Diffusion
Importance of the Substance Transportation
It is vital for substances to be transported in and out of a range of organisms.
These substances include:
1.
2.
3.
4.
5.
6.
oxygen
carbon dioxide
water
dissolved food molecules
mineral ions
urea
Cells must remove waste substances and to take in useful substances in order for the cell to
function. These substances exchange through the cell membrane through diffusion,
__osmosis __and __active transport. __
Surface Area Volume Ratio
Multicellular organisms__ require exchange surfaces and a transport system__ in order for
substances to pass in and out of the cells.
The larger the surface area is compared to the volume, the quicker and more substances can
pass through the membranes.
1. Small and thin structures have a larger surface area compared to the volume.
2. Large and thick structures have a smaller surface area compared to the volume.
Multicellular organisms have specialised exchange surfaces__ __- such as lungs, gills or
villi in the intestines so that there is a larger surface area for the transport of substances to
take place.
Surface area = Number of sides x (length x length)
Volume = length x length x length
So, as the volume increases, the surface area does not increase at the same rate. If there was a
sugar cube that was 5g, it will have a larger SA:V ratio to 5g of powdered sugar.
Alveoli Adaptations
Alveoli__ __are highly adapted for gas exchange by diffusion between air in the lungs, and
blood in the capillaries.
1. They are folded in order to maximise the surface area to volume ratio.
2. This allows more gas exchange to take place.
3. The alveoli wall linings are moist, allowing oxygen and carbon dioxide to dissolve
easily.
4. It has__ thin walls__ so that the distance in which the gases have to diffuse across is
short.
5. Capillaries have a rich blood supply, which intensifies the concentration gradient.
Rate of Diffusion
The following factors have an affect on the rate of diffusion:
Surface Area
1. The larger the surface area to volume ratio, the quicker the rate of diffusion takes
place.
2. The substance simply has more area to diffuse across, despite being the same volume.
Concentration Gradient
1. The higher the concentration differences in the gradient, the higher the diffusion rate.
2. If there is an equal concentration on both sides of the membrane, the rate of diffusion
will be extremely slow.
3. E.g. if the alveoli has a much higher concentration of oxygen than in the blood, then
the oxygen will diffuse into the blood much quicker.
Diffusion Distance
1. Shorter the distance between the material in which the substance is diffusing through,
the faster the diffusion rate. It simply has a shorter journey to travel.
Temperature
1. By increasing the temperature, the rate of diffusion increases as there is more energy
in each particle.
2. These particles will now bounce against each other more frequently.
Ficks Law
The rate of diffusion can be calculated using Fick’s Law:
__Rate of diffusion is proportional to: (surface area × concentration difference ) / thickness of
membrane __
This will mean that the rate of diffusion will double if the surface area / concentration
difference is doubled, or if the thick of membrane in which exchange takes place is halved.
Based on Ficks Law, what will happen to the rate of diffusion if the surface area or
concentration difference is doubled?
Your answer should include: Diffusion / Double
Explanation: Rate of diffusion will double
What will happen to the rate of diffusion if the temperature is increased?
Increase
Explanation: Increase, as the particles have more energy
What is the advantage of the alveoli’s thin walls?
Short diffusion distance
Effect of surface area:volume ratio on rate of
diffusion
2/19/2016
0 Comments
Equipment required;

A block of stained agar
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Single edged razor
blade/scalpel
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Vernier callipers
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5cm3 syringe

White tile

Ruler

Stopwatch

Hydrochloric acid 2M (1M in
our experiment)

Forceps
Method
1. Cut the agar into 5 different-sized cubes, use the razor blade to cut. The measurements
should be made with vernier callipers or a clear ruler.
2. Use the razor blade to place the agar block into equally sized test tubes
3. Use the syringe to immerse the blocks in 3cm3 of 1M HCl
4. As the HCl is added start the stopwatch
5. When the block becomes colourless, immediately record the time.
6. Record the results in a table and draw a graph.
Safety

Wear safety googles, HCl is an irritant.

