IB Biology HLTopic 1 — Life’s Building Blocks & VarietyPaper 1 & 2Core idea~10 min read
Osmosis
Water does not need a pump or a drop of energy to cross a membrane. It drifts — but it drifts in a predictable direction. Once you see why a water molecule is slightly lopsided, everything about osmosis, tonicity and water potential falls into place.
📚 What you need to know
A solution is a solute dissolved in a solvent. Water is an excellent solvent because it is dipolar.
The hydrogen side of a water molecule is slightly positive and the oxygen side slightly negative, so water forms hydrogen bonds with polar solutes and ions.
The interaction between solvent and solute is solvation; water molecules surround each solute particle in a hydration shell.
Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution across a partially permeable membrane.
Put the other way round: water moves from lower solute concentration to higher solute concentration.
The membrane lets small molecules such as water through, but not larger solute molecules.
A solution can be hypotonic (more dilute than the cell), hypertonic (more concentrated) or isotonic (the same).
Why water is such a good solvent
A water molecule is not symmetrical. The oxygen atom pulls the shared electrons of the covalent bonds closer to itself, so that end carries a slight negative charge while the two hydrogen ends carry a slight positive charge. A molecule with separated charges like this is dipolar.
That small imbalance is the whole story. Because water has both a positive end and a negative end, it can line itself up against anything charged — a polar molecule, a positive ion, a negative ion — and hold on through hydrogen bonds or ion-dipole attractions.
The bent shape matters. If water were straight, the two pulls would cancel out and it would not be dipolar at all.
Hydration shells
When a salt such as sodium chloride dissolves, the water molecules turn to face each ion: negative oxygen ends towards the sodium ions, positive hydrogen ends towards the chloride ions. Each solute particle ends up wrapped in a layer of water called a hydration shell, and that is what keeps it dissolved and separated.
The flip side. Non-polar molecules, such as long hydrocarbon chains, have no charged regions for water to grip. No hydration shell can form, so they stay clumped together away from the water — they are hydrophobic.
Osmosis and the partially permeable membrane
Every cell is surrounded by a membrane that is partially permeable: small molecules like water slip through, but larger solute molecules cannot. That one-sided permeability is what makes osmosis possible.
Definition to learn word for word
Osmosis is the diffusion of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane
Water still moves down a concentration gradient — a gradient of water concentration, not solute. The dilute side has more free water molecules, so more of them cross to the concentrated side than come back the other way. The result is a net movement of water from low solute concentration to high solute concentration.
The solute cannot cross to balance the two sides, so the water has to do all the work.
Students lose marks writing “water moves from high concentration to low concentration” without saying concentration of what. Say “from a dilute solution to a concentrated solution”, or spell it out: high water concentration to low water concentration.
Hypotonic, hypertonic and isotonic
These three words describe the solution outside the cell, compared with the cytoplasm inside it. Get that reference point right and the direction of water movement follows automatically.
Outside solution
Compared with the cytoplasm
Net movement of water
Hypotonic
Lower solute concentration (more dilute)
Into the cell
Hypertonic
Higher solute concentration (more concentrated)
Out of the cell
Isotonic
The same solute concentration
No net movement
Pink dots are solute. Count them either side of each cell and the direction of the arrow explains itself.
This one catches people out every year. In an isotonic solution water molecules still move in and out constantly, because they still have kinetic energy. What is zero is the net movement in one direction. Never write “no water moves”.
Worked examples
WORKED EXAMPLE
Define osmosis. [2]
Give the movement and the gradient
Osmosis is the net movement of water molecules from a region of lower solute concentration (a dilute solution) to a region of higher solute concentration.
Name the barrier
This movement happens through a partially permeable membrane.
2 marksLeave out “partially permeable membrane” and you have defined diffusion, not osmosis.
WORKED EXAMPLE
Explain why sodium chloride dissolves readily in water but a lipid does not. [3]
Point 1 — the property of water
Water is dipolar, with a slightly negative oxygen end and slightly positive hydrogen ends.
Point 2 — the salt
These charged ends are attracted to the sodium and chloride ions, forming hydration shells around them and holding them in solution.
Point 3 — the lipid
A lipid is non-polar, so there are no charged regions for water to be attracted to and no hydration shell can form.
3 marks“Like dissolves like” is a handy shortcut, but write out the charges to earn the marks.
💡 Exam tip
Say net movement every single time. It is the most common missing word in osmosis answers.
Describe the gradient in terms of solute concentration so there is no doubt about what is dilute.
Use partially permeable, the wording IB mark schemes expect.
Hypotonic means the outside is more dilute; hypertonic means the outside is more concentrated. Always compare to the cell.
A silly but effective hook: hippotonic solutions are full of water; hypertonic means hyper amounts of solute.
Osmosis is passive. Never mention ATP or active transport in an osmosis answer.
⚠ Common mix-up
Water moves towards the solute, not away from it. The concentrated side gains water.
Isotonic does not mean nothing happens. There is movement, just no net movement.
Dipolar is not the same as ionic. Water carries partial charges, not full ones.
Hypo and hyper are easy to swap. Hypo is below (less solute), hyper is above (more solute).
Osmosis only refers to water. Solute moving down a gradient is plain diffusion.
The membrane does not “choose”. It blocks larger molecules by size and chemistry, not by decision.
Up next: Osmosis in Cells — what actually happens to an animal cell and a plant cell when the tonicity is wrong, and why one bursts while the other does not.
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