Component 3.2 - Structure And Transport In Phloem

Question 1

How are the end walls of the phloem different than in the xylem?

Answer 1

The end walls do not break down. Instead, the end walls and sometimes parts of the side walls are perforated in areas called sieve plates.

Question 2

What types of cells and tissue is the phloem made up of?

Answer 2

It is a living tissue and consists of several types of cells, including sieve tubes and companion cells.

Question 3

What adaptation do sieve tube elements have to make them suited to their function?

Answer 3

They lose their nucleus and most of their other organelles during development, allowing space for transporting materials.

Question 4

How is the metabolism of a sieve tube element controlled?

Answer 4

It is controlled by at least one neighbouring companion cell.

Question 5

What features of a companion cell suggest they are very biochemically active?

Answer 5

This is indicated by the large nucleus, dense cytoplasm containing much RER and many mitochondria.

Question 6

How are companion cells and sieve tube elements connected?

Answer 6

Plasmodesmata

Question 7

How is transport in the xylem different to in the phloem?

Answer 7

Products of photosynthesis are translocated from “source” to “sink”. Means unlike xylem which can only transport upwards, phloem can translocate up, down and sideways.

Question 8

What main materials does the phloem transport?

Answer 8

Sucrose and amino acids

Question 9

What is the technique for a ringing experiment?

Answer 9

Cylinders of outer bark tissue are removed from all the way around a Woody stem. This would remove the phloem. The plant would then be left some time, while it photosynthesised, then the contents above and below the ring would be analysed.

Question 10

What are the results of a ringing experiment?

Answer 10

Above the ring, there was a lot of sucrose, suggesting it had been translocated in the phloem from source to sink. Below the ring, there was no sucrose, suggesting it had been used by plant tissues but not replaced, because the ring prevented it moving downwards.

Question 11

How is a radioactive tracer and autoradiography experiment set up to investigate organic substances translocated in phloem?

Answer 11

A stem section is placed on a photographic film which is exposed, if there is a radiation source, producing an autoradiograph.

Question 12

What are the results of the autoradiography experiment?

Answer 12

A plant photosynthesis in the presence of 14C in carbon dioxide 14CO2. The position of exposure and therefore the radioactivity, coincides with the position of the phloem, because it is the phloem that translocates the sucrose made of 14CO2 in photosynthesis.

Question 13

What is the technique for using aphids to investigate the organic substances transported in phloem.

Answer 13

An aphid has a styler, this is inserted into a sieve tube and the phloem contents exudes under pressure into the stylet. The aphid was anaesthetised and removed.

Question 14

What are the results of the aphid experiment?

Answer 14

As the sap in the phloem is under pressure it exuded from the styles and was collected and analysis showed the presence of sucrose.

Question 15

What did aphid and radioactive tracer experiments show?

Answer 15

The radioactivity and therefore, the sucrose made in photosynthesis, moved at a speed of 0.5-1 m h^-1.

Question 16

Why were the results of the aphid and radioactive tracer experiment unusual?

Answer 16

The speed that the sucrose moves at is much faster than the rate of diffusion alone so some additional mechanism had to be considered.

Question 17

What is the mass flow hypothesis?

Answer 17

Suggests a passive flow of sugars from the phloem of the leaf, where there is the highest concentration (the source), to growing tissues, where there is a lower concentration (the sink).

Question 18

Describe process of mass flow?

Answer 18

In leaf cells, sucrose is made by photosynthesis. The sucrose makes the water potential very negative and water passes into the cell by osmosis.

As water enters the leaf hydrostatic pressure builds up, forcing sucrose in solution into the phloem joining source to sink.

The pressure pushes the sucrose solution down the phloem and this movement is called mass flow.

In the sink the sucrose is converted to insoluble starch so water potential is higher and then water is forced up the xylem back to the source.

Question 19

What aspects of translocation does mass flow not explain?

Answer 19

1) Rate of phloem transport is 10000 times faster than diffusion.
2) Does not take into account sieve plates
3) Sucrose and amino acids move at different rates in different directions
4) Phloem has a high oxygen consumption, translocation is slowed at low temperatures or when cyanide is applied.
5) Companion cells are very active but mass flow does not suggest a role for them.

Question 20

What do other theories suggest instead of mass flow?

Answer 20

1) Active process - cyanide and low temperatures inhibit translocation - indicates energy generated by respiration is used.
2) Protein filaments - pass through sieve pores so different solutes are carried along different routes though same sieve tube.
3) Cytoplasmic streaming - could be responsible for movement in different directions. Providing there was some mechanism to transport solutes.