Take care with the razor blade cut away from the body.
Experiment
Results
The results show that as the surface area:volume ratio the time it takes for the agar block
to become colourless decreases, therefore the rate of diffusion increases. This is due to the
surface area being greater compared to the volume of the agar jelly. More HCl molecules
can diffuse into the jelly at the same time, therefore the HCl molecules can reach the whole
of the agar in a shorter amount of time. Resulting in the agar block becoming colourless
more quickly with a greater SA:Vol ratio.
Questions
Identify 3 factors controlled;
1. Temperature
2. Size of reciprocal HCl is in
3. Concentration difference of HCl to the agar.
Increase reliability and accuracy;
To increase the reliability the experiment should be repeated at least 3 times for each size
of block. Then averaging the results and discounting any anomalies.
To increase accuracy the agar blocks should be cut more accurately, a laser cutter would be
ideal as it would have much more accurate cutting than a razor blade. As you can see in the
pictures of the experiment our agar blocks did not have very straight edges which would
affect our surface area:volume ratio. Multiple stopwatches should be used, and each
experiment should be done one at a time, so that readings for time are closer to the true
value.
Explain effect of SA:Vol ratio on rate of diffusion, relate to living organisms
As the surface area:volume ratio increases rate of diffusion also increases. This allows
single celled organisms that have a large SA:Vol, such as amoeba, to rely on just diffusion to
provide nutrients for cell processes. Whereas multicellular organisms whom have a small
SA:Vol have evolved transport systems as they cannot rely on skin surface diffusion to
provide nutrients to the cells in the body quickly.
Limitations of experiment to compared to a living organism
The experiment uses a lifeless agar jelly block, which lacks a cell membrane which allows
diffusion to take place in all living cells. The cell membrane is a complex mix of a
phospholipid bilayer, protein carrier channels, ion channels, carbohydrates, proteins. The
agar block lacks this important feature. Other than that its is a good way to show the
comparison between surface area:volume ratio and rate of diffusion
5.2C: Diffusion
1. Last updated
Nov 19, 2019
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2.
5.2B: Selective Permeability
o 5.2D: Facilitated transport
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Contributed by Boundless
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General Microbiology at Boundless
Diffusion is a process of passive transport in which molecules move from an area of higher
concentration to one of lower concentration.
Learning Objectives
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Describe diffusion and the factors that affect how materials move across the cell membrane.
Key Points
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Substances diffuse according to their concentration gradient; within a system, different
substances in the medium will each diffuse at different rates according to their individual
gradients.
After a substance has diffused completely through a space, removing its concentration
gradient, molecules will still move around in the space, but there will be no net movement
of the number of molecules from one area to another, a state known as dynamic
equilibrium.
Several factors affect the rate of diffusion of a solute including the mass of the solute, the
temperature of the environment, the solvent density, and the distance traveled.
Key Terms
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diffusion: The passive movement of a solute across a permeable membrane
concentration gradient: A concentration gradient is present when a membrane separates
two different concentrations of molecules.
Diffusion
Diffusion is a passive process of transport. A single substance tends to move from an area of
high concentration to an area of low concentration until the concentration is equal across a
space. You are familiar with diffusion of substances through the air. For example, think about
someone opening a bottle of ammonia in a room filled with people. The ammonia gas is at its
highest concentration in the bottle; its lowest concentration is at the edges of the room. The
ammonia vapor will diffuse, or spread away, from the bottle; gradually, more and more people
will smell the ammonia as it spreads. Materials move within the cell ‘s cytosol by diffusion,
and certain materials move through the plasma membrane by diffusion. Diffusion expends no
energy. On the contrary, concentration gradients are a form of potential energy, dissipated as
the gradient is eliminated.
Figure 5.2C.15.2C.1:
Diffusion: Diffusion through a permeable membrane moves a substance from an area of high
concentration (extracellular fluid, in this case) down its concentration gradient (into the cytoplasm).
Each separate substance in a medium, such as the extracellular fluid, has its own concentration
gradient independent of the concentration gradients of other materials. In addition, each
substance will diffuse according to that gradient. Within a system, there will be different rates
of diffusion of the different substances in the medium.
Factors That Affect Diffusion
Molecules move constantly in a random manner at a rate that depends on their mass, their
