Introduction to Diffusion, Osmosis, Transport and Active Transport

See also Surface exchange of substances in animal organisms

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

What is diffusion? Why does diffusion happen?

What is osmosis? How does osmosis work?

Why is osmosis so important in plants and animals?

What is active transport? How does active transport work?

Why is active transport needed in plants and animals?

You should appreciate that it is important that dissolved substances must be able to get in and out of a cell through the cell membranes, otherwise the cell could not live or reproduce!


Experiments to show diffusion (adapted from my states of matter page)

Particles always moving at random and this causes them to spread throughout in a container if a gas or spread out throughout a solution if dissolved in a solvent.

It is this continuous random movement of particles that allows diffusion to take place.

Diffusion is the natural net movement of particles from a higher concentration to a lower concentration.


In both experiment you start with a container of a colourless medium (air or water), add a coloured material (gas or soluble solid), make sure the container is sealed to prevent any air disturbance (or gas escaping).

The container is left to stand, preferably at a constant temperature to prevent mixing due to convention. Immediately the coloured particles spread (gases mix, solid dissolves and spreads) due to random natural particle movement, from an area of high concentration to one of low concentration.

The spreading is self-evident and direct experimental evidence for the natural constant random movement of particles (molecules or ions).

After many hours, due to diffusion, the colour is evenly distributed due to the random movement of ALL the particles in the gas or liquid mixture.

As you can see, diffusion readily occurs in liquids or gases and it is faster in gases because of the greater distance between the particles.

Diffusion is almost impossible in solids because of the stronger interparticle bonding forces holding the particles in fixed positions.

A particle model of diffusion in gases and liquids: This picture could represent diffusion of molecules or ions in cell fluids or blood stream or gases in the lungs. Imagine the diffusion gradient from left to right for the green particles added to the blue particles on the left. The blue particles could we water and the green particles could be a sugar, protein or carbon dioxide molecule. So, for the green particles, net migration is from left to right and will continue, in a sealed container, until all the particles are evenly distributed (as pictured). BUT, as in living organism, if the green particles are removed or used in some process on the right, then net migration (net diffusion) would continue until there was not enough green particles to create a diffusion gradient from left to right i.e. become evenly very dilute.

Be able to define diffusion as the movement of particles from an area of high concentration to an area of lower concentration.

You experience the gas diffusion experiment (or the diffusion particle picture above!) if somebody sprays perfume or deodorant into a room (green particles in the diagram above!).

Even without draughts or convection, the odour will eventually enter your nose and be detected by your sense of smell in any area of the room.

Similarly you can smell petrol or diesel fumes throughout garage due to the diffusion of fuel vapour molecules,

You should know that all liquid or dissolved particles have kinetic energy and so in constant random motion in all directions and tend to spread in all directions, BUT, on average, they will tend to migrate from a region of higher concentration to a region of lower concentration.

The two experiments described above illustrate this random spreading, but by the nature of the experiment design you will see initially the spreading on average is upwards because the coloured substance starts off at the bottom of the container where the concentration will be very high.


(i) The bigger the concentration difference between two adjacent regions, the steeper the diffusion gradient and the faster the rate of diffusion takes in terms of the net transfer of a particular molecule or ions (eg sugar or sodium ions etc.).

(ii) If the system is warmer, at a higher temperature, the particles gain kinetic energy and can on average move faster and so diffusion is faster.


1b. The action of cell membranes - selective diffusion

Although cell membrane holds the cell together it lets substances in and out, but these substances must be dissolved in water in order pass to and fro through the cell membrane by diffusion.

However, only small molecules and ions can diffuse through the cell membrane e.g. glucose and oxygen for respiration, waste carbon dioxide from respiration, amino acids for protein synthesis and of course water itself, as well as being the solvent.

BUT big molecules cannot get through the cell membrane e.g. starch and proteins.

In the particle model of a cell membrane on the right, the thick dotted line represents the membrane.

Think of the grey circles as the larger molecules like proteins or starch which cannot pass from left to right through the cell membrane.

Imagine the blue circles are water - they can pass through the membrane in any direction.

Imagine the green circles are small molecules or ions - they can also pass through the cell membrane in either direction, but the concentration is greater on the left than the right.

Therefore the diffusion gradient is from left to right and there is a net movement of the green particles (smaller molecules) from the left higher concentration to the right lower concentration passing through the cell membrane in the process.

Also bare in mind that the larger the surface area of a membrane, the faster the net rate of diffusion of a particular molecule or ion.

Examples of diffusion in living organisms

The process of respiration.

The thin cell membranes allow the diffusion of small molecules in and out of cells.

Since the capillaries are thin and numerous, the diffusion distance from cells is short, so transfer of nutrients in, and waste products out, is as efficient as possible.

As the cells respire they use up oxygen/glucose, so their concentration falls in the cell. Therefore the external concentrations (e.g. in capillaries) is higher, so more oxygen/glucose will diffuse into the cell.

At the same time, the concentration of the waste product carbon dioxide builds in the cell, and so carbon dioxide will then naturally diffuse out of the cell to the lower concentration region in the capillaries.

For more details on gas exchange see Surface exchange of substances in animal organisms



Know and understand that water often moves across boundaries by osmosis - a special case of particle diffusion down a concentration gradient.

Know that osmosis is the diffusion or bulk movement of water from a dilute to a more concentrated solution through a partially permeable membrane (semi-permeable membrane) that allows the passage of very small molecules like water (diagram on right).

A partially permeable membrane has extremely small pores or holes that only allow the tiniest of molecules like water through e.g. even relatively small molecules like sucrose will not pass through a partially permeable membrane (see experiment in section 2b. below).

