More solute does not always mean water moves in
- The idea
- Predict
- See it
- Why
- Check
The idea
“Water moves into the cell with more solute, or out of the cell with more pressure.”
More solute does pull water in, and pressure does push it out. Both happen at once.
The short version
The mistake: Water moves into the cell with more solute, or out of the cell with more pressure.
What's actually true: Water moves from higher to lower total water potential, which is solute potential plus pressure potential.
- Solute potential is zero for pure water and drops as solute is added. Pressure potential is the wall's push, positive in a turgid cell.
- Water potential is their sum. Water moves from higher water potential to lower, so compare the totals.
- A cell with positive pressure still gains water from pure water, as long as its total is below zero.
On the exam: Add solute and pressure potential for each side. Water moves toward the lower total.