TL;DR:
Osmosis is the net movement of water across a selectively permeable membrane toward the side with more dissolved solute.
Hypertonic means more solute outside the cell than inside, so the cell loses water. Hypotonic means less outside, so the cell gains water.
In a potato slice experiment, the solution where mass does not change is the isotonic point.
Leave a potato slice in plain water and it turns firm. Leave another in strong salt water and it goes limp and shrinks. Same potato, same time, opposite results. The explanation sits in the membrane around every cell.
The idea in plain words
A cell membrane is selectively permeable: water passes through easily, while many dissolved particles do not. Water molecules are always moving, and they move both ways across the membrane. What matters is the net direction.
Osmosis is the net movement of water across such a membrane from the side with a lower concentration of dissolved solute to the side with a higher one. Put simply, water moves toward the saltier or sweeter side.
Hypertonic, hypotonic, isotonic
These words compare the solution outside the cell with the inside of the cell.
Hypertonic outside: more solute outside. Water leaves the cell. The cell shrinks.
Hypotonic outside: less solute outside. Water enters the cell. An animal cell swells and may burst. A plant cell becomes firm (turgid) because the cell wall resists.
Isotonic: equal concentration. Water moves both ways at the same rate and there is no net change.
The prefixes help: hyper means over, hypo means under. The comparison always runs from the point of view of the solution outside.
Why water moves
Dissolved particles interact with water molecules, so water in a concentrated solution is less free to move. Water's free movement tends to even out across the membrane, which means it passes toward the side where it is least free. No energy from the cell is needed. It is a form of diffusion for water.
Worked example: a potato slice in salt water
A student cuts four identical potato cylinders, each weighing 5.00 g. Each goes into a different salt solution for an hour. The masses afterwards are an invented practice data set.
Salt (g per 100 mL) | Final mass (g) | Change in mass (g) | Percentage change |
|---|---|---|---|
0 | 5.60 | +0.60 | +12 |
1 | 5.15 | +0.15 | +3 |
2 | 4.70 | −0.30 | −6 |
3 | 4.40 | −0.60 | −12 |
Find the changes.
5.60 − 5.00 = +0.60,5.15 − 5.00 = +0.15,4.70 − 5.00 = −0.30,4.40 − 5.00 = −0.60.Convert to percentage change. Dividing by the same 5.00 g start makes the slices comparable even if their starting masses differ:
0.15 ÷ 5.00 = 0.03,−0.30 ÷ 5.00 = −0.06,−0.60 ÷ 5.00 = −0.12.Interpret. In pure water the slice gains mass: water entered, so the water outside was hypotonic. At 3 g per 100 mL it loses mass, so the solution was hypertonic.
Find the isotonic point. The change is +3 at 1 and −6 at 2, so it crosses zero between them. Linear interpolation: the drop from +3 to −6 is 9 units, and 3 of those are needed to reach zero, so the crossing is at
1 + 3/9 ≈ 1.3g per 100 mL.
That value estimates the concentration inside the potato cells. It is an estimate because we drew a straight line between two points. A graph with a line of best fit through all the points is better.
Plants, animals and the cell wall
The same process gives different outcomes in different cells. An animal red blood cell in pure water takes in water until it bursts, because it has no wall. A plant cell in pure water also takes in water, but the wall pushes back, and the cell becomes firm and healthy. That firmness is why a watered plant stands upright.
In a hypertonic solution a plant cell loses water until its membrane pulls away from the wall. This is called plasmolysis, and it is why a plant wilts when the soil is very salty or very dry.
How to check your answer
The sign of the change should match the direction of water movement. Gain means water in.
The interpolated value must lie between the two concentrations you used.
Percentage change uses the starting mass, not the final mass.
The trend must fall as salt rises, since more salt outside pulls more water out.
Common mistakes
Saying salt moves into the cell to balance things. The membrane blocks most solutes, so water moves, not salt.
Mixing up hypo and hyper. Always say 'relative to the cell' and which side you describe.
Forgetting the cell wall. Plant cells survive hypotonic solutions because the wall stops them bursting.
Comparing raw masses. Slices rarely start at exactly the same mass, so use percentage change.
Lab safety: Take care when you cut potatoes: use a cutting board, cut away from your hand, and ask for help with sharp tools.
Where you will use it next
Osmosis explains why plants wilt, why drinking sea water does not hydrate you, and how kidneys balance water. It also pairs with the enzyme topic, because both depend on how molecules behave in solution: see enzymes and why temperature and pH matter.
Frequently asked questions
Is osmosis the same as diffusion?
Osmosis is diffusion of water across a selectively permeable membrane. The word is used for the water case in particular.
Why not use sugar solutions instead of salt?
You can. Many classes use sucrose, and the method is the same: change one solute concentration, measure the change in mass.
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