Diffusion, Osmosis, Transport and Active Transport
See also Surface
exchange of substances in
Doc Brown's Biology Revision Notes
Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent
What is diffusion? Why does diffusion
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
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.
able to define diffusion as the movement of particles from an area of high
concentration to an area of lower concentration.
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
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
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
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
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.
diffusion in living organisms
The process of
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
For more details on gas exchange see
Surface exchange of substances in
Know and understand that water often moves across boundaries by
osmosis - a special case of particle diffusion down a
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
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
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.
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
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 %
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
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
Other similar experiment
You can repeat the experiments using
common salt (sodium chloride, NaCl) and you should get a similar pattern
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.
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.
Examples of surfaces for the exchange of substances in
What is 'active transport'
Substances are sometimes absorbed against a
concentration gradient - a net transfer against the normal diffusion gradient
action described above in
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
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
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
For more on the gut see
Surfaces for the exchange of substances in
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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