Describe diffusion and osmosis, explain how they move substances across cell membranes, and predict the effect of osmosis on plant and animal cells

What this dot point is asking

SEAB wants you to describe diffusion and osmosis, to explain how they move substances across the cell membrane, and to predict what happens to plant and animal cells placed in different solutions. The central idea is that substances move in and out of cells without using energy when there is a difference in concentration.

The answer

Diffusion

Diffusion is the net movement of particles from a region where they are more concentrated to a region where they are less concentrated, down a concentration gradient. It happens because particles are always moving randomly, so over time they spread out evenly. Diffusion does not need energy from the cell. It is how oxygen enters cells and carbon dioxide leaves them.

Osmosis

Osmosis is a special case of diffusion that involves water. It is the net movement of water molecules from a dilute solution (lots of water) to a more concentrated solution (less water) through a partially permeable membrane. The membrane lets water through but not larger dissolved particles. Like diffusion, osmosis needs no energy from the cell.

The cell membrane

The cell membrane is partially permeable: it allows some substances through while holding others back. This control lets the cell take in what it needs (such as oxygen and water) and remove waste, while keeping important contents inside.

Effect of osmosis on cells

What happens to a cell depends on the solution it is in:

• in pure water (a dilute solution), water enters the cell by osmosis. A plant cell becomes firm (turgid) because the cell wall stops it bursting; an animal cell may swell and burst because it has no wall.
• in a concentrated solution, water leaves the cell. A plant cell becomes floppy (the contents shrink away from the wall); an animal cell shrinks.

Examples in context

Example 1. Why salad goes limp when salted. Sprinkling salt on cut salad makes the solution outside the cells more concentrated, so water leaves the cells by osmosis. The cells lose their firmness and the salad becomes limp and watery.

Example 2. How gas exchange works in the lungs. Oxygen is more concentrated in the air in the lungs than in the blood, so it diffuses into the blood. Carbon dioxide is more concentrated in the blood, so it diffuses the other way into the lungs to be breathed out. Both movements are diffusion down a concentration gradient.

Try this

• Cue. Define osmosis. The net movement of water from a dilute to a more concentrated solution through a partially permeable membrane.
• Cue. State what happens to an animal cell placed in pure water. Water enters by osmosis and the cell may swell and burst.
• Cue. Explain why carbon dioxide leaves a respiring cell by diffusion. Its concentration is higher inside the cell, so it diffuses out down the gradient.