Osmosis vs Diffusion: The Difference That Trips Everyone Up

Quick answer: Diffusion is the net movement of any particles from a higher to a lower concentration, down a concentration gradient, with no energy needed. Osmosis is a special case of diffusion: the movement of water molecules only, from a dilute solution to a more concentrated one, through a partially permeable membrane. Active transport is the opposite direction — low to high concentration — and it needs energy from respiration.

When I first met these three in Year 10, I could recite all three definitions and still mix them up the moment a question phrased things sideways. The trap is that "concentration" means two things at once in osmosis — the concentration of the solute and the concentration of the water — and examiners love catching you swap them. Once that clicked, the potato required practical stopped feeling like a random lab and started making sense.

Here's the version I wish I'd had: the clean distinctions, the exact wording mark schemes reward, and a walk-through of the required practical with real numbers.

The three processes, side by side

Diffusion:

• What moves: any particle, often gases or dissolved solutes — oxygen, carbon dioxide, glucose.
• Direction: high to low concentration (down the gradient).
• Energy: none — it's passive.
• Example: oxygen diffusing from the alveoli in your lungs into the blood.

Osmosis:

• What moves: water molecules only.
• Direction: from a dilute solution (lots of water) to a concentrated solution (little water), across a partially permeable membrane.
• Energy: none — also passive.
• Example: water moving into a root hair cell from the soil.

Active transport:

• What moves: particles, often mineral ions or glucose.
• Direction: low to high concentration — against the gradient.
• Energy: required, released by respiration.
• Example: root hair cells absorbing mineral ions from very dilute soil water; the small intestine absorbing the last of the glucose after a meal.

The one line that ties it together: diffusion and osmosis are both passive and go down the gradient, so they cost nothing; active transport swims upstream, so it has to pay in energy from respiration.

The wording examiners actually want

Marks here are won and lost on precision. Two phrases that must appear:

• For osmosis: "through a partially permeable membrane." Leave it out and you drop the mark, even if the rest is perfect. AQA, Edexcel and OCR all insist on it.
• For active transport: "against the concentration gradient" and "using energy from respiration." Both halves, every time.

The classic slip is writing "water moves from a high concentration to a low concentration" for osmosis. High concentration of what? If you mean solute, you've said the exact opposite of the truth. Say "from a dilute solution to a concentrated solution," and never leave "concentration" floating on its own. Say your definition out loud, then check it against Explain anything — it will flag the missing phrase before an examiner does.

Water potential, and which board needs it

Water potential is just a measure of how much water wants to move. Pure water has the highest water potential. Add solute and the water potential drops (it gets more negative). Water always moves from a higher water potential to a lower one — which is the same rule as "dilute to concentrated," only dressed up.

Here's the board-specific bit worth knowing:

• GCSE (AQA, Edexcel, OCR): stick with "dilute solution to concentrated solution." You do not need the term water potential and shouldn't lean on it.
• Cambridge A-Level and AP Biology: water potential is expected, and you use it properly with negative values.

Knowing which language your paper wants stops you overcomplicating a GCSE answer or underselling an A-Level one.

What osmosis does to cells

• An animal cell (no cell wall) in pure water swells and bursts — it can't resist the water rushing in.
• A plant cell in pure water takes in water until it's firm and turgid; the cell wall stops it bursting. That turgor is what holds a plant upright.
• A plant cell in a concentrated solution loses water, goes flaccid, and eventually the membrane peels away from the wall — plasmolysis.

That's exactly why the potato practical works: potato cells behave like tiny osmosis detectors.

The osmosis required practical, step by step

The task: investigate the effect of different sugar (sucrose) concentrations on the mass of potato tissue.

Variables to get right (a near-guaranteed exam question):

• Independent variable: the concentration of the sugar solution.
• Dependent variable: the mass of the potato, measured as percentage change.
• Control variables: same potato, same length and surface area of cylinder, same volume of solution, same temperature, same time in solution.

