General Biology I · Diffusion and osmosis
Predict net movement of a solute or water across a membrane and explain when a cell gains or loses volume. Review concentration as amount per volume.
Lesson and summary
Particles move in both directions. For a permeable neutral solute, a concentration difference drives net diffusion from higher to lower concentration. At equilibrium random movement continues, but net flux is zero. Osmosis is net movement of water across a selectively permeable membrane. With equal pressure and an impermeant solute, water moves toward the more concentrated solution.
Worked example
Two compartments contain 0.10 M and 0.30 M sucrose at equal pressure. The membrane passes water but blocks sucrose. Water moves toward the 0.30 M side, increasing its volume. If the liquid levels can change, the resulting pressure difference can balance the osmotic drive before concentrations match.
Common mistake
Tonicity describes sustained effects on cell volume. A solute that crosses the membrane freely does not have the same sustained effect as a nonpenetrating solute, even at the same initial concentration.
Practice
An animal cell has more nonpenetrating solute inside than the surrounding solution. Predict its initial volume change and describe what happens to water molecules in both directions.
Practice solution
The cell initially swells. Water crosses both ways, but more enters than leaves. The surrounding solution is hypotonic to the cell.
Mini-exam · 10 points
A membrane passes water and glucose but blocks sucrose. Initially, A has 0.20 M glucose and no sucrose. B has 0.20 M sucrose and no glucose. Identify which way glucose diffuses (3 points). Which solute sustains an osmotic difference while sucrose remains unequal, and why? (7 points)
Exam answer key
Glucose initially diffuses from A to B. Sucrose sustains the osmotic difference because it is trapped on B. Glucose can redistribute, so its initial gradient cannot sustain the same effect.
Virtual lab · Predict osmosis
Predict the initial net movement of water from a concentration difference.
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Procedure
Assume equal pressure and a membrane that passes water but blocks sucrose. Start with 0.10 M on the left and 0.30 M on the right. Swap the concentrations, then make them equal. Before each run, predict the direction of net movement.
Expected results and answer check
Water moves net toward the more concentrated side under these assumptions. Swapping concentrations reverses the net direction. At equal concentrations, molecules still move both ways and net movement is zero.
This model predicts the initial direction only. It does not model pressure changes, membrane permeability rates, or actual cell volume.