Osmosis in Plant and Animal Cells

What is osmosis?

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.

Three words in that definition earn marks, and most dropped marks come from leaving one out:

• Net — water molecules cross the membrane in both directions the whole time. Osmosis is the imbalance between the two flows, not a one-way trip.
• Water — only water moves. The solute stays put, because its molecules are too large to pass through the membrane.
• Partially permeable — the membrane lets small water molecules through but blocks larger solute molecules. Some specifications say semi-permeable; the two mean the same thing, and either is accepted.

Osmosis is a passive process. It needs no energy from respiration and no ATP — the molecules are already moving because of their own kinetic energy. If a question mentions energy or carrier proteins, it is asking about active transport, not osmosis.

How to use this simulation

1. The cell inside starts at roughly 5% solute. Use the slider to change the concentration of the solution outside the cell.
2. Set the outside to 0% (hypotonic). Watch what happens to the plant cell and the animal cell — and notice that they do not behave the same way.
3. Set it to 5% (isotonic). The cells stop changing. Water is still crossing the membrane, but equally in both directions.
4. Set it to 15% (hypertonic). Both cells lose water, but again the outcomes differ.
5. Use Export Data (CSV) to pull the numbers into a spreadsheet and plot them, or Export Diagram (PNG) for your notes.

The single most useful habit: before you move the slider, say out loud which way water will move and why. Then check. Getting the prediction wrong is more informative than getting it right.

What happens in each solution

Hypotonic — outside more dilute than the cell

The solution outside has the higher water potential, so water moves into the cell.

A plant cell swells. The vacuole fills, the cytoplasm and membrane press outward against the cellulose cell wall, and the cell becomes turgid — firm, but not damaged. The wall is strong enough to resist the pressure, so the cell cannot burst. This turgor pressure is what holds up non-woody plants; lose it and the plant wilts.

An animal cell has no cell wall, so nothing resists the swelling. It keeps taking in water until the membrane fails and the cell bursts — lysis. In red blood cells this is called haemolysis. This is precisely why fluids given to patients in hospital are made isotonic with blood.

Isotonic — outside the same as the cell

Water potentials are equal, so water enters and leaves at the same rate. There is no net movement and neither cell changes size. Note the wording carefully: movement has not stopped, it has balanced.

Hypertonic — outside more concentrated than the cell

The solution outside has the lower water potential, so water moves out of the cell.

A plant cell loses water from its vacuole and becomes flaccid. If it keeps losing water, the membrane peels away from the cell wall — plasmolysis. The wall itself keeps its shape, which is why a plasmolysed cell still looks rectangular under the microscope while the contents have shrunk inward. The point at which the membrane just begins to pull away is called incipient plasmolysis.

An animal cell shrinks and its surface becomes wrinkled and spiky — crenation.

The potato practical

Almost every specification includes an osmosis practical using potato cylinders, and it is a reliable exam topic. The method:

1. Cut cylinders of potato of equal length and diameter using a cork borer.
2. Blot each one dry and record its initial mass.
3. Place one cylinder in each of a range of sucrose concentrations (typically 0.0 to 1.0 mol/dm³).
4. Leave for a fixed time — 20–30 minutes at minimum, longer is better.
5. Remove, blot dry again, and record the final mass.
6. Calculate the percentage change in mass for each: (final − initial) ÷ initial × 100.

Why percentage change rather than the actual change in grams? Because the cylinders never start at exactly the same mass. Converting to a percentage makes the comparison fair — this is a very common two-mark question.

Why blot dry? Surface solution clinging to the cylinder adds mass that has nothing to do with osmosis.

Reading the graph. Plot percentage change in mass (y) against sucrose concentration (x). The line crosses the x-axis at the point of zero change — where the external solution has the same water potential as the potato cell sap. That intercept is the answer when a question asks you to estimate the concentration inside the cells.

Control variables: temperature, time in solution, volume of solution, size and surface area of the cylinders, and using the same potato throughout — different potatoes, and even different parts of one potato, vary in solute concentration.

Common exam mistakes

• Saying the solute moves. In osmosis, only water moves. If sugar crossed the membrane it would be diffusion, not osmosis.
• Describing one-way movement. Always write net movement.
• Saying a plant cell bursts. It cannot — the cell wall prevents it. Only animal cells burst.
• Confusing flaccid with plasmolysed. Flaccid means limp from water loss; plasmolysed means the membrane has actually pulled away from the wall. Plasmolysis is the more extreme state.
• Writing "water moves to where there is more solute". True, but it scores poorly. Use water potential, or "from a dilute solution to a concentrated solution".
• Calling osmosis active. It requires no energy. Never mention ATP.
• Forgetting the membrane. The definition is incomplete without "through a partially permeable membrane".

Exam-style questions