of translocation is constant throughout the plant regardless of the concentration
4. The rings on the trees is where the phloem vessels are and by peeling off the bark you can see the phloem vessels
5. The feeding of the insects such as aphids suck out the water and sucrose as the insects need sugar
6. The radioactive carbon can be taken in by plants and then they can see where the carbon is transported to through x-rays