Membrane Transport in a Salt-Tolerant Crop
A researcher compares two cultivars of a salt-tolerant grass. Cultivar A grows in soil with a solute potential of -0.8 MPa; cultivar B grows in soil at -0.4 MPa. Root hair cells of both cultivars have an internal solute potential of -1.2 MPa at dawn. By midday, cultivar A cells accumulate proline until internal solute potential reaches -1.6 MPa, while cultivar B cells accumulate no new solutes. A tracer experiment shows that water crosses both root hair membranes faster than expected from simple diffusion alone, and that this rate drops sharply when a mercury compound (which blocks aquaporins) is added. The researcher measures turgor pressure and finds cultivar A cells remain turgid at midday while cultivar B cells lose turgor.
1. Explain, using water potential, why cultivar B cells lose turgor at midday while cultivar A cells do not.
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Water moves from higher to lower water potential. Water potential = solute potential + pressure potential.
Cultivar B at midday: external solute potential is -0.4 MPa, internal is -1.2 MPa. The gradient favours water entry, but as water enters, pressure potential rises and opposes further entry. Turgor depends on the cell maintaining a positive pressure potential that offsets the internal solute potential. If transpiration demand exceeds supply, water leaves faster than it enters, pressure potential falls toward zero, and turgor is lost.
Cultivar A at midday: proline accumulation lowers internal solute potential to -1.6 MPa. This steeper gradient (from -0.8 MPa outside to -1.6 MPa inside) maintains water inflow against transpirational loss, so pressure potential stays positive and cells remain turgid.
The key judgement: turgor is not set by solute potential alone but by the balance between solute potential and pressure potential, and that balance shifts with the external environment and transpiration.
2. The tracer data show water movement is partly protein-mediated. Identify the proteins involved, state the evidence supporting this, and predict the effect of the mercury compound on cultivar A's ability to maintain turgor.
Read the worked answer
The proteins are aquaporins - channel proteins that facilitate osmosis across membranes.
Evidence: water crossed faster than simple diffusion predicts, and the rate fell sharply when mercury was added. Mercury binds sulfhydryl groups on proteins, blocking aquaporin channels, so a protein-dependent component is demonstrated.
Prediction for cultivar A: blocking aquaporins reduces membrane water permeability, so water cannot enter fast enough to offset transpirational loss. Even with proline lowering solute potential to -1.6 MPa, the rate of water uptake falls, pressure potential declines, and turgor is reduced or lost. Note the solute gradient still exists, so some water movement continues by simple diffusion through the lipid bilayer - the effect is on rate, not on the direction of flow. A strong answer distinguishes the thermodynamic driving force (water potential gradient) from the kinetic pathway (aquaporins).