environment, and the amount of thermal energy they possess, which in turn is a function of
temperature. This movement accounts for the diffusion of molecules through whatever medium
in which they are localized. A substance will tend to move into any space available to it until
it is evenly distributed throughout it. After a substance has diffused completely through a space
removing its concentration gradient, molecules will still move around in the space, but there
will be no net movement of the number of molecules from one area to another. This lack of a
concentration gradient in which there is no net movement of a substance is known as dynamic
equilibrium. While diffusion will go forward in the presence of a concentration gradient of a
substance, several factors affect the rate of diffusion:
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Extent of the concentration gradient: The greater the difference in concentration, the more
rapid the diffusion. The closer the distribution of the material gets to equilibrium, the slower
the rate of diffusion becomes.
Mass of the molecules diffusing: Heavier molecules move more slowly; therefore, they
diffuse more slowly. The reverse is true for lighter molecules.
Temperature: Higher temperatures increase the energy and therefore the movement of the
molecules, increasing the rate of diffusion. Lower temperatures decrease the energy of the
molecules, thus decreasing the rate of diffusion.
Solvent density: As the density of a solvent increases, the rate of diffusion decreases. The
molecules slow down because they have a more difficult time getting through the denser
medium. If the medium is less dense, diffusion increases. Because cells primarily use
diffusion to move materials within the cytoplasm, any increase in the cytoplasm’s density
will inhibit the movement of the materials. An example of this is a person experiencing
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dehydration. As the body’s cells lose water, the rate of diffusion decreases in the cytoplasm,
and the cells’ functions deteriorate. Neurons tend to be very sensitive to this effect.
Dehydration frequently leads to unconsciousness and possibly coma because of the
decrease in diffusion rate within the cells.
Solubility: As discussed earlier, nonpolar or lipid-soluble materials pass through plasma
membranes more easily than polar materials, allowing a faster rate of diffusion.
Surface area and thickness of the plasma membrane: Increased surface area increases the
rate of diffusion, whereas a thicker membrane reduces it.
Distance travelled: The greater the distance that a substance must travel, the slower the rate
of diffusion. This places an upper limitation on cell size. A large, spherical cell will die
because nutrients or waste cannot reach or leave the center of the cell. Therefore, cells
must either be small in size, as in the case of many prokaryotes, or be flattened, as with
many single-celled eukaryotes.
A variation of diffusion is the process of filtration. In filtration, material moves according to
its concentration gradient through a membrane; sometimes the rate of diffusion is enhanced by
pressure, causing the substances to filter more rapidly. This occurs in the kidney where blood
pressure forces large amounts of water and accompanying dissolved substances, or solutes, out
of the blood and into the renal tubules. The rate of diffusion in this instance is almost totally
dependent on pressure. One of the effects of high blood pressure is the appearance of protein
in the urine, which is “squeezed through” by the abnormally high pressure.
Lab 1 - Surface
Area to Volume
Ratios
Lab 1 - Surface Area to Volume Ratio WORKSHEET
Major Concepts
1.
2.
3.
4.
5.
6.
As the surface area and the surface -to-volume ratio of a cell
increases, so does its ability to exchange materials. The rate
increased almost linearly with increasing surface -tovolume ratio.
As cells perform their normal functions, they need to
exchange materials with the world outside of the cellular
membrane. Cells with larger surface -to-volume ratios are able
to exchange materials more efficiently.
The smaller a cell, the larger its surface -to-volume ratio. As a
cell grows larger, the volume increases faster than the surface
area. Small cells are more efficient at exchanging
materials than large cells because they have more surface area
to work with in relation to their size.
In the small intestine, th e surface is covered by epithelial
cells that have increased their surface areas by forming tiny
folds of their cellular membrane called microvilli. The
microvilli dramatically increase the surface area of these cells
and their ability to absorb nutrients. In general, cells adapt to
the task of absorption and excretion by increasing the size of
their cellular membrane while keeping their volume low.
As cells grow larger, their volume increases faster than their
surface area. This leads to a decrease in the efficiency of the
cell at exchanging materials across the cell membrane.
To be able to efficiently exchange materials, cells must divide
by mitosis and cytokinesis when they become too large.
An experiment to investigate the effect of surface area :
volume ratio on the diffusion rate.
2/9/2016
3 Comments
The objectives of this experiment was to calculate the surface area to volume to ratio of pieces of
agar and determine what effect this ratio had on the rate of diffusion.
Equipment:

