Transport in Plants- Phloem

What is the function of phloem tissue?

Phloem tissue transports solutes round plants. Like xylem, phloem is formed from cells arranged in tubes

What are cell types in phloem tissue?

• Sieve tube elements are living cells that form the tube for transporting solutes. They have no nucleus and few organelles

• There’s a companion cell for each sieve tube element. They carry out living functions for sieve cells, e.g. providing the energy needed for the active transport of solutes.

What is translocation?

• Translocation is the movement of solutes to where they’re needed in a plant. Solutes are sometimes called assimilates

• It is an energy-requiring process that happens in the phloem

• Translocation moves solutes from ‘sources’ to ‘sinks’. The source is where it is made the sink it where it is used up

How is the concentration gradient maintained?

• Enzymes maintain a concentration gradient from the source to the sink by changing the solutes at the sink

• This makes sure there is always a lower concentration at the sink than at the source

What is the first stage of the mass flow hypothesis?

1. Active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source

2. This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells

3. This creates a high pressure inside the sieve tubes at the source end of the phloem

What is the second stage of the mass flow hypothesis?

1. At the sink end, solutes are removed from the phloem to be used up

2. This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis

3. This lowers the pressure inside the sieve tubes

What is the third stage in the mass flow hypothesis?

1. The result is a pressure gradient from the source end to the sink end.

2. This gradient pushes solutes along the sieve tubes towards the sink

3. When they reach the sink the solutes will be used or stored

What is supporting evidence for mass flow?

• If a ring of bark is removed from a woody stem, a bulge forms above the ring. The fluid has a higher concentration of sugars than the fluid below the ring- evidence that there is a downward flow of sugars

• A radioactive tracer can be used to track the movement of organic substances in a plant

• Pressure in the phloem can be investigated using aphids. They pierce the phloem and sap is let out. Sap flows out quicker nearer the leaves than further down the stem- shows there is a pressure gradient

• If a metabolic inhibitor is put in the phloem, translocation stops- evidence active transport is involved.

What are objections against mass flow?

• Sugar travels to many different sinks, not just the one with the highest water potential as the model suggests

• The 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.

How can the translocation of solutes be demonstrated experimentally?

• Translocation of solutes can be modelled in an experiment using radioactive tracers

• This is done by supplying part of a plant with an organic substance that has a radioactive label, e.g. CO₂ containing ¹⁴C. This can be supplied to a leaf by being pumped into a container which completely surrounds the leaf

• The radioactive carbon will then be incorporated into organic substances produced by the leaf, which will be moved around the plant by translocation

What is autoradiography?

• The movement of these substances can be tracked using a technique called autoradiography

• To reveal where the radioactive tracer has spread to in a plant, the plant is killed and then the whole plant is placed on photographic film- the radioactive substance is present wherever the film turns black

What do the results of the experiment demonstrate?

It demonstrates the translocation of substances from source to sink over time- for example, autoradiographs of plants killed at different times show an overall movement of solutes from the leaves towards the roots