Biology › Plant transport and mineral nutrition

Plant transport and mineral nutrition

Unit B08 · 3 lessons.

A plant has no pump, so every movement in this unit is explained by a gradient. The first lesson follows water from the root hair cell into the xylem and explains how the column holds together as it rises. The second is the leaf, where evaporation draws the column up and guard cells control how fast water is lost. The third is the phloem, where sucrose loaded at a source moves to the sinks by mass flow.

Revise diffusion and osmosis first: water potential is used throughout, with pure water at 0 kPa and every solution negative. Phloem loading uses active transport and co-transport. AQA (3.3.4.2), OCR A (3.1.3), Cambridge International (topic 7) and Pearson Edexcel Biology B (Topic 4) examine all three lessons as AS content. Pearson Edexcel Biology A uses the xylem and phloem lessons in Topic 4 but does not examine transpiration as a topic.

Notes

Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.

• Xylem and the transpiration stream
  Root hair cells and the uptake of water and mineral ions, the apoplast and symplast routes across the cortex, the Casparian strip at the endodermis…
• Transpiration and stomatal control
  Transpiration defined and explained as a consequence of gas exchange, the effect of light, temperature, humidity and air movement on the rate, the…
• Phloem translocation and the mass-flow hypothesis
  The structure of sieve tube elements, sieve plates and companion cells, active loading of sucrose by proton pumping and co-transport, the mass flow…

Practical work

The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.

• Water relations of plant tissue: osmosis and transpiration