Translocation 9.4 Flashcards
How is glucose transported?
After glucose is made by photosynthesis, it is converted into sucrose by a hydrolysis reaction to be transported. When it reaches the cells that need it, it is converted back into glucose by a condensation reaction so it can be used as a respiratory substrate or storage. It can be transported both ways.
Where does glucose get transported to?
It goes from a source to a sink in a process called translocation.
What are assimilates?
Assimilates are the products of photosynthesis produced by the sources that are then transported. Although glucose is made, the main assimilate is sucrose which makes up a big amount of phloem sap.
Main sources
These are where the assimilates come from and where the plant respires the most. they can be: green leaves or stems, food stores in seed when they germinate and storage organs at the beginning of the growth period.
Main sinks
These are where the assimilates go and where the plant respires the most. These can be: root hair cells that need to actively transport mineral ions in, meristems that are actively dividing, developing seeds
Phloem loading
This is the process of taking the assimilates from the sources and loading them into the phloem. It is an active process and there are two ways it can be done, through the symplast and apoplast pathway.
Why is sucrose transported instead of glucose?
Sucrose is the main carbohydrate as it is less reactive and won’t be easily metabolised like glucose will, so is better to be transported
Apoplast Route
This is the active process of moving assimilates to the phloem. Sucrose will travel through the cell walls and intermembrane spaces to the companion cells and sieve elements by diffusion down a concentration gradient which is maintained by the removal of sucrose from the phloem by phloem unloading.
How the assimilates are translocated
Once the assimilates have travelled through the cell walls, they will reach the companion cells. Here the sucrose will be moved in by an active process. First hydrogen ions will be actively pumped out of the companion cells by a proton pump and will decrease the concentration gradient. The hydrogen ions will then return to the companion cells by being cotransported with sucrose by a cotransporter. This will increase the concentration of sucrose inside of the companion cells as well as the sieve tube elements as the sucrose will travel through the plasmodesmata into them. Because of this increased concentration, the water carrying the rest of the assimilated will move into the companion cells by osmosis, which will increase the turgor pressure, it will then move into the sieve tube elements and will be in the phloem. Accumulation of the solutes in the phloem will lead to mass flow pushing them to areas of lower pressure which are the sinks and it can move up or down. The pressure differences lead to the assimilates being transported very quickly over many metres
How are companion cells adapted for translocation?
They have many mitochondria which will produce many ATP in order to carry out the active transport and they have a big surface area so more sucrose can be actively transported at the same time.
Phloem Unloading
This is when the sucrose in the phloem has reached the point where the sinks are or the cells that need it. Sucrose will diffuse out of the phloem and enter the surrounding cells where it will either be converted into glucose or it will diffuse into the next cell. This maintains the concentration gradient of sucrose between the phloem and the cells so more can diffuse out when needed. At the same time as the solutes diffusing out, the water potential in the phloem will increase so water will move out by osmosis. Some of the water used in translocation will travel into the transpiration stream.
Evidence for translocation
- Microscopy can be used to analyse the structure of companion cells and see how they are adapted
- If the mitochondria of the companion cells are poisoned, translocation will stop
- It moves very fast compared to if it was diffusing which suggests it’s an active process
- Aphids will penetrate the plant and drink the phloem sap, so if the aphids are transquilised and removed, you will see the phloem sap pressure and sucrose concentration is higher closer to the source than the sink and you can analyse the sap that comes out
