State that water moves into and out of cells by osmosis through the cell membrane.

3.2 Osmosis

Water moves into and out of cells by osmosis through the cell membrane. Osmosis is the passive movement of water from an area of lower solute concentration to an area of higher solute concentration, until equilibrium is reached.

What is Osmosis?

Think of a cell as a tiny water‑sponge surrounded by a semi‑permeable membrane. The sponge can only let water in or out, not the solutes (like salt or sugars).

If the outside of the cell has more solute than the inside, water will leave the cell to dilute the outside solution. If the outside has less solute, water will enter the cell to balance the concentrations.

Key Terms

• Solute – a substance dissolved in a solvent (e.g., salt in water).
• Solvent – the liquid in which the solute is dissolved (usually water).
• Concentration gradient – the difference in solute concentration across the membrane.
• Osmotic pressure – the pressure required to stop the net movement of water.

Examples & Analogies

1. 🧪 Red blood cells in a salt solution: They shrink because water leaves the cell to balance the high external salt concentration.
2. 🌊 Freshwater fish in a saltwater aquarium: Their cells lose water and shrink, showing the effect of a higher external solute concentration.
3. 💧 Plant cells in a sugary solution: They swell and may burst because water enters the cell, increasing internal pressure.

Exam Tip Box

When answering exam questions, remember to:

• Identify the solute concentration on each side of the membrane.
• State the direction of water movement (inward or outward).
• Explain the resulting cell shape change (e.g., plasmolysis, crenation).
• Use the correct terminology (osmotic pressure, concentration gradient).

Osmosis in a Table