Transport in plants - phloem Flashcards
What does the phloem transport?
Organic solutes
What are solutes?
Dissolved substances
What are the main solutes that the phloem transports?
Sugars
How is the phloem arranged?
In long tubes
What three parts make up the phloem?
Sieve tube elements
Sieve plates
Companion cells
What are sieve tube elements?
living cells which form the tube for transport
What does every sieve tube element have paired with it?
Companion cell
What is the purpose of a companion cell?
To carry out all functions for the sieve tube elements
Why do sieve tube elements need companion cells?
They have no nucleus
What do companion cells provide?
Energy for transporting the solutes
What is the purpose of sieve plates?
Not known
What is translocation?
The movement of solutes to where its needed
During translocation, where do the solutes move from? Which way along the concentration gradient?
Source to sink
High to Low
How are enzymes used in translocation?
They maintain a concentration gradient from source to sink by changing solutes at the sink so that theres always a lower concentration gradient at the sink
What is the best supporting theory for translocation?
Mass flow hypothesis
What happens at the source according to the mass flow hypothesis?
1) Active transport actively loads solutes from companion cells into the sieve tubes of the phloem
2) This lowers the water potential inside the sieve tubes
3) Water enters via osmosis from xylem and companion cells
4) Creating a high pressure inside the sieve tubes at the source end of the phloem
What happens at the sink end according to the mass flow hypothesis?
1) Solutes are removed from the phloem increasing water potential inside the sieve tubes
2) Water leaves the tubes by osmosis
3) Lowers the pressure inside the sieve tubes
The higher the concentration of sucrose at the source…
The higher the rate of translocation
What does translocation create? What does this cause?
Pressure gradient from the source end to the sink
Causing the solutes to be pushed along the sieve tubes towards the sink
What do companion cells contain lots of?
Mitochondria
Why is the ATP released by companion cells needed?
To actively load the solutes into the phloem at the source
How is bark used as supporting evidence for the mass flow hypothesis?
The fluid from the bulge, created by the removal of a ring of bark, has a higher concentration of sugars than below the ring
The sugars can’t move past the area where the bark has been removed
What is evidence that there is a pressure gradient during translocation?
When pierced, the sap flows out quicker nearer the leaves than further down the stem
What is used as evidence that there is active transport involved in translocation?
If a metabolic inhibitor, which blocks ATP production, is put in then translocation stops
What is evidence against the mass flow hypothesis?
The sugar travels to many different sinks, not just the one with the highest water potential like suggested
How are sieve plates used as an argument against mass flow hypothesis?
They create a barrier so a lot of pressure would be needed
