IB Biology SLTopic 2 — Osmosis & Water PotentialPaper 1 & 2Core idea~9 min read
Osmosis
Osmosis gets called “water moving to where there is more solute”, which is true but tells you nothing about why. The real answer starts with the shape of a water molecule — and once you see that, everything else on this page follows.
📘 What you need to know
Water is dipolar: the oxygen side is slightly negative and the hydrogen side slightly positive. That makes it an excellent solvent.
Water molecules surround dissolved particles to form hydration shells. The interaction between solvent and solute is called solvation.
Cell membranes are partially permeable — small molecules such as water pass through, larger solute molecules do not.
Osmosis is the diffusion of water molecules from a less concentrated (dilute) solution to a more concentrated solution across a partially permeable membrane.
The same thing said the other way round: water moves from a region of lower solute concentration to a region of higher solute concentration.
Water moves down a water potential gradient: a dilute solution has a higher water potential, a concentrated one has a lower water potential.
Why water dissolves so much
A solution is a solute dissolved in a solvent. Water is the best solvent most cells will ever meet, and the reason is its shape.
The two hydrogen atoms are not opposite each other — they sit at an angle of about 105°. Inside each polar covalent bond the shared electrons are pulled closer to the oxygen atom, so the oxygen end carries a slight negative charge and the hydrogen end a slight positive charge. A molecule with charges separated like this is dipolar.
Every dissolved particle in a cell is wearing a coat of water molecules. That is what “dissolved” actually looks like.
Because water is dipolar it can form hydrogen bonds with other polar molecules and with ions. Polar solvent molecules orientate themselves towards a solute particle, and a shell of water builds up around it — a hydration shell. That whole interaction between solvent and solute is called solvation.
The flip side: non-polar substances such as long hydrocarbon chains have no charged regions, so water has nothing to attract. They are hydrophobic and stay undissolved — which is exactly why a membrane made of lipid tails works as a barrier.
Partially permeable membranes
Every cell is surrounded by a membrane that is partially permeable. That means it lets small molecules like water pass through, but not larger solute molecules such as sucrose.
This is the bit that makes osmosis possible. If the solute could move too, it would simply diffuse until both sides were equal and nothing dramatic would happen. Because it cannot, only the water can move to even things out.
What osmosis actually is
Definition worth learning word for word
osmosis = the diffusion of water molecules from a less concentrated solution to a more concentrated solution, across a partially permeable membrane
Notice that osmosis is a type of diffusion. The water molecules are moving down their own concentration gradient — there are more free water molecules in the dilute solution, so more of them cross to the concentrated side than come back.
Water moves both ways the whole time. What we call osmosis is simply the imbalance — more molecules going right to left than the other way.
Water potential
There is a tidier way to describe the same thing, and it is the one this topic is named after. Water potential is a measure of how free the water molecules in a solution are to move.
Pure water has the highest water potential of all.
Adding solute lowers water potential, because water molecules become tied up in hydration shells around the solute particles.
Water always moves from a region of higher water potential to lower water potential — down the water potential gradient.
Two ways of saying it, one idea. “From dilute to concentrated” and “from high water potential to low water potential” describe the same movement. Use whichever the question uses, and you will never get the direction the wrong way round.
Worked examples
WORKED EXAMPLE
Predict the direction of movement
Two solutions are separated by a partially permeable membrane. Side A is 0.1 mol dm−3 sucrose and side B is 0.6 mol dm−3 sucrose. State the direction of the net movement of water and explain your answer.
Step 1: Compare the solute concentrations
B has the higher solute concentration, so A is the more dilute solution.
Step 2: Compare water potentialsA = higher water potential, B = lower water potentialStep 3: State the direction
Water moves down the water potential gradient.
Net movement from A to BSay “net movement”. Water crosses both ways; only the balance is one-directional.
WORKED EXAMPLE
Explain why water is a good solvent
Explain why water is able to dissolve ionic and polar substances.
Point 1: The structure
Water is dipolar — the oxygen end is slightly negative and the hydrogen ends slightly positive.
Point 2: The interaction
These charged regions attract charged and polar solute particles, forming hydrogen bonds or ion–dipole forces.
Point 3: The result
Water molecules surround each particle in a hydration shell, separating the particles so the substance dissolves.
Dipolar, so it surrounds charged particlesThe three steps are structure, interaction, result. That pattern answers most “explain why” questions.
WORKED EXAMPLE
Why the membrane matters
A membrane is replaced with one that lets both water and sucrose molecules pass. Predict what happens to the two solutions either side, and explain why this is not osmosis.
Step 1: What can now move
Sucrose can diffuse as well as water, down its own concentration gradient.
Step 2: The outcome
Sucrose moves from the concentrated side to the dilute side until both sides reach the same concentration.
Step 3: Why it is not osmosis
Osmosis requires a partially permeable membrane that holds the solute on one side.
Simple diffusion of solute, not osmosisThe partially permeable membrane is part of the definition, not an optional detail.
💡 Exam tip
Learn the full definition, including “partially permeable membrane”. Leaving that out loses a mark on its own.
Always write net movement of water, not just “movement”.
Talk about water molecules moving, not “water concentration moving”.
Use the word dipolar for water and hydration shell for what forms around a dissolved particle.
Remember osmosis is a form of diffusion — it is passive and needs no ATP.
If a question gives water potentials rather than concentrations, remember that the less negative value is the higher water potential.
⚠ Common mix-up
Saying water moves from high to low concentration of water without saying which side that is. A dilute solution is the one with more free water.
Getting the direction backwards. Water moves towards the more concentrated solute solution.
Thinking osmosis needs energy. It is passive; no ATP is involved.
Saying the solute moves in osmosis. The solute cannot cross a partially permeable membrane.
Confusing partially permeable with fully permeable. A plant cell wall is freely permeable; the cell membrane is not.
Describing water as non-polar. Its polarity is the entire reason any of this works.
Up next: Osmosis in Cells — what actually happens to an animal cell and a plant cell placed in solutions that are too strong or too weak.
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