A-Level Biology AQA Notes

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Mass Transport in Plants

The xylem transports water & mineral ions up the plant against gravity
Water evaporates from the leaves creating tension (transpiration), and the cohesive nature of water moves the whole column of water up the xylem (cohesion-tension theory)
The rate of transpiration is affected by: light, temperature, humidity & wind.
The phloem transports assimilates from sources to sinks via translocation
Sucrose is actively transported into the companion cells and moves via diffusion into the sieve tube followed by water. Assimilates move from area of high to low pressure (mass flow). At the sink the solutes are removed, water leaving by osmosis.
To track the movement of sugars through the phloem, scientists’ radioactive isotopes are used in tracer experiments with radioactive isotopes
Ringing- removal of the bark and phloem, theoretically prevents translocation to the sinks below the ring

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