9.2 (Structure of phloem sieve tubes (Cell wall that resists high…
Structure of phloem sieve tubes
Cell wall that resists high pressures inside sieve tube
sieve plate - cross wall that strengthens the sieve tube with pores that allow sap to pass through in either direction
Lumen of sieve tube with no organelles so that the flow of sap is not interrupted
Cell membrane with proton pumps and co-transporter proteins
The function of the phloem
Transports organic compounds like sugars
Takes place in phloem sieve tubes
Sugars and amino acids are loaded into phloem sieve tubes by active transport in parts of the plant called sources
These are then unloaded at sinks which need the nutrients
Measuring phloem transport rates
Aphids have long piercing mouthparts called stylets.
These are inserted into leaves, pushing inwards through plant tissues until the stylet pierces a sieve tube
The high pressure in the phloem sap pushes the sap out through the stylet
To sample phloem sap, the aphid is cut off from its stylet when it has started feeding and the stylet is left as a very narrow tube through which sap continues to emerge.
Radioactive CO2 -14 will be loaded to the leave, which will eventually lead to the phloem.
The time taken for radioactive sucrose to emerge from severed aphid stylets at different distances from the leaf can be used to measure the rate of translocation.
Loading phloem sieve tubes
The main sugar carried by phloem sieve tubes is sucrose, and it is loaded via active transport into the phloem, but not by pumping sucrose molecules directly.
Instead protons are pumped out of phloem cells by active transport to create a proton gradient and co-transporter proteins in the membrane of phloem cells use this gradient to move the sucrose molecules into the cell by simultaneously allowing protons out down the concentration gradient
To speed up the process adjacent phloem cells also absorb sucrose by co-transport and then pass it to sieve tubes via plasmodesmata