Key facts
- Diffusion can involve many particles and does not inherently require a membrane.
- Osmosis specifically describes net water movement through a selectively permeable membrane.
- In hypotonic surroundings, animal cells may swell while plant cells become turgid.
- In hypertonic surroundings, plant cells may lose turgor and undergo plasmolysis.
Diffusion and osmosis both describe passive movement, but they are not identical. Diffusion is the net movement of particles from an area of higher concentration toward lower concentration. Osmosis is the net movement of water across a selectively permeable membrane, driven by a difference in water potential.
Diffusion vs. osmosis: the essential differences
| Feature | Diffusion | Osmosis |
|---|---|---|
| What moves? | Particles such as dissolved molecules or gases | Water molecules |
| Must a membrane be present? | No | Yes, a selectively permeable membrane |
| Energy needed? | Not directly; passive diffusion does not consume ATP | Not directly; osmosis is passive |
| Common example | Oxygen moving along a concentration gradient | Water entering or leaving plant cells |
How diffusion works in everyday life
Drop a small amount of food coloring into still water: over time its molecules spread, even without stirring, until the color becomes more uniform. Random molecular motion continues at equilibrium, but there is no longer net movement in one direction. In biological membranes, simple diffusion describes certain small molecules moving through the membrane down their concentration gradient. Other substances require membrane proteins; that is facilitated diffusion, which is also passive.
How osmosis works inside cells
Cell membranes let water pass, while restricting some solutes. A difference in total solute conditions changes the direction of net water movement. Water moves from higher water potential toward lower water potential. In basic classroom terms, where pressure effects are ignored, water generally moves from the more dilute solution toward the more concentrated one across the membrane.
Hypotonic, hypertonic and isotonic solutions
- Hypotonic external solution: The environment has a lower concentration of effectively nonpenetrating solutes than the cell interior. Net water enters the cell. Animal cells may swell; plant cells typically become turgid because the cell wall resists expansion.
- Hypertonic external solution: The external solution has a higher effective concentration of nonpenetrating solutes. Net water exits the cell; plant cells can undergo plasmolysis when the protoplast pulls away from the cell wall.
- Isotonic external solution: Effective solute concentrations balance across the membrane, so there is no sustained net water movement. Water molecules still move in both directions.
Tonicity describes the effect of a solution on cell volume and depends on solutes that do not readily cross the membrane. It is not always identical to a simple comparison of total dissolved solute concentrations.
Try a basic potato osmosis investigation
Question: Does a salty solution change the mass of potato tissue? Prepare equal-size raw potato pieces, a cup of plain water and a cup of water with dissolved table salt. Dry and weigh each piece before immersion. Place one piece in each cup for an equal period, remove, gently blot with the same method, and weigh again.
Prediction: Potato in plain water may gain mass if water moves inward; potato in sufficiently salty water may lose mass. Actual results depend on concentration, temperature, time, tissue variability and blotting. Calculate percentage mass change as (final mass − starting mass) ÷ starting mass × 100. For example, a change from 10.0 g to 9.2 g is a loss of 8%. Use repeats and keep sample size, immersion time and drying consistent. Do not eat laboratory samples.
Our featured photo shows living onion epidermal cells under a microscope: a real plant-cell structure appropriate for understanding osmosis, not a photograph of the potato experiment itself.
Common exam mistakes
- Osmosis moves water, not salt molecules.
- Diffusion does not require a membrane, even though it can occur across one.
- Equilibrium does not mean molecular motion stops.
- A plant cell’s rigid wall helps prevent bursting in hypotonic conditions.
- Water channels called aquaporins can facilitate water movement, but osmosis remains passive.
Five-question practice quiz
- What moves during osmosis? Answer: Water.
- Does diffusion always need a membrane? Answer: No.
- Which solution tends to make an animal cell shrink: hypertonic or hypotonic? Answer: Hypertonic.
- What is plant-cell plasmolysis? Answer: Shrinkage of the protoplast away from the cell wall as it loses water.
- Why can salt water reduce potato mass? Answer: Net water movement out of potato cells.
Continue studying with verified resources
Build on this lesson with our photosynthesis guide and mitosis vs. meiosis lesson. For the science, read OpenStax Biology 2e: Passive Transport and OpenStax Anatomy & Physiology: Passive Transport.
Educational information; observations vary with experimental conditions.
Sık Sorulan Sorular
Is osmosis a type of diffusion?
Yes. Osmosis is the passive net diffusion of water across a selectively permeable membrane.
Does osmosis use ATP?
Passive osmosis does not directly require cellular ATP; the water moves in response to a water-potential gradient.
Why does a potato shrink in salt water?
When the surrounding solution has a lower water potential, water leaves potato cells by osmosis; the tissue can become less firm and lose mass.