Transporting in Plants - Phloem Flashcards
Give two important cell types in the phloem
- Sieve tube elements
2. Companion cells
What are sieve tube elements and companion cells?
Sieve tube elements are living cells that form the tube for transporting solutes. They have no nucleus and few organelles so there’s a companion cell for each sieve tube element. The companion cells carry out living functions for sieve cells
What is translocation?
Translocation is the movement of solutes to where they’re needed in a plant
What are solutes also called?
Assimilates
What does translocation require to happen?
Energy
Where does translocation moves solutes?
Translocation moves solutes from ‘sources’ to ‘sinks’. The source of a solute is where it’s made, so it’s at a high concentration there. The sink is where solutes are used up, so there’s a low concentration there
How is a concentration gradient maintained between the source and sink?
Enzymes maintain a concentration gradient from the source to the sink by changing the solutes at the sink (by breaking them down or making them into something else). This ensures there’s a lower concentration at the sink than at the source
What theory explains how solutes are transported from source to sink
The mass flow hypothesis
Explain the mass flow hypothesis
Active transport is used to actively load the solutes from companion cells into the sieve tubes of the phloem at the source. This lowers the water potential inside the sieve tubes, so water enters the tubes via osmosis from the xylem and companion cells. This creates a high pressure inside the sieve tube at the source end of the phloem. At the sink end, the solutes are removed from the phloem to be used. This increases the water potential inside the sieve tubes, so water leaves the tubes via osmosis. This lowers the pressure inside the sieve tubes. The results is a pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes towards the sink. When the solutes reach the sink they will be used up
What are the four main evidence points for supporting mass flow?
- If a ring of bark is removed from a woody stem, a bulge forms above the ring. the fluid from the bulge has a higher concentration of sugars than the fluid from below the ring - this is evidence of a downwards 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. The sap flows out quicker nearer the leaves than further down the stem - this is evidence of a pressure gradient
- If a metabolic inhibitor is put into the phloem, then translocation stops - evidence the active transport is involved
What are the two main point objecting mass flow?
- Sugar travels to many different sinks, not just the one with the highest water potential as the model would suggest
- 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 radioactive tracers be used to model translocation of solutes in plants?
- Carbon dioxide containing the radioactive isotope 14C is supplied to a single leaf by being pumped into a container that 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
- The movement of these substance is tracked using autoradiography. To see where the radioactive tracer has spread, the plant is killed and place on photographic film - the radioactive substance is present wherever the film turns black
How do the results of autoradiography show translocation?
The results demonstrate the translocation of substances from source to sink over time. Autoradiographs of plants killed at different times show an overall movement of solutes from the leaves towards the roots
