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Evidence for Translocation

Evidence for the Translocation Mechanism

The cells surrounding the companion cells have a low pH (high conc. of H+ ).This supports the mechanism described because: It proves that hydrogen ions are transported out of the companion cells.

Sucrose is at a higher concentration in the sieve element than in the surrounding tissues.This supports the mechanism described because: It proves that sucrose is transported in the phloem and not in the xylem and it’s transport into the phloem must be controlled.

 In spring time sucrose moves in the phloem from the stores in the roots to the growing leaves, however in summer sucrose moves from the photosynthesising leaf to the storage organs in the root. This supports the mechanism described because: Sucrose moves from source to sink.  In summer the leaves gain more sunlight and therefore are the main source releasing sucrose into the phloem.  In spring time when light is lower intensity the photosynthesising leaves will not be able to meet demand and therefore the major source will be stores in the roots.                      


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It is an active process by which amino acids are transported through phloem.  Sugar is produced in the photosyntheic tissues and must be transported from these sources to areas of need, such as areas that have large energy requirements.

The sugars are transported in the phloem which consists of two types of cell, the sieve tube and companion cell. Unlike xylem, the cells of the phloem are living.

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Key words

Translocation- The mass flow of Sucrose in the Phloem tissue from a ‘Source’ to a ‘Sink’ .

Mass Flow- The movement of a body of suagrs from an area of high pressure to an area of lower pressure.

Source- Any part of the plant that is producing Sucrose and therefore the sap in the Phloem is under high pressure. Eg: Photosynthesising leaf.

Sink- A part of the plant that is using Sucrose and therefore the sap in the Phloem is at low pressure.Eg: Growing root.

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Evidence explained

 Explain why?

1. When a plant is put into an atmosphere containing CO2 made from radioactive C14 radioactive sucrose appears in the phloem?  Because the carbon in CO2 is incorporated into the sucrose produced.

2. a) If a ring of bark is cut away around a tree trunk, swelling will form above/below the ring.- Swelling will form above the ring as sugars cannot pass where the ring (and therefore phloem) has been removed.  This will lower the water potential in this region and cause water to enter by osmosis causing further swelling.

 b) Above the ring in summer and below in spring?- Above in summer as leaves are the main source and sucrose is transported down to the roots for storage. Below in spring as the roots are the main source and sucrose is transported up from the roots storage.

c) Transpiration will continue as normal?- Removing a ring of bark removes the phloem only.  The xylem is left intact.

3. A poison which stops respiration will cause Translocation to stop but not Transpiration?Respiration produces ATP which is required for active loading of sucrose into the phloem.  If respiration stops then there will be no ATP for active loading and translocation will cease.Transpiration relies on the evaporation of water from the leaves and does not require ATP.

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1.  Companion cells use ATP to actively transport Hydrogen ions out of the cell into the surrounding tissue

2. This sets up a concentration gradient so the hydrogen ions diffuse back into the companion cells through a protein that acts as a carrier for both hydrogen ions and sucrose at the same time.

3. The sucrose molecules are carried through this co-transporter protein into the companion cell against the concentration gradient

4. The sucrose molecules can then move from the companion cell into the sieve tube, through the plasmodesmata that connect them. This is called active loading.

5. Sucrose lowers the water potential of the sieve tube element

6. Causes water to enter by osmosis from the xylem which increases the hydrostatic pressure and forces the water (and sucrose) along the phloem.

7. When sucrose is removed from the phloem at a sink, the water potential increases which causes water to leave by osmosis and decreases the hydrostatic pressure creating a pressure gradient

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How to sample phloem sap

How to sample phloem sap? Aphids feed on phloem sap. They do this by inserting their sharp mouthpiece (stylet) into the phloem sieve elements on the outer edge of the stem.

If you allow an aphid to feed like this, its head can be easily cut off leaving the stylet inserted into the phloem.

As the hole formed is only very small, translocation continues. Phloem sap (sucrose) will  ooze out of the stylet by mass flow and can be easily collected for analysis.

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Summary Questions

Q. Describe the evidence that supports the theory that translocation occurs in the phloem.

 When a plant is supplied with labelled radioactive carbon dioxide labelled carbon can be observed in the phloem soon after being supplied to the plant. The rate of flow of sugars in the phloem is higher than diffusion. An insect such as an aphid feeds by inserting its proboscis (mouth parts) into the phloem

 Q. State what is meant by the terms source and sink.

 Source  = site where sucrose / sugars are loaded into phloem 

Sink = site where sucrose / sugars are unloaded / removed from phloem

Q. Suggest reasons why a stem would swell above an area where a ring including the phloem had been removed.

  • Sugars cannot pass the cut
  • Accumulating sugars decrease the water potential
  • Causing water to move into cells
  • Damage triggers increased cell division
  • To produce cells to store sugars
  • Cut causes infection
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