A strip of red onion skin under the lens. Add salt water, wait two minutes, and the colour shrinks back from every cell wall. From that retreat, and a ruler you build inside the eyepiece, you can work out the concentration sealed inside the cell.
Two lenses sit between your eye and the cell. The eyepiece magnifies ten times. The objective you swing into place magnifies another forty.
Multiply them. Ten times forty is four hundred, the cell arrives four hundred times larger than life.
But four hundred times what? The view carries no scale until you give it one.
So you drop a tiny ruler into the eyepiece, the graticule, and teach it real units by lining it up against a stage micrometer. Calibrate once, and every cell after that can be measured.
Three parts decide how large the cell looks, and let you measure it. Match each label to where it sits.
Calibrated at ×400, one eyepiece division is worth 2.5 µm, found by lining 100 divisions against the stage micrometer's 250 µm.
Drag the two markers to the edges of the cell, then read its width. Width = divisions × 2.5 µm.
One strip rests in pure water. The other sits in salt water far stronger than the cell. Two minutes pass.
In the salt bath, which way does water move, and why?
Out, down its own gradient. Water isn't pulled, and it isn't trying to fix anything, it moves from where free water is plentiful, inside the cell, to where it is scarcer, in the salty solution outside.
If you've ever salted a cucumber and watched beads of water rise to the surface, you have already run this experiment.
IB exam tip: A question that asks you to explain this wants the direction and the mechanism, solute lowers water potential, so water diffuses to the lower value. A question that asks you to state it only wants the direction.
The membrane lets water through but holds back salt. So water drifts to the side where it is scarcer, toward the salt.
Press play and watch the net flow build. Water moves; the solute can't follow.
Water potential is just how freely water can move. Add solute and you lower it. Water always flows toward the lower value.
Raise the outside concentration and watch the living contents pull away from the wall. Drag the slider.
The cell wall is rigid scaffolding. It holds its shape when water leaves, it does not shrink.
What shrinks is the living part: the plasma membrane and everything it encloses. It peels inward, and the space behind it fills with the solution from outside.
That separation has a name, plasmolysis. It is the visible proof that water has left the cell.
Water has left this cell. Drag each label to the part it points to.
Bathe strips of onion across a range of concentrations. In each, count what fraction of cells have plasmolysed.
Plot fraction against concentration. The curve climbs from almost none to almost all.
Find where exactly half have gone. There, the solution outside matches the cell inside, so that concentration is the one you couldn't see.
Take your lab worksheet, prepare the mounts, calibrate the graticule for real, and gather your numbers. When you have your data, come back for the last stretch.
A student bathed strips of red onion epidermis in sodium chloride solutions and recorded the percentage of plasmolysed cells in each. Cells were viewed with a ×10 eyepiece and a ×40 objective. The student found that 50% of cells were plasmolysed at a concentration of 0.30 mol dm⁻³.
STATE = a specific answer, no working needed. Award 1 mark.
OUTLINE = a brief account of the main steps. Award 1 mark per point, max 2.
Accept: "calibrate the graticule, then measure". Do not accept: using a ruler at high power / measuring without calibration.
DEDUCE = reach a conclusion from the data. EXPLAIN = give a reason. Award 1 mark per point, max 3.
Accept: "water potential of cell ≈ water potential of 0.30 mol dm⁻³ solution". Do not accept: "the cell is 0.30 M of salt" without reference to internal concentration.
Fill in the blanks. Stuck? Tap Reveal answers.
Osmosis is the net movement of down a gradient of water , across a partially permeable . When a plant cell in a strong solution loses water, its membrane peels from the wall, called . The outside concentration that plasmolyses half the cells matches the cell's own contents: its point.
Plasmolysis turns a concentration you can't see into a number you can measure.
Eyepiece times objective. Ten times forty is four hundred, and the calibrated graticule turns that into real micrometres.
IB: State = ×400. Explain = ×10 eyepiece × ×40 objective, scaled to µm via the graticule.
Water moves down its own gradient, never pulled. Solute lowers the potential; water leaves toward the lower value.
IB: State = water moves out. Explain = solute lowers water potential, so water diffuses to the lower value.
Where half the cells plasmolyse, outside matches inside. That concentration is the cell's own.
IB: Deduce = internal conc ≈ that solution. Explain = isotonic, no net water movement.