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Transport: 1. DIFFUSION what is it? how to demonstrate it and its importance in biology

Doc Brown's Biology exam study revision notes

There are various sections to work through, after 1 they can be read and studied in any order.

INDEX of notes: Transport in organisms

(1) DIFFUSION what is it? how to demonstrate it!

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 GCSE chemistry notes page)

Particles are 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 and, apart from active transport movements, there is a net transfer of particles from a higher concentration to a lower concentration.

This is the process of spreading, called diffusion, naturally occurs in gases and liquids, because all the particles (molecules or ions) have sufficient kinetic energy to move around freely at random.

Diffusion is almost impossible in solids because the particles cannot move freely from one position to another.

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

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

Diffusion of a specific material will continue until an equilibrium is reached when the concentration is evenly distributed and a concentration gradient for that material no longer exists.

The pictures below illustrate a diffusion gradient in a gas or a liquid.

The green particles are more concentrated to the left, so an average, with random movement, they will spread out more to the right. Similarly, blue green particles are more concentrated to the right, so an average, with random movement, they will spread out more to the left.

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(c) doc b

An experiment to show the diffusion of air and bromine into each other in the fume cupboard of a chemistry laboratory  and ...

showing diffusion of a dissolved solid in a liquid, but it could be glucose in fluids between cells, semi-permeable membranes discussed further down the page.

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.


Another 'biological' demonstration of diffusion - with a bit of added chemistry!

1. Agar gel cubes are prepared with a little sodium hydroxide solution and a few drops of phenolphthalein indicator added. Phenolphthalein turns pink-red in alkaline solution.

The agar jelly cubes are placed in a beaker of dilute hydrochloric acid.

2. When the beaker is left to stand, the acid will slowly diffuse into the agar jelly cubes and neutralise the alkali ...

hydrochloric acid + sodium hydroxide ===> sodium chloride + water

HCl(aq)  +  NaOH(aq)  ===> NaCl(aq)  +  H2O(l)

... and the agar jelly cubes begin to turn colourless because the indicator phenolphthalein turns colourless in acid.

3. Gradually, all of the alkali is neutralised, and the whole of the agar gel turns colourless as the acid diffuses right to the centre of the cubes.


It is also true to say that the alkali can also diffuse out of the cubes and be neutralised in the acid solution. Either way, it doesn't matter, all the particles are on the move constantly at random in all directions.

At the start, the diffusion gradient for the acid is into the agar gel cubes.

You can do an investigation with different concentrations of acid to change the diffusion gradient and time the results. You need to keep the recipe of the agar jelly cubes constant and conduct the experiments at the same temperature. You should be able to predict the pattern of results!

Keywords, phrases and learning objectives for this part on transport in organisms

Be able to describe the process of transferring substances by diffusion.

Be able to explain what diffusion is and how to demonstrate it with experiments and its importance in biology.



INDEX of biology notes on transport in organisms


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