See Diffusion and Osmosis Happen, Molecule by Molecule

How to use this simulation

1. Pick one of three scenes under "What to watch": diffusion, osmosis, or a cell placed in a solution.
2. In diffusion, give the two halves different concentrations and watch the large coloured particles jostle randomly until they fill the tank evenly. On the graph, the two lines meeting is the end of diffusion.
3. In osmosis the dashed orange line in the middle is a semi-permeable membrane: small blue water molecules pass, large solute particles bounce back. The water level on the stronger side rises, then stops at a fixed Δh.
4. Choose "A cell in a solution" and a hypertonic solution: the plant cell's protoplast peels away from its wall (plasmolysis). Then switch to hypotonic and watch it recover (deplasmolysis).
5. Switch the cell type to a red blood cell: in a hypotonic solution it swells and bursts, in a hypertonic one it shrivels into a spiky shape.
6. Move the temperature between 5 °C and 60 °C and compare how quickly everything happens. Pause at any moment and note the readings.

Perfume, sugar in tea and salted mango: one mystery

Light an incense stick in one corner of a room and a few minutes later the whole room smells of it, although nobody fanned the smell across. Drop one drop of food colouring into a glass of still water and, without stirring, the whole glass slowly turns pink. How did the smell and the colour spread by themselves? The answer has a name: diffusion.

Now a second scene. Sprinkle salt on slices of raw mango and after a while water pools in the bowl and the slices go soft and wrinkly. Leave a handful of dry raisins in water overnight and by morning they are plump. Water leaves the fruit in one case and enters it in the other. Which way it goes is decided by osmosis.

Roots pull water out of soil, leaves swap oxygen and carbon dioxide with the air, blood picks up oxygen in the lungs and the kidneys clean the blood. Behind almost every job a living thing does, these two quiet processes are at work. That is why the chapter appears in Class 8 science, in every board exam and in medical entrance tests.

From zero: molecules never stand still

Everything starts from one small fact: every substance is made of enormous numbers of tiny particles, and those particles are always moving. In liquids and gases they rush about, collide with one another and change direction every time they are hit. This endless random jiggling is thermal motion.

In 1827 the botanist Robert Brown looked through a microscope at pollen grains floating in water and saw them trembling for no visible reason. Invisible water molecules were knocking into each grain from every side. That jittery motion is now called Brownian motion, and every dot in the simulation moves exactly like that: no favourite direction, a new random nudge every instant.

Here is the clever part. No single molecule knows where the crowd is. But where there are more molecules, more of them happen to wander out; where there are fewer, fewer wander back. Add it up and you get a net flow from high concentration to low. Nothing pushes; it is just counting.

When both sides reach the same concentration, do the molecules stop? No! They keep moving, but as many cross from left to right as from right to left, so the net flow is zero. This is a dynamic equilibrium. In the readings panel the "→" and "←" counts end up roughly equal; watch for it.

What is diffusion?

Definition: diffusion is the net movement of molecules or ions of a substance from a region where they are more concentrated to a region where they are less concentrated, at the same temperature and pressure, continuing until the concentration is the same everywhere.

Diffusion happens in gases, liquids and even solids, fastest in gases and slowest in solids, because gas molecules have lots of empty space around them and move very quickly. It needs no membrane. Even if a membrane is present, if it lets every particle through (a fully permeable membrane, such as a plant cell wall) the process is still plain diffusion.

In the "Diffusion" mode of the simulation there is no barrier. Start with few solute particles on the left and many on the right, and they gradually drift left. The two graph lines, the left and right concentrations, move toward each other until they meet, and the state reads "Evenly spread".

• A drop of ink spreading through water
• The smell of cooking or perfume filling a house
• A tea bag colouring still water without stirring
• CO₂ entering and O₂ leaving a leaf through the stomata
• Oxygen passing from the air sacs (alveoli) of the lungs into the blood
• Glucose and ions moving within a cell

Diffusion pressure and rate of diffusion

Where molecules are packed closely, they have a stronger tendency to spread outward. The pressure that tendency creates is called diffusion pressure. The higher the concentration, the higher the diffusion pressure, and a substance always moves from higher to lower diffusion pressure.

Pure water has the highest diffusion pressure of water. Dissolve sugar or salt in it and each solute particle holds some water molecules around itself, leaving fewer free to wander. So the water in a solution has a lower diffusion pressure than pure water. That shortfall used to be called the diffusion pressure deficit (DPD); today it is expressed as water potential, ψ.

