Drop a tea bag into hot water and, without stirring, the colour spreads until the whole cup is amber. Leave a wilted lettuce leaf in cold water and it becomes crisp again. Both are the same idea at work: particles move from where they are crowded to where they are not. In biology, that idea appears as diffusion and osmosis — two processes behind almost everything a cell does.
This article explains both processes, the difference between them, what tonicity means, and how to reason through the classic exam questions on the topic.
The starting point: particles are always moving
Molecules in liquids and gases are in constant random motion. They collide, bounce and spread out. Nobody directs this movement, and it doesn’t require energy from the cell — it is simply the result of thermal motion.
Because the motion is random, particles tend to spread from a crowded region into a less crowded one. That difference in crowding is the concentration gradient, and movement “down the gradient” is what both diffusion and osmosis describe.
Diffusion
Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, until they are evenly spread. Once the distribution is even, movement continues, but there is no longer a net change — the system has reached equilibrium.
In living systems, oxygen diffuses from the air in the lungs into the blood because oxygen is more concentrated in the air spaces than in the blood arriving there. Carbon dioxide diffuses in the opposite direction for the same reason.
Several factors change how quickly diffusion happens:
- Steeper gradient — a bigger difference in concentration means faster net movement.
- Higher temperature — particles move faster.
- Larger surface area — more space for particles to cross, which is why lungs and intestines are heavily folded.
- Shorter distance — thin exchange surfaces speed the process considerably.
- Smaller particle size — small molecules generally diffuse faster than large ones.
Simple and facilitated diffusion
The cell membrane is a lipid bilayer, so small non-polar molecules such as oxygen and carbon dioxide pass straight through it. That is simple diffusion.
Charged particles and larger polar molecules, such as ions and glucose, cannot cross the lipid interior easily. They pass through specific channel or carrier proteins instead. This is facilitated diffusion. It still moves substances down the gradient and still uses no cellular energy — the protein just provides a route.
Both differ from active transport, which pushes substances against the gradient and requires energy in the form of ATP.
Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane — one that lets water through but restricts certain solutes. Water moves from the side with a higher water concentration (fewer dissolved particles) to the side with a lower water concentration (more dissolved particles).
A useful way to hold this in mind: water follows solute. If one side of a membrane has more dissolved material, water tends to move toward it.
| Diffusion | Osmosis | |
|---|---|---|
| What moves | Any solute particle | Water only |
| Membrane needed? | No | Yes, selectively permeable |
| Direction | High to low solute concentration | High to low water concentration |
| Energy required | None | None |
Tonicity: what happens to a cell
Tonicity describes a solution’s solute concentration compared with the inside of a cell placed in it. It predicts which way water will move.
| Solution | Compared with the cell | Net water movement | Effect on an animal cell |
|---|---|---|---|
| Hypotonic | Fewer solutes outside | Water enters the cell | Swells, may burst |
| Isotonic | Equal solute concentration | No net movement | Stays the same |
| Hypertonic | More solutes outside | Water leaves the cell | Shrinks |
Plant cells behave differently because of their rigid cell wall. In a hypotonic solution, water entering the vacuole pushes the membrane against the wall, creating internal pressure that keeps the plant firm — this is turgor. In a hypertonic solution, water leaves, the membrane pulls away from the wall, and the cell becomes plasmolysed. That is exactly what wilting looks like at the cellular level.
Everyday examples
- Salting food to preserve it. A hypertonic environment draws water out of microbial cells, making it hard for them to grow.
- Wrinkled fingertips after a long bath. Water movement in the outer skin layers changes their volume.
- Crisping vegetables in cold water. Water enters plant cells and restores turgor.
- Medical fluids given intravenously. They are designed to be close to isotonic so red blood cells neither swell nor shrink.
- Smelling coffee from another room. Aroma molecules diffusing through the air.
How to approach exam questions
Most questions on this topic can be solved with three steps. First, identify what is moving — if it is water across a membrane, it’s osmosis; if it’s a solute, it’s diffusion. Second, compare the two sides and find where the solute is more concentrated. Third, remember that water moves toward the higher solute concentration, and predict whether the cell gains or loses water.
One common trap: describing water as moving “from low to high concentration”. Always state clearly whether you mean the concentration of water or of solute, since they are inverse. Another is forgetting that neither process requires energy — that distinction belongs to active transport.
Conclusion
Diffusion and osmosis are simple ideas with enormous reach: gas exchange, nutrient absorption, plant support and fluid balance all rest on them. Once you can identify what is moving and in which direction the gradient runs, most questions on the topic become straightforward.
To keep building on cell transport, membranes and related topics, explore the free biology courses available on Cursa.
