7. Transport in Plants - Phloem

Question 1

What is the function of the phloem?

Answer 1

Its a tissue which transports organic solutes (sugars like sucrose) around the plant.

Question 2

What is the structure of the phloem like?

Answer 2

It formed from cells arranged in tubes.

Sieve tube elements and companion phloem tissue.

Question 3

What are sieve tube elements?

Answer 3

They are living cells that form the tube for transporting solutes.
They have no nucleus and few organelles so there is a companion cell for each sieve tube element.

Question 4

What are companion cells?

Answer 4

They carry out living functions for sieve cells eg providing energy needed for active transport of solutes.

Question 5

Define translocation?

Answer 5

Its the movement of solutes to where they’re needed in the plant.
Energy requiring process that occurs in the phloem.
It moves solutes from the sources to the sinks.

Question 6

What are solutes sometimes called?

Answer 6

Assimilates.

Question 7

What is the source?

Answer 7

Where assimilates are produced.

High concentration here.

Question 8

What is the sink?

Answer 8

Where assimilates are used up

Lower concentration here.

Question 9

EXAMPLE.

What is the source and sink of sucrose?

Answer 9

Source = leaves.
Sink = other part of the plant, especially food storage organs and meristems in the roots, stems and leaves.

Question 10

Meristems?

Answer 10

Areas of growth.

Question 11

How are enzymes involved in translocation?

Answer 11

They maintain concentration gradient from the source to the sink by changing the solutes at the sink.
eg breaking them down or making them into something else.
This makes sure there is always a lower concentration at sink rather than at source.

Question 12

EXAMPLE.

What happens in potatoes?

Answer 12

Sucrose is converted into starch in the sink, so there is always a lower concentration of sucrose at the sink than inside the phloem.
Makes sure there is a constant supply of new sucrose reaching the sink from the phloem.

Question 13

What theory do scientists use to explain how solutes are transported?

Answer 13

The Mass Flow Hypothesis

1. Source
2. Sink
3. Flow

Question 14

The Mass Flow Hypothesis

1. Source

Answer 14

Active transport is used to actively load the solutes (sucrose) from companion cells into sieve tubes of phloem at the source (leaves)
This lowers water potential inside sieve tube so water enters the tubes by osmosis from xylem and companion cells - creates high pressure at the source end of phloem.

Question 15

The Mass Flow Hypothesis

2. Sink

Answer 15

At sink end, solutes are removed from the phloem to be used up.
This increases water potential inside sieve tube, so water also leaves the tubes by osmosis.
This lowers the pressure inside sieve tubes.

Question 16

The Mass Flow Hypothesis

3. Flow

Answer 16

The result is a pressure gradient from source end to the sink end.
Gradient pushes solutes along sieve tubes towards sink.
When they reach sink the solutes will be used (respiration) or stored (as starch).
Higher conc of sucrose at source=higher rate of translocation.

Question 17

What organelles are present in companion cells?

Answer 17

Mitochondria.
Meaning they make lots of ATP.
ATP is needed to actively load solutes into the phloem at source.

Question 18

What is the water potential like at the source and the sink?

Answer 18

Source - low water potential

Sink - High water potential

Question 19

What is the pressure like at the source and the sink?

Answer 19

Source - High pressure

Sink - Low pressure.

Question 20

Supporting Evidence 1

Answer 20

If a ring of bark is removed from a woody stem, a bulge forms above the ring. Fluid in bulge has higher concentration of sugars than the fluid from below the ring. Sugars can’t move past area where bark has been removed - evidence of downward flow of sugars.

Build up of sugar above ring causes a decrease in water potential so water moves into cells adding to bulge.

Question 21

Supporting Evidence 2

Answer 21

Pressure in phloem can be investigated using aphids (they pierce the phloem, then their bodies are removed leaving mouthparts behind which allows sap to flow out).
Sap flows out quicker nearer the leaves than further down the stem - evidence of pressure gradient.

Question 22

Supporting Evidence 3

Answer 22

Radioactive tracer such as radioactive carbon (14C) can be used to track the movement of organic substances in a plant.

Question 23

Supporting Evidence 4

Answer 23

If a metabolic inhibitor (which stops ATP production) is put inot the phloem, then translocation stops - this is evidence that active transport is involved

Question 24

Objection Evidence 1

Answer 24

Sugars travel to many different sinks, not just the one with the highest water potential as the model would suggest.

Question 25

Objection Evidence 2

Answer 25

Sieve plates would create a barrier to mass flow.

A lot of pressure would be needed for the solutes to get through at a reasonable rate.

Question 26

Where are the sugars made?

Answer 26

Leaves

thats why there is a donwward flow of sugars from source to sink

Question 27

how do we use radioactive tracers?

Answer 27

Supplying a part of a plant (leaf) with an organic substance that has a radioactive label - then track its movement.
Movement tracked using autoradiography.
Plant is then killed (freezing using liquid nitrogen)and then whole plant is placed on photographic film.
Wherever the film turns black the radioactive substance is present.

Question 28

What does the movement of radioactive tracers represent?

Answer 28

The translocation of the substances from source to sink over time
Autoradiography of plants killed at different times show an overall movement of solutes from the leaves towards roots.