Transport in Plants and Plant Water Relations
Water movement
The exam tests water potential, osmosis, plasmolysis, imbibition, root pressure, cohesion-tension, transpiration and phloem transport.
Core rules
- Water potential ψw = ψs + ψp; pure water at atmospheric pressure is zero, solutes lower ψs, and turgor raises ψp.
- Water moves from higher water potential to lower water potential.
- Osmosis is diffusion of water across a semipermeable membrane; plasmolysis occurs when a cell loses water in a hypertonic solution.
- Imbibition is water absorption by colloids such as seed coats.
- Root hairs absorb water; the apoplast and symplast are parallel pathways.
- Ascent of sap is explained mainly by cohesion-tension and transpiration pull; root pressure contributes in some plants.
- Stomatal opening involves K+ uptake into guard cells, water entry and turgor increase.
- Phloem transport follows the pressure-flow hypothesis from source to sink.
Worked example
A cell with ψw = -0.8 MPa is placed in a solution with ψw = -0.3 MPa. Reasoning: -0.3 is higher than -0.8, so water enters the cell. If the cell had ψw = -0.3 MPa and the solution -0.8 MPa, water would leave and the cell would plasmolyse.
Traps
Do not compare solute concentration alone; compare water potential. Transpiration pull, not root pressure, is the main force in tall trees. Xylem transport is mainly upward; phloem is source-to-sink and can be bidirectional. Guttation is not transpiration.