A-Level Biology OCR Notes

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The Phloem

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.

Water Transport in Plants

The xylem transports water & mineral ions up the plant against gravity. It is made of dead cells and lignified.
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)
Water moves up the xylem due to capillary actin, root pressure and transpiration pull
The rate of transpiration is affected by: light, temperature, humidity & wind.
Xerophytes are plants adapted to living in dry conditions. They can reduce water loss by having : hairs, waxy cuticle, small
Hydrophytes are plants adapted to living in water. Their adaptations include: stomata on the upper epidermis, using hydathodes, large air spaces for buoyancy and oxygen diffusion.

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