9.2 Phloem Transport Flashcards
What is (active) translocation?
The movement of organic compounds from source to sink.
What is the source, what are the sinks?
Source: where the organic compounds get synthesised, in the photosynthetic leaves
Sink: Where the organic compounds are stored/needed: root, fruits,seeds…
What are sieve elements in the phloem structure and the functions?
Sieve elements are long, narrow cells, which form the sieve tubes. As they have no nucleus and few organells, they maximise space for translocation.
What are companion cells in the phloem structure and their functions?
The companion cells provide metabolic support for the sieve elements, and they facilitate material exchange at sources and sinks. Furthermore, they have appropriate transport proteins within the membrane, as well as mitochondria for active transport energy provision.
How are organic compounds loaded into sieve elements/the phloem?
The companion cells actively pump protons out, so that a concentration gradient builds up. As the H+ flow back in, they pass through a co-transport protein, which requires that the proton takes a sucrose along.
How does the sap travel to the sink?
All the solutes (sucrose etc) make the phloem hypertonic, so water will want to move in from the xylem. As water is incompressible, this increases the hydrostatic pressure, which forces the sap to move towards areas of lower pressure (=sink).
How does the phloem unload at the sink?
The solutes are unloaded by companion cells, making the sap hypotonic again, so the water there moves back to xylem via osmosis, ensuring that the hydrostatic pressure is always lower at the sink.
What are stylets of aphids?
The mouthpiece of aphids, which are insects that eat sap. The stylet works like a tube, so when it is severed, the sap will continue to flow out due to the hydrostatic pressure.
Outline the experiment to determine the phloem transport rate using aphid stylets.
A plant is put in radioactive CO2, so that it makes radioactive sucrose. Aphids are encouraged to eat the sap and once they start, their stylet is severed. The sap that comes out of the stylets is analyzed for radioactivity and the time it takes for the radioactivity to travel from stylet 1 to stylet 2 is the flow rate.