Method:

1. Use a cork borer to cut several potato cylinders, then trim them to the same length with a scalpel (say 3 cm).
2. Blot each cylinder dry and measure its starting mass on a balance. Record it.
3. Set up a range of sucrose concentrations — for example 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 mol/dm3 — one tube each, equal volumes.
4. Put one cylinder in each tube and leave them all for the same time (often 24 hours, or 20 to 30 minutes in a lesson).
5. Remove each cylinder, blot it dry the same way, and measure its final mass.
6. Calculate the percentage change in mass for each concentration.

Why blot dry both times? Surface water would add mass that has nothing to do with osmosis, and that would wreck the comparison.

Why percentage change, not just "grams gained"? Because no two cylinders start at exactly the same mass. Percentage change puts them all on the same scale so the concentrations can be compared fairly. That reasoning is itself a mark.

Worked example: calculating percentage change

A potato cylinder is left in 0.4 mol/dm3 sucrose solution.

1. Note the starting mass: 4.20 g.
2. Note the final mass after 24 hours: 3.78 g.
3. Find the change: 3.78 - 4.20 = -0.42 g. It's negative, so the cylinder lost mass.
4. Divide by the starting mass: -0.42 / 4.20 = -0.10.
5. Multiply by 100: -0.10 x 100 = -10%.

So the cylinder lost 10% of its mass. Water left the potato cells by osmosis, which tells you the solution outside was more concentrated than the cell contents — a lower water potential outside, so water moved out.

The same maths on a cylinder in distilled water — 4.00 g rising to 4.40 g — gives +10%: it gained water, because the distilled water was more dilute than the cells.

Plot percentage change (y-axis) against concentration (x-axis) and you get a line sloping downwards. Where it crosses zero — no change in mass — the solution's concentration equals the concentration inside the potato cells. That crossing point is how you estimate the internal concentration, and it's a favourite "interpret the graph" question. If any step of that felt shaky, drop the numbers into Explain anything and ask it to show the percentage-change working line by line for your exact board.

Test yourself

1. A potato cylinder starts at 5.00 g and ends at 4.50 g. What is the percentage change in mass, and what does it tell you?
2. Give the two phrases an examiner needs to see in a definition of active transport.
3. A plant cell and an animal cell are both placed in pure water. Describe what happens to each.

Quick answers:

1. (4.50 - 5.00) / 5.00 x 100 = -10%. The cylinder lost water by osmosis, so the solution was more concentrated than the cell contents.
2. "Against the concentration gradient" and "using energy from respiration."
3. The animal cell swells and bursts (no cell wall); the plant cell becomes turgid but does not burst, because the cell wall resists the pressure.

Want more of these, auto-marked? Generate a set on this exact topic with the quiz maker, or turn the definitions into flashcards for spaced practice.

FAQ

Is osmosis a type of diffusion?

Yes. Osmosis is the diffusion of water specifically, across a partially permeable membrane. Both are passive and move particles down a gradient — osmosis is just the water-only, membrane-crossing version.

Does osmosis need energy?

No. Osmosis and diffusion are both passive, so they need no energy from the cell. Only active transport uses energy from respiration, because it moves particles against the gradient.

Why do we calculate percentage change in mass instead of the actual change?

Because the potato cylinders don't all start at the same mass. Percentage change scales every result to its own starting point, so different cylinders can be compared fairly.

What is the difference between "dilute" and "high water potential"?

They describe the same thing. A dilute solution has more water and fewer solute particles, so it has a higher (less negative) water potential. Water moves from dilute to concentrated, which is the same as high water potential to low. GCSE uses the first phrasing; Cambridge A-Level and AP use the second.

In short: Diffusion moves any particle down a gradient for free, osmosis moves water across a partially permeable membrane for free, and active transport pushes particles up the gradient using energy — and the potato practical is simply osmosis made measurable through percentage change in mass.