The amount of substance that spreads per unit time is the rate of diffusion. The bigger the concentration difference, the faster the rate, which is why particles cross quickly at the start of the simulation and only trickle back and forth near the end. For gases, Graham's law adds that the rate is inversely proportional to the square root of the molar mass: light gases spread faster.

What is osmosis?

Definition: when two solutions of different concentration are separated by a semi-permeable membrane, the solvent (usually water) moves through the membrane from the less concentrated solution into the more concentrated one. This process is osmosis.

A semi-permeable membrane lets small water molecules through but holds back larger solute particles such as sugar. Cell membranes, the thin skin inside an eggshell, a fish swim bladder, cellophane and parchment paper are examples. A plant cell wall is not one: it is fully permeable, letting both water and solutes pass.

Look at the "Osmosis" mode. Small blue dots are water, large coloured dots are solute. The large dots bounce off the membrane; the small ones slip through. The right side holds more solute, so fewer of its water molecules are free. More water crosses left to right than comes back, and the right-hand level rises.

Does the level rise forever? No. As water piles up on the right, the extra column presses down harder: hydrostatic pressure. That pressure pushes water back. Eventually the two pushes balance and the level stops. In the simulation Δh settles at a fixed value and the state reads "Equilibrium". The pressure needed to stop the flow is the osmotic pressure.

• A semi-permeable membrane must be present
• The concentrations on the two sides must differ
• Only the solvent moves, never the solute
• Water moves from the dilute solution to the concentrated one

Osmotic pressure: how hard a solution pulls

Separate a solution from pure water with a semi-permeable membrane and water tries to flow in. The exact pressure you would have to apply to the solution to stop that inflow is its osmotic pressure, π. The stronger the solution, the larger π.

For dilute solutions van 't Hoff gave a neat formula that looks just like the gas law. C is the molar concentration, R the gas constant, T the temperature in kelvin, and i the number of particles each formula unit splits into (1 for sugar, about 2 for NaCl). The formula shows that osmosis depends on the number of particles, not on what they are.

Plant biologists describe water movement with water potential, ψ. Pure water has ψ = 0; adding solute makes ψ negative. Water always moves from higher ψ (less negative) to lower ψ (more negative). A cell's ψ is the sum of its solute potential and its pressure potential.

Difference between diffusion and osmosis

"Write five differences between diffusion and osmosis" is a near-annual exam question. Read this table once carefully and check the reason behind each row in the simulation.

Hypotonic, isotonic and hypertonic solutions

Put a cell into a solution and the direction water moves depends on how the concentration outside compares with the inside. That comparison is called tonicity. In the simulation the inside of the cell is about 0.3 mol/L and the three presets outside are hypotonic 0.05, isotonic 0.30 and hypertonic 0.60 mol/L.

Hypotonic means the outside is weaker than the cell sap: water flows in. Isotonic means both are equal: as much water leaves as enters and the net flow is zero. Hypertonic means the outside is stronger: water flows out.

Plant and animal cells react differently, because a plant cell sits inside a rigid wall that will not let it burst. An animal cell such as a red blood cell has no wall, so too much water makes it burst.

Plasmolysis and deplasmolysis

What is plasmolysis? When a living plant cell is placed in a hypertonic solution, water leaves the cell sap by exosmosis. The vacuole shrinks and the protoplast contracts, pulling away from the cell wall and gathering toward the middle. This is plasmolysis.

Because the wall is permeable, the space between the wall and the shrunken protoplast fills with the outside solution. In the "A cell in a solution" mode choose hypertonic and watch the green protoplast peel away from the corners of the wall and round up while the blue vacuole shrinks. The state reads "Incipient plasmolysis", then "Plasmolysed".

What is deplasmolysis? If a plasmolysed cell is moved into water or a hypotonic solution before it dies, water re-enters by endosmosis, the protoplast swells and presses against the wall again. That is deplasmolysis. The simulation deliberately does not restart when you change the outside solution, so you can watch this recovery yourself.

• Plasmolysis happens only in living cells; a dead cell's membrane is no longer semi-permeable
• A cell kept plasmolysed for long dies and cannot deplasmolyse
• Too much fertiliser makes soil water hypertonic and crops wilt: that is plasmolysis too
• Salt and sugar preserve pickles and jams because microbes are plasmolysed

Imbibition

Dry wood, seeds and raisins swell in water, and wooden doors stick in the rainy season. No solution or membrane is involved here: dry colloidal materials such as cellulose, protein and starch soak up water directly. This is imbibition.

Imbibition is the first step of seed germination: the seed takes up water, swells, splits its coat and switches on its enzymes. Imbibition pressure can be huge; quarry workers once drove dry wooden wedges into cracks in rock and wetted them to split the stone. Imbibition releases heat and increases the volume of the material.

