Biology Set 3: Osmosis & Diffusion

What is diffusion?

The movement of molecules from high concentration to low concentration — down the concentration gradient.

Does diffusion require energy?

No. It is a passive process.

What drives diffusion?

The concentration gradient — molecules naturally spread from where there are more of them to where there are fewer.

Give one example of diffusion in the body.

Oxygen moving from the lungs into the bloodstream, or CO2 moving from the blood into the lungs.

What is the difference between osmosis and diffusion?

Diffusion is the movement of any molecule down a concentration gradient. Osmosis is specifically the movement of water across a selectively permeable membrane.

What moves during osmosis?

Water molecules only.

What does osmosis require that diffusion does not?

A selectively permeable membrane.

In which direction does water move during osmosis?

From low solute concentration (high water concentration) to high solute concentration (low water concentration).

What is a hypotonic solution?

A solution with lower solute concentration than the cell. Water moves into the cell — the cell swells.

What is a hypertonic solution?

A solution with higher solute concentration than the cell. Water moves out of the cell — the cell shrinks.

What is an isotonic solution?

A solution with equal solute concentration to the cell. No net movement of water — the cell stays the same size.

What happens to a plant cell in a hypotonic solution?

Water enters the cell by osmosis. The cell swells and becomes turgid. The cell wall prevents it from bursting. This is the normal healthy state for plant cells.

What happens to a plant cell in a hypertonic solution?

Water leaves the cell by osmosis. The cell membrane pulls away from the cell wall — this is called plasmolysis. The cell becomes flaccid.

What happens to an animal cell in a hypotonic solution?

Water enters by osmosis and the cell swells. Without a cell wall it can burst — this is called lysis.

What happens to an animal cell in a hypertonic solution?

Water leaves by osmosis and the cell shrinks and shrivels — this is called crenation.

What is turgor pressure?

The pressure of water pushing outward against the cell wall in a plant cell. It keeps the plant firm and upright.