Translocation

Question 1

What is translocation for?

Answer 1

sucrose/amino acids transported

Question 2

How are companion cells adapted?

Answer 2

• Lots of mitochondria – for active transport
• Lots of ribosomes/rER – to make proteins for transport (e.g. carrier proteins)

Question 3

What is the mass flow hypothesis?

Answer 3

1. Active Transport: Solutes are
   actively loaded from the source
   into the sieve tubes by the companion cells.
2. Osmosis In: As the water
   potential in the sieve tubes falls,
   water enters them from the xylem
   and the companion cells.
3. Pressure Rises: This creates high
   hydrostatic pressure at the source end of the phloem.
4. Osmosis Out: At the sink end, the solutes are removed from the phloem and used up. This increases water potential inside the sieve tube so water leaves by osmosis.
5. Pressure Falls: As the water leaves the phloem, pressure at the sink end decreases.

Question 4

Translocation/Mass Flow Model Answer

Answer 4

• Sucrose (and other solutes) move by active transport into phloem by companion cells (the SOURCE)
• This lowers the water potential in the phloem and water moves in from the xylem by osmosis
• This creates a higher hydrostatic pressure leading to mass flow to respiring cells/storage organs (the SINK)
• Sucrose moves from the phloem into cells by active transport
• This increases the water potential, so water moves from the phloem into the xylem by osmosis

Question 5

How does the mass flow hypothesis link to translocation?

Answer 5

creates a pressure
gradient from source to
sink.
The idea is that this
pressure gradient pushes
the solute up and down
the phloem.

Question 6

The ringing experiment- showing phloem transport food

Answer 6

•Removing of bark would remove phloem.
• accumulation of food substances would cause swelling above the cut region.

Question 7

Aphids experiment- showing phloem transport food

Answer 7

Aphid penetrates the stem into
the phloem using its mouthpart
called stylet and sucks the
plant sap
A feeding aphid can be
anaesthetized and the stylet
cut off
The phloem sap flows out
through the stylet and can be
analysed. It is found to contain
sugars and other organic
s u b s t a n c e s

Question 8

2 pieces of evidence for mass flow hypothesis

Answer 8

• Sap flows out faster near the leaves than further down the stem if the
phloem is pierced (e.g. by aphids). This suggests a pressure gradient.
• Metabolic inhibitors (which stop ATP production and therefore active
transport) stop translocation.

Question 9

Evidence against mass flow hypothesis and translocation

Answer 9

• A lot of pressure would be needed to force solutes through the sieve plates at sufficient speed.
• Sugars travel to many different sinks at once, not just the one with the highest water potential as the mass flow hypothesis would suggest