A block of stained agar

Scalpel/single razor blade

4 test tubes

2M hydrochloric acid

5 cm3 syringe

White tile

Callipers

Stopwatch
Method:

Cut the agar into 4 different sized cubes (10mm, 7mm, 5mm and 1mm) using a sharp razor, callipers
and a white tile to provide a neutral background to aid cutting.

Using the razor blade to lift the agar blocks, place each different sized cube into different test tubes
and label clearly.
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Using a syringe, immerse the blocks in 2M HCl by adding 3 cm3 of the acid.

Immediately start the stopwatch and time how long the pink colour takes to disappear and for each
block to turn colourless.

To ensure reliability, repeat this experiment 3 times.
Safety precautions:

HCl is an irritant so eye protection should be worn.

Cut away from oneself and take care with sharp edges.
What happens when the agar is placed in the HCl?
As time goes on the colour of the agar block becomes increasingly orange as it loses its pink colour.
The reason for this is because of the hydrochloric acid diffusing into the block and making it lose its
colour. The acid travels from an area of high concentration of HCl (in the test tube) to an area of low
concentration of HCl (the agar cube) along a concentration gradient and the point at which the
original colour has completely gone is when an even distribution of particles has been achieved.
This experiment was designed to then see how this diffusion process occurred in different sized
cubes and to investigate the difference I looked at the diffusion rate, i.e. how long it took for the pink
colour to disappear.
My prediction would be that the rate of diffusion would speed up as the surface area to volume ratio
was increased, i.e. the cube was made smaller.
Conclusion
From these results I can see that the surface area to volume ratio ad an effect on how fast diffusion
occurred. As the agar cubes were cut smaller the ratio of surface area to volume became larger and
this affected the diffusion rate by making it faster. The results show a clear trend that supports my
knowledge on how the diffusion rate speeds up when there is a larger surface area to volume ratio.
The time decreased which highlights how diffusion occurred quicker as there was more surface area
in relation to its volume over which the HCl could diffuse over. Another reason for this increase in
rate was due to the fact that the HCl particles had to travel longer distances in those agar cubes
which had a larger volume in comparison to its surface area. Overall my results support the idea that
as the surface area to volume ratio increases so does the rate of diffusion.
This experiment relates to real life because single-celled organisms use diffusion as their means of
getting the substances they need and expelling those they don’t. Diffusion works efficiently for
single-celled organisms such as amoeba due to a large surface area to volume ratio as there is a
relatively large surface area over which substances can diffuse into or out of the organism. This is
demonstrated in my experiment as the smaller cubes with a larger surface area to volume ratio
resemble single-celled organisms. My results also support the idea that large organisms cannot use
diffusion as their means of transporting substances in and out, as shown by the 10mm by 10mm
cube which took 10 minutes and 10 seconds for the HCl to diffuse all the way through. Diffusion
would not be efficient enough for larger organisms as the surface area is not large enough for
substances to diffuse over, as a result they require a complex transport system.
This experiment does have its limitations as the organism’s cell is represented by a lifeless agar cube. It
lacks a semi-permeable membrane and only allows for simple diffusion unlike real-life membranes. Cell
surface membranes are much more sophisticated as they consist of a combination of phospholipids,
carbohydrates and carrier/channel proteins. As well as this the shape of the agar cube does not resemble
an organism’s cell. Apart from these limitations this experiment is a good way of demonstrating the effect
of surface area to volume ratio on the rate of diffusion.
jor Concepts
1.
2.
3.
4.
5.
6.
As the surface area and the surface -to-volume ratio of a cell
increases, so does its ability to exchange materials. The rate
increased almost linearly with increasing surface -tovolume ratio.
As cells perform their normal functions, they need to exchange
materials with the world outside of the cellular membrane. Cells
with larger surface-to-volume ratios are able to exchange
materials more efficiently.
The smaller a cell, the larger its surface -to-volume ratio. As a
cell grows larger, the volume increases faster than the surface
area. Small cells are more efficient at exchanging materials than
large cells because they have more surface area to work with in
relation to their size.
In the small intestine, the surface is covered by epithelial cells
that have increased their surface areas by forming tiny folds of
their cellular membrane called microvilli. The
microvilli dramatically increase the surface area of these cells
and their ability to absorb nutrients. In general, cells adapt to
the task of absorption and excretion by increasing the size of
their cellular membrane while keeping their volume low.
As cells grow larger, their volume increases faster than their
surface area. This leads to a decrease in the efficiency of the cell
at exchanging materials across the cell membrane.
To be able to efficiently exchange materials, cells must divide
by mitosis and cytokinesis when they become too large.