Diffusion and Osmosis: Which Way Does the Water Go?

A dried raisin put in water swells. Salt on a slice of cucumber draws water out. One rule explains both. This is a brief note on How Substances Move In and Out of Cells.

Three movements

• Diffusion. Particles spread from where there are more to where there are fewer. No energy is needed. It is faster when warmer, and its time grows with the square of the distance.
• Osmosis. The movement of water through a selectively permeable membrane. No energy is needed.
• Active transport. Particles moved from fewer to more, by carrier proteins, using energy from respiration.

One rule for water

Water moves towards the side with more dissolved substance.

A plant cell's wall stops it from bursting: it becomes turgid. In a strong solution its membrane pulls away from the wall: plasmolysis.

A worked example

A potato chip of $8.0\ \text{g}$ is $7.2\ \text{g}$ after an hour in a solution.

1. Change. $7.2\ \text{g} - 8.0\ \text{g} = -0.8\ \text{g}$.
2. Percent. $\dfrac{-0.8\ \text{g}}{8.0\ \text{g}} \times 100\% = -10\%$.
3. Meaning. The chip lost water, so the solution was hypertonic to its cells.

Practice with new numbers

A program to try

The program applies the one rule to three solutions outside a cell.

Mistakes to avoid

• "Water moves towards the weaker solution." It moves towards the stronger one.
• "Osmosis needs energy." Neither diffusion nor osmosis does; only active transport does.
• "A plant cell bursts in pure water." Its wall holds it.
• Dividing by the wrong mass. Always divide by the mass before.

Check yourself

1. A cucumber slice is put in strong salt water. What happens to its cells?

2. Why is a patient given a salt solution of the right strength, not pure water, into the blood?

3. A molecule takes 1 s to diffuse across a cell. About how long across a cell 4 times as wide?

4. Which of diffusion, osmosis and active transport needs energy, and why?