Is osmosis active or passive?

Osmosis is passive, which means it does not require energy from respiration. Water moves down a water potential gradient on its own. This is different from active transport, which moves particles against the gradient and does need energy in the form of ATP.

Why does a plant cell not burst in pure water?

When a plant cell takes in water by osmosis it becomes turgid, but the strong cellulose cell wall resists further expansion and stops the cell bursting. An animal cell has no cell wall, so in pure water it can swell and burst, a process called lysis.

Osmosis and Diffusion Explained: IGCSE Biology (0610)

Q: What is the difference between osmosis and diffusion?

Diffusion is the net movement of any particles from a higher to a lower concentration. Osmosis is a special case that only involves water molecules moving across a partially permeable membrane, from a dilute solution to a more concentrated one. Osmosis is really just the diffusion of water through a membrane.

Short answer: Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated, until they are evenly spread. Osmosis is a special type of diffusion that involves only water molecules moving across a partially permeable membrane, from a dilute solution to a more concentrated one. Both are passive, meaning they need no energy. The key exam trap is remembering that osmosis is always about water crossing a membrane.

Why these two topics matter

Osmosis and diffusion turn up across the whole 0610 course: in the lungs, the small intestine, the kidneys, root hair cells, and plant support. Get the definitions precise now and you unlock marks in many different questions later. Examiners are strict about the exact wording, so this article gives you the definitions Cambridge accepts.

Diffusion, defined properly

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random movement.

A few points that earn marks:

The word net matters. Particles move both ways, but overall more move from high to low concentration.
It happens because particles (in gases and liquids) are always moving randomly.
It is passive, so it does not use energy from respiration.
It continues until the particles are evenly distributed, reaching equilibrium.

Everyday example: a drop of food colouring spreading through still water. Biological example: oxygen diffusing from the air in the alveoli into the blood, and carbon dioxide diffusing the other way.

Osmosis, defined properly

Osmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a concentrated solution), through a partially permeable membrane.

Notice what is different from diffusion:

It only involves water molecules.
It must happen across a partially permeable membrane (sometimes called semi-permeable). This membrane lets water through but blocks larger solute molecules like sugar.
The direction is from dilute to concentrated in terms of solute, which is the same as high to low water potential.

Water potential is simply a measure of how free the water molecules are to move. Pure water has the highest water potential. Add solute and the water potential falls.

Osmosis vs diffusion at a glance

Feature	Diffusion	Osmosis
What moves	Any particles (gases or dissolved solutes)	Water molecules only
Membrane needed	No	Yes, partially permeable
Direction	High to low concentration of that particle	High to low water potential (dilute to concentrated)
Energy required	None (passive)	None (passive)
Example	Oxygen into blood at the alveoli	Water into a root hair cell

What happens to cells in different solutions

This is a favourite exam topic, and plant and animal cells behave differently because of the cell wall.

Plant cells:

In pure water or a dilute solution, water enters by osmosis. The cell swells and pushes against the cell wall. It becomes turgid. The rigid cellulose wall stops it bursting, and turgor gives the plant support.
In a concentrated solution, water leaves by osmosis. The cell becomes flaccid, and if a lot of water leaves, the membrane pulls away from the wall. This is plasmolysis, and the cell is plasmolysed.

Animal cells (no cell wall):

In pure water, water enters and the cell can swell and burst, called lysis or haemolysis in red blood cells.
In a concentrated solution, water leaves and the cell shrinks and looks crinkled (crenated).

This is why the concentration of blood plasma is kept carefully controlled: it stops red blood cells bursting or shrivelling.

A worked osmosis example

Imagine a potato cylinder placed in three beakers overnight:

Distilled water: the water potential outside is higher than inside the cells, so water moves in. The cylinder gains mass and becomes firmer.
A concentrated sugar solution: the water potential outside is lower, so water leaves the cells. The cylinder loses mass and becomes floppy.
A solution of equal water potential: no net movement, so the mass stays about the same.

If an exam asks you to explain a change in mass, always name osmosis, state the direction of water movement, and link it to the water potential difference.

Where active transport fits in

Diffusion and osmosis are passive, but sometimes a cell needs to move particles against the concentration gradient, from low to high concentration. That requires active transport, which uses energy from respiration (ATP) and carrier proteins in the membrane. A classic example is root hair cells absorbing mineral ions from soil where the ion concentration in the soil is lower than inside the root. This does not happen by diffusion, because diffusion cannot go against the gradient.

Quick revision checklist

Diffusion: any particles, no membrane needed, down the gradient, passive.
Osmosis: water only, partially permeable membrane, dilute to concentrated, passive.
Turgid, flaccid, plasmolysis for plant cells; lysis and crenation for animal cells.
Active transport when movement is against the gradient and needs energy.

Learn the exact wording of these definitions and half the battle is won, because examiners reward the precise terms.

If you want to test whether you have the wording right, ExamPal is an AI tutor that already knows the 0610 syllabus and checks your definitions and answers against the Cambridge mark scheme, so you find out exactly which words you are missing.