Lesson 2 of 4 / Movement of substances
Osmosis
Which way will water move across a cell membrane?
In this lesson: Predict the net movement of water using water potential and membrane permeability.
About 4 min
The key ideaOsmosis is the net movement of water molecules from higher to lower water potential through a partially permeable membrane.
Water potential describes the tendency of water to move from one place to another. At the same pressure, a more dilute solution has a higher water potential than a more concentrated solution.
Explore the idea
Which way will water move overall?
Higher water potentialInside
Lower water potential
The membrane lets water through, but not these solute particles.
Net movement into the cell.The outside solution has a higher water potential. Water crosses the partly permeable membrane in both directions, with a net movement into the cell.
- Water molecule: can cross
- Solute particle: cannot cross here
- Partly permeable membrane
The examples compare the same solute at equal pressure. More dissolved solute lowers water potential under these conditions. The picture shows starting conditions; it does not model changes in pressure or volume. Particle numbers and arrow widths are illustrative.
Explanation
A partially permeable membrane allows some substances to pass through more easily than others. In a simple osmosis model, water can cross the membrane but the dissolved solute cannot. The membrane separates two solutions.
For solutions at the same pressure, dissolving more solute lowers the water potential. A dilute solution has a higher water potential than a more concentrated solution. Compare the two solutions before deciding the direction of water movement.
Water molecules move through the membrane in both directions. When water potential differs, the net movement is from higher to lower water potential. Osmosis does not require energy supplied by respiration.
If the water potentials are equal, water still moves both ways, but there is no net movement. Without a partially permeable membrane, the movement described is not osmosis.
Step by step
- 1
Identify the membrane
The question must involve a partially permeable membrane separating the solutions.
- 2
Compare water potential
For the simple solutions here, at the same pressure, the more dilute solution has the higher water potential.
- 3
Explain the net movement of water
Water moves by osmosis from higher to lower water potential through the membrane.
Worked example
A membrane bag in sugar solution
A bag contains a dilute sugar solution. It is placed in a more concentrated sugar solution. The membrane allows water, but not sugar, to pass. Predict what happens to the bag.
One way to explain it
The bag loses water and its mass decreases. Water moves by osmosis from the higher water potential inside the bag to the lower water potential outside, through the partially permeable membrane.
Why this answer works
- Compare the two water potentials.
- Name water as the substance moving.
- Connect water loss to the decrease in mass.
Is this true? "The more concentrated sugar solution has the higher water potential."
For these solutions at the same pressure, more dissolved sugar means lower water potential. Be precise: solute concentration and water potential change in opposite directions.