What changes the rate of diffusion and osmosis

In the simulation you can change the two concentrations and the temperature. In real life a few more things matter, and every one of them comes back to how fast the molecules move and how far they must travel.

• Concentration difference: bigger difference, faster rate (Fick's law)
• Temperature: warmer molecules move faster and water becomes runnier, so the rate rises
• Size or mass of the molecule: small, light molecules spread faster (Graham's law)
• Medium: fastest in gases, slower in liquids, slowest in solids
• Distance and area: thin membranes and large surfaces speed it up, which is why lungs have millions of alveoli and the gut has villi
• Pressure: for gases, a pressure difference also drives diffusion

Hands-on experiments: potato, raisins and egg

The textbook experiments work at home. Predict each result first, then compare it with the simulation.

Try these in the simulation

Do not just watch: play with a question. Before each experiment below, write down what you expect.

• In osmosis mode set both concentrations equal (say 0.30 each). Will the level rise? The flow reads "Equilibrium", because the two solutions are isotonic.
• Set 0 on the left and 1.00 on the right: Δh is largest. Now set the right to 0.50: does Δh roughly halve? Osmotic pressure is proportional to concentration.
• In diffusion mode run once at 5 °C and once at 60 °C, and time how long the two graph lines take to meet.
• In cell mode, plasmolyse a plant cell with a hypertonic solution, then switch to hypotonic: it recovers (deplasmolysis).
• Take a red blood cell into a hypotonic solution: its volume climbs until it bursts, and an isotonic solution afterwards cannot bring it back. That is why a saline drip is 0.9 %.

Solved problems

Questions from this chapter mix chemistry and biology. Work each answer out yourself first, then check.

Mistakes almost everyone makes

Watch for these in written answers; examiners take marks off for exactly these.

• Writing "in osmosis the solute moves" is wrong. Only the solvent moves.
• "Water moves from the concentrated to the dilute solution" is backwards. It moves from dilute to concentrated.
• Calling the cell wall semi-permeable is wrong. The wall is permeable; the cell membrane is semi-permeable.
• At equilibrium the molecules do not stop moving; only the net flow becomes zero.
• Calling imbibition osmosis is wrong. Imbibition needs no membrane and no solution.
• A plant cell in an isotonic solution is flaccid, not turgid.

Diffusion and osmosis in everyday life

These processes are at work around us every day. A few examples make the application part of any question easy to answer.

• Salting fish and meat preserves them: microbes are plasmolysed
• Pickles, jams and jellies keep because of their high salt or sugar
• Oral rehydration solution (ORS) in diarrhoea helps the gut absorb water and salts
• Root hairs absorb soil water, and water passes from cell to cell by osmosis
• Transpiration and gas exchange through the stomata of leaves
• Kidney dialysis: wastes diffuse out of blood across a semi-permeable tube
• Reverse osmosis (RO) filters push salty water the "wrong" way to get pure water
• Limp vegetables freshen in water, and a wilted plant stands up after watering

Exam corner: Class 9–10, CBSE and board exams

Diffusion and osmosis appear in Class 8 and 9 science ("the fundamental unit of life"), in Class 11 plant physiology ("transport in plants") and in NEET, SSC and HSC biology. Definitions and the difference table are the commonest short questions.

Short questions ask "What is diffusion?", "What is a semi-permeable membrane?", "What is plasmolysis?". Reasoning questions ask "Why is osmosis called a special type of diffusion?" or "Why is the cell wall permeable?". Application questions give a scenario, a farmer over-fertilising or a potato in sugar solution, and ask you to explain what happens.

• Learn the definitions in textbook wording, but give examples in your own words
• For a difference question, write at least five points as a table
• In a diagram of a plasmolysed cell label the wall, the shrunken protoplast, the vacuole and the gap
• Entrance tests ask MCQs on osmotic pressure, water potential and tonicity; work each formula by hand once

Revision at a glance

Read only this part the night before the exam and the whole chapter comes back.

• Diffusion: particles spread by themselves from high to low concentration; no membrane needed
• Osmosis: solvent moves through a semi-permeable membrane from dilute to concentrated solution
• Osmotic pressure π = iCRT; water potential ψ = ψs + ψp
• Hypotonic: water enters; isotonic: no net flow; hypertonic: water leaves
• Plasmolysis: in a hypertonic solution the protoplast leaves the wall; deplasmolysis: it comes back
• Imbibition: colloids soak up water; the first step of germination
• The rate rises with a bigger concentration difference, higher temperature, smaller molecules, thinner membranes and larger areas

Frequently asked questions