What is osmosis? Diffusion, osmosis and why cells burst

What is diffusion?

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration as a result of their random thermal motion. No particle "knows" which way to go; each moves randomly, and because there are more particles on the crowded side, more happen to cross away from it than towards it.

Equilibrium is reached when the concentrations are equal, and at that point the motion does not stop — only the net movement does. That distinction shows up in exam mark schemes surprisingly often, because "the particles stop moving" is a common and costly phrasing.

What is osmosis?

Osmosis is the diffusion of water across a semipermeable membrane, from the side where water is more concentrated to the side where it is less concentrated. Because water is more dilute where there is less solute, this is the same thing as saying water moves towards the more concentrated solution.

The membrane is what makes it a special case. It lets water through but not the dissolved solute, so the solute cannot even out its own concentration by moving — the water has to do the evening out instead. If the membrane let the solute through as well, the process would simply be diffusion in both directions.

What is the difference between diffusion and osmosis?

Diffusion describes any particle moving down its own concentration gradient; osmosis describes water moving across a membrane. Every case of osmosis is a case of diffusion, but not the other way round.

What do hypotonic, hypertonic and isotonic mean?

The terms describe the surrounding solution relative to the inside of the cell, and this reference point is where most of the confusion comes from. A hypotonic solution has less solute than the cell, so water moves in. A hypertonic solution has more solute, so water moves out. An isotonic solution has the same, so there is no net movement.

A useful check when you are stuck: find the more concentrated side and move the water towards it. In a hypertonic surrounding, that is outside the cell, so the cell shrinks. The vocabulary is describing the direction of water, not the direction of solute.

Why does a red blood cell burst in pure water?

A red blood cell placed in pure water bursts because water moves in by osmosis towards the higher solute concentration of the cytoplasm, and the cell has no wall to resist the pressure. The membrane stretches, then fails — a process called haemolysis.

The same cell in a strongly salty solution does the opposite: water leaves, the cell shrinks and the surface becomes spiky, which is called crenation. This is why intravenous fluids are isotonic. Giving pure water into a vein would burst red blood cells, and this is the clinical reason the topic sits on every first-year physiology syllabus.

Why do plant cells behave differently?

Plant cells survive in pure water because the rigid cell wall pushes back as the cell takes up water, and the cell becomes turgid rather than bursting. Turgor pressure is what keeps a non-woody plant upright; a wilting plant is one whose cells have lost it.

In a hypertonic solution the plant cell loses water, the membrane pulls away from the wall, and the cell is said to be plasmolysed. The classic practical — red onion epidermis in salt solution under a microscope — is designed so you can see the membrane detach, which is the visible proof that water left the cell.

How does osmosis work in the kidney and the gut?

The kidney uses osmosis to reclaim water from the filtrate: ions are actively pumped out of the nephron tubule, which makes the surrounding tissue more concentrated, and water then follows passively. The pumping costs energy; the water movement does not. This split between an active step and a passive consequence is the whole design of the loop of Henle.

Oral rehydration solution works on the same principle in reverse. Sodium and glucose are absorbed together by the gut wall, which raises the solute concentration on the far side of the epithelium, and water follows by osmosis. This is why a rehydration sachet contains sugar as well as salt — the glucose is not there for energy but to drive water absorption.

Which osmosis exam questions come up most often?

The recurring question is direction: given a cell in a named solution, state which way water moves and what happens to the cell. Answer it by naming the more concentrated side first, then the direction, then the consequence, and you will hit most of the mark points.

The other recurring ones are worth rehearsing:

• Explain why osmosis does not require energy.
• Explain why the potato-cylinder practical uses mass change rather than length.
• Explain why a plant cell becomes turgid while an animal cell bursts.
• Explain why a seedling wilts in a salted soil.

Rehearsing an answer beats rereading it — Karpicke and Roediger (2008) showed that retrieving material produces substantially stronger retention than repeated study, and Dunlosky (2013) rated practice testing among the most effective study techniques available. Turn each of those questions into a flashcard from your own lecture notes and answer it from memory before checking. Our articles on active recall and spaced repetition explain why that works.