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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
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INDEX
of biology notes on transport in organisms
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 (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.
==>
==>
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.
Notes:
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!
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