Topic 9.2 Phloem Transport Flashcards
What is translocation
The movement of organic compounds from source to sink
What is a source
Where organic compounds are synthesized (eg leaves)
What are sinks
Where compounds are delivered to for use or storage (eg roots)
Phloem composed of two main cells what are they
Sieve element cells and Companion cells
Sieve element cells
No nuclei to maximize the amount of material for translocation
Thick and rigid walls to withstand hydrostatic pressure which facilitates flow
Companion cells
Provide metabolic support for sieve element cells (because have no nuclei) and facilitate loading and unloading of materials.
Lots of mitochondria to fuel active transport of materials between sieve element cells and source or sink
What is phloem loading
Organic compounds produced at source loading into phloem sieve tubes by companion cells
This is an active transport process requiring ATP expenditure
Factors affecting translocation rate
Rate of photosynthesis
Rate of cellular respiration
Rate of transpiration
MASS FLOW at sink
Solutes in phloem unloaded into sinks leading to sap solution to have low solute concentration
Water is then drawn out the phloem and back into the xylem
This keeps hydrostatic pressure lower at the sink than the source meaning sap always move towards the sink
APHID EXPERIMENT
A plant grown sealed containing radioactive carbon dioxide
Leaves conver this CO2 into radioactive susgars
Aphids then encouraged to feed on the phloem sap then the stylet is chooped off and sap continues to flow
Sap is then analysed for radioactive sugars and phloem transport rate can be measured by time taken to detect these radioactive sugars.
MASS FLOW at source
Active transport of solutes make solution have low solute concentration.
This causes water to be drawn from the xylem causing hydrostatic pressure to increase forces sap to move towards sink