So, in living organisms, only water gets through and depending on the relative concentration of dissolved substances either side of the membrane, osmosis can happen in either direction.

Although the water molecules (and any other particles) are moving around at random, there will be a net transfer of water in one direction at a time through a partially permeable membrane.

The net direction of diffusion of water is from a less concentrated solution to a more concentrated solution i.e. from the higher concentration of water molecules to a lower concentration of water molecules across the membrane.

In other words a more concentrated solution becomes more dilute in the process.

This osmosis diffusion can occur in either direction depending on the relative concentration of the solutes in the cell fluids or tissue fluids and concentrated solutions e.g. of sugars, will tend become diluted by water passing through the partially permeable membrane.

The soft cell wall, or outer membrane of an animal cell, acts as a partially permeable membrane.

The water surrounding cells, the tissue fluid, contains the dissolved molecules the cell needs to survive eg sugars, amino acids, oxygen, as well as waste carbon dioxide etc.

If the cells are short of water ('partially dehydrated'), the concentration of dissolved substances increases, so water diffuses through the cell membrane into the cells to dilute the cell fluids until equilibrium is established. Conversely, if the cell solution is too dilute, then water will diffuse out from osmotic action across the semi-permeable membrane of the cell wall.

See Homeostasis - osmoregulation - ADH - water control  gcse biology revision notes


2b. A simple demonstration of osmosis

Set-up and method

You can do a simple experiment to demonstrate osmosis by placing blocks or cylinders of potato into pure water and then a series of sugar solutions (e.g. glucose/sucrose) increasing in concentration (increasingly higher molarity mol/dm3 e.g. from 0.2 to 1.0) i.e. from a dilute to a very concentrated sugar solution.

The dependent variable is the potato mass and the independent variable is the concentration of the sugar solution.

All the other variables should be kept constant - so make sure the original potato blocks are identical in size and mass, same temperature, same time left to change, same sugar and same volume of liquid - all about a 'fair test'.

You measure and record the mass of the potato blocks and place them individually in pure water and the range of sugar solutions in a series of beakers.

Leave the beakers for 24 hours.

Carefully remove the potato blocks from the liquid, dry them with a paper towel and re-weigh them.


From the weighings work out the mass loss from each potato block.

You can convert the weighing into % mass loss.

The block in pure water should have hardly changed in mass.

The other potato 'chips' will shrink as the cells lose mass (water) to the more concentrated external sugar solution.

You can plot a graph of mass loss (g or %) versus the sugar concentration (mol/dm3), you should get a rising positive gradient correlation.

The more concentrated the sugar solution, the greater the mass loss of the potato block.


Since the potato blocks have lost mass, the direction of osmosis is for water to leave the potato and dilute the external sugar solution.

The water in the potato cells will undergo osmosis and diffuse through the partially permeable membranes of the potato cells to try and dilute the external sugar solution.

The greater the concentration, the greater the osmotic effect - a higher concentration of sugar will draw out more water - a greater rate of diffusion.

Sources of error

Obviously, all experiment can be repeated e.g. groups of students in the same class, this reduces errors - some groups of pupils might be more careful than others. You can then use mean values in your numerical analysis.

If the potato blocks are not completely dried, your will record a smaller mass of water loss than actually happened.

Especially if the room is warm, evaporation would increase the concentration of the sugar solution, this would increase the mass loss. This error can be eliminated by putting 'lids' over the beaker - paper covers held with an elastic bands will do.

Other similar experiment

You can repeat the experiments using common salt (sodium chloride, NaCl) and you should get a similar pattern of results.


Drinks and hydration

Most soft drinks contain water, sugar and ions.

Sports drinks contain sugars to replace the sugar used in energy release during the activity.

They also contain water and ions to replace the water and ions lost during sweating.

Know and understand that if water and ions are not replaced, the ion / water balance of the body is disturbed and the cells do not work as efficiently.



See also Examples of surfaces for the exchange of substances in animal organisms

What is 'active transport'

Substances are sometimes absorbed against a concentration gradient - a net transfer against the normal diffusion gradient action described above in section 1.

This means transfer occurs in the opposite direction to the natural direction of the diffusion gradient.

e.g. active transport enables cells to absorb ions from very dilute solutions.

BUT, this movement of chemicals across a cell membrane, requires the use of energy from respiration and the overall process is called active transport.

The diagram illustrates the movement of molecules (green spheres) being moved through the membrane in the opposite direction to the natural diffusion gradient.

Remember that absorption by diffusion down the concentration gradient through membranes doesn't require energy from respiration


Examples of active transport

The gut

Active transport is required to absorb nutrients like amino acids, sugars like glucose etc. from the gut when the concentration in the gut is lower than their concentrations in the blood supply, and a healthy body requires these nutrients all the time.

If the concentrations of nutrients (e.g. sugars, amino acids) in the gut is higher than that in the blood stream, then the nutrients will naturally diffuse into the blood stream because of the direction of the concentration gradient (more concentrated ==> less concentrated).

If the concentration gradient flow is in the direction of the blood stream (higher) to gut (lower), then respiration powered active transport must be used to work against the natural diffusion flow.

So active transport enables the gut to move nutrients like into the blood even though the natural concentration gradient (diffusion gradient) is the wrong way round.

Glucose can be transferred into the blood stream, even if its concentration is higher in the blood stream, and so conveyed to cells for respiration.

need extra gut diagram

For more on the gut see Surfaces for the exchange of substances in animal organisms



Active transport is used in the absorption of nitrates and other ions by plant roots.

For details see Transport and gas exchange in plants, transpiration, absorption of nutrients etc.



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