B7. Transport in Plants - Phloem Flashcards
Structure and function of the phloem
What are solutes and what is an example?
What does phloem tissue consist of?
What is this and how is it adapted?
What does it have and how is it adapted what does this do?
Figure 1: A phloem tissue
Solutes are dissolved substances. Phloem tissue transports organic solutes (mainly sugars like sucrose) round plants. Like xylem, phloem is formed from cells arranged in tubes.
Sieve tube elements and companion cells are important 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, so 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?
What are example of solutes and what are they also called?
What is different about it to transpiration?
What places are involved in it and what do these mean?
Example being a leaf?
Translocation is the movement of solutes (e.g. amino acids and sugars like sucrose) to where they’re needed in a plant. Solutes are sometimes called assimilates. It’s an energy-requiring process that happens in the phloem. Translocation moves solutes from ‘sources’ to ‘sinks’. The source is where assimilates are produced (so they’re at a high concentration there).
The sink is where assimilates are used up (so they’re at a lower concentration there).
Example
The source for sucrose is usually the leaves (where it’s made), and the sinks are the other parts of the plant, especially the food storage organs and the meristems (areas of growth) in the roots, stems and leaves.
What do enzymes do and how?
What does this make sure of?
Example being a potato?
Enzymes maintain a concentration gradient from the source to the sink by changing the solutes at the sink (e.g. by breaking them down or making them into something else). This makes sure there’s always a lower concentration at the sink than at the source.
Example
In potatoes, sucrose is converted to starch in the sink areas, so there’s always a lower concentration of sucrose at the sink than inside the phloem. This makes sure a constant supply of new sucrose reaches the sink from the phloem.
Tip: Assimilates are …
Tip: Assimilates are substances that become incorporated into the plant tissue.
The mass flow hypothesis (3 steps with examples throughout)
- Source
- Active transport is used to actively load the solutes (e.g. sucrose from photosynthesis) from companion cells into the sieve tubes of the phloem at the source (e.g. the leaves).
- This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells.
- This creates a high pressure inside the sieve tubes at the source end of the phloem.
- Sink
- At the sink end, solutes are removed from the phloem to be used up.
- This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis.
- This lowers the pressure inside the sieve tubes.
- Flow
- The result is a pressure gradient from the source end to the sink end.
- This gradient pushes solutes along the sieve tubes towards the sink. When they reach the sink the solutes will be used (e.g. in respiration) or stored (e.g. as starch).
- The higher the concentration of sucrose at the source, the higher the rate of translocation.
Tip: Companion cells contain many ____________, which means they can make lots of ___. ____ is needed to …
Tip: Companion cells contain many mitochondria, which means they can make lots of ATP. ATP is needed to actively load the solutes into the phloem at the source.
Figure 3: How the mass flow hypothesis works.
Mass flow evidence - Supporting evidence (4 things)
- If a ring of bark (which includes the phloem, but not the xylem) is removed from a woody stem, a bulge forms above the ring-see Figure 4. The fluid from the bulge has a higher concentration of sugars than the fluid from below the ring. This is because the sugars can’t move past the area where the bark has been removed - this is evidence that there can be a downward flow of sugars.
- Pressure in the phloem can be investigated using aphids (they pierce the phloem, then their bodies are removed leaving the mouthparts behind, which allows the sap to flow out… gruesome). The sap flows out quicker nearer the leaves than further down the stem - this is evidence that there’s a pressure gradient.
- A radioactive tracer such as radioactive carbon (C) can be used to track the movement of organic substances in a plant (see below).
- If a metabolic inhibitor (which stops ATP production) is put into the phloem, then translocation stops - this is evidence that active transport is involved.
Mass flow evidence - Objections (2 things)
- Sugar travels to many different sinks, not just to 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.
Evidence from radioactive tracers
Translocation of solutes can be modelled in an experiment using radioactive tracers. This can be done by supplying part of a plant (often a leaf) with an organic substance that has a radioactive label, then tracking its movement.
The movement of these substances can be tracked using a technique called _______________. To reveal where the radioactive tracer has spread to in a plant, the plant is ______(e.g. by freezing it using liquid nitrogen) and then the whole plant (or sections of it) is placed onto photographic film - wherever the film turns ______, the radioactive substance is present
The results demonstrate the translocation of substances from source to sink over time - for example, autoradiographs of plants killed at different times show an _________ ___________of ________(e.g. products of photosynthesis) from the leaves towards the roots.
Translocation of solutes can be modelled in an experiment using radioactive tracers. This can be done by supplying part of a plant (often a leaf) with an organic substance that has a radioactive label, then tracking its movement.
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 (e.g. by freezing it using liquid nitrogen) and then the whole plant (or sections of it) is placed onto photographic film - wherever the film turns black, the radioactive substance is present
The results demonstrate 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 (e.g. products of photosynthesis) from the leaves towards the roots.
Correlation and causal relationships - Example
Scientists carried out a ringing experiment on a particular species of woody plant. A varying amount of bark was left connecting the upper and lower parts of the stems (see below). The plants were left for 24 hours, then the amount of carbohydrate in the plant below the ringing was measured.
Correlation?
Conclusions?
Correlation
The results in the table above show a positive correlation - as the width of the bark strip remaining increased, the amount of carbohydrate transported to the lower part of the stem (i.e. below the ringing) also increased.
Conclusions
From the results, you might conclude that removing the bark caused a reduction in the amount of carbohydrate transported down the stem. This may be because removing more bark, removes more phloem, which reduces the amount of carbohydrate that can be transported. This provides evidence in support of the mass flow hypothesis because the phloem is transporting carbohydrates down from a source in the leaves to a sink in the roots.
However, you have to be careful when drawing conclusions, especially when there’s not much data. The results don’t prove that there is a downward flow of sugars in the phloem - there could be other factors affecting the results. For example, it could be that the sugars are actually transported in the xylem, but the xylem tissue was accidently damaged when the bark was removed. The experiment has also been carried out on only one species of plant, so you can’t conclude that this would be the case for all plant species.
However, so many studies have now been done on mass flow, that the correlation shown by this experiment is accepted to be a causal relationship, i.e. removing more of a plant’s phloem causes less carbohydrate to be transported downwards, towards a plant’s roots.
