Phloem transport
UNDERSTANDINGS
Plants transport organic compounds from sources to sinks
Active transport is used to load organic compounds into phloem sieve tubes at the source
High concentrations of solutes in the phloem at the source lead to water uptake by osmosis. Incompressibility of water allows transport along hydrostatic pressure gradients
Raised hydrostatic pressure causes the contents of the phloem to flow towards sinks
Translocation is the movement of organic compounds (From source to sink)
organic compounds are synthesised at the source (Photosynthetic leaves)
Compounds are delivered to the sink (For use or storage)
transported through a vascular tube (Phloem),usually transported as sucrose (Disaccharide)
Organic compounds produced at the source are actively loaded into phloem sieve tubes by companion cells
Materials pass through into sieve tubes through the plasmodesmata
organic solutes such as sucrose are loaded into sieve tube elements by companion cells
companion cell actively transports protons (Hydrogen ions) out which sets up a proton gradient across the plasma membrane
movement of protons back into the companion cell drives the transport of sucrose using a sucrose-proton co-transporter, this results in a build up of sucrose within the phloem sieve tube for transport from the source (Via active transport)
active transports moves against the concentration gradient which requires ATP
The active transport of organic compounds into the phloem by companion cells makes the sap solution hypertonic
This causes water to enter the phloem from the xylem through osmosis as water travels from high solute concentration to low solute concentration
build up of water in the phloem causes the hydrostatic pressure to increase
increase in hydrostatic pressure forces the phloem sap to move towards areas of lower pressure, thereby transporting compounds to the sink
Organic compounds unloaded by companion cells, causing sap solution at the sink to be hypotonic and water to diffuse back into the xylem
ensures that the hydrostatic pressure at the sink is always lower than the hydrostatic pressure at the source
Translocation rate
could be monitored by introducing radioactive carbon dioxide in the plant. Aphids have a stylet that extracts the sap from the phloem
aphid is killed but the sap will continue to be extracted due to hydrostatic pressure
sap is then analysed for the presence of radioactively-labelled sugars
rate of phloem transport (translocation rate) can be calculated based on the time taken for the radioisotope to be detected at different positions along the plant’s length