Cells- Diffusion

Diffusion is the spreading out of the particles of any substance in solution, or

particles of a gas, resulting in movement from an area of higher
concentration which is where there's more particles (the socks) to an area
of lower concentration, where there's fewer particles (the air surrounding it).

Living organisms need different substances to be able to survive and function.

These substances need to be transported into and out of their cells through
diffusion. During diffusion, molecules move from an area of high concentration to
an area of low concentration. They are said to move down a concentration
gradient. Particles diffuse until they are evenly spaced apart. Diffusion is
a passive process, which means that no energy is needed and it happens
naturally.

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Diffusion in organisms
The cell membrane has an important job to do. It is selective. This means it won't
just let any random particle through - it can monitor which particle is allowed
through it and also how many particles. Pretty clever, right?!

In unicellular organisms such as the amoeba (organisms made up of only one

cell), diffusion is how the organism gets its food and oxygen, and how it gets rid
of waste products. As it's only made up of one cell, the particles of food or water,
for example, don't have far to travel, so diffusion is the simplest way to enter the
cell. They don't need complex transport systems.

In multicellular organisms, surfaces and organ systems are specialised for

exchanging materials. This is to allow sufficient molecules to be transported into
and out of cells for the organism’s needs. Diffusion is the main way that
substances move over short distances in organisms.

Breathing involves exchanging gases in the lungs - this requires diffusion. When
you breathe in, oxygen in the inhaled air diffuses through the tiny alveoli (air
sacs) in your lungs into your bloodstream and is transported around your
body. Carbon dioxide is the waste gas produced by respiration. Carbon dioxide
diffuses from cells into the bloodstream and is exhaled by the lungs.

The alveoli have a few adaptations that make gas exchange very efficient. For
example, they're only one cell thick, making them very thin! This allows gases to
pass through easily and quickly!

Another example of diffusion is in the small intestine. Digested food is broken

down into small molecules such as glucose and it's transported around the body
in the blood. The small intestine is lined with many finger-like projections
called villi. The glucose molecules diffuse through the villi of the small intestine
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into the blood to be transported around the body. The villi are adapted by being
very thin and having finger-like projections that increase the surface area.

Diffusion also occurs in plants. Plants take in carbon dioxide

for photosynthesis and produce oxygen. These enter and leave the plant through
diffusion. The leaf is adapted for gas exchange. There are tiny pores,
called stomata, in the surface of the leaf which allow carbon dioxide to enter the
leaf for photosynthesis. Molecules of carbon dioxide diffuse from a region of
higher concentration (the atmosphere) to a lower concentrated area (leaf). The
stomata also allow oxygen to diffuse out.
One way to demonstrate how diffusion happens is using something called agar.
Agar is a little like jelly and is full of nutrients, which is why it's often used in the
science lab to grow things such as bacteria in petri dishes.

We can use different sized cubes of agar dyed pink to represent organisms - small,
medium and large.

Placing each cube in a petri dish and adding hydrochloric acid to it

will remove the pink colour from the cube. The hydrochloric acid is able to
diffuse into the agar cube and react with the dye, turning it white. If no acid
diffuses into the cube then it will remain pink.

We can time how long it takes for the cube to turn from pink to white. This tells
us how quickly diffusion has happened! Pretty smart right?

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What do we think will happen in this investigation? What prediction can we
make?
The smaller the cube the faster the acid will diffuse into it, turning it white
quicker.

Why?

The size of a cell and its surface area affects how quickly substances can get
in and out.

The smaller cube has a larger surface area to volume ratio. This means it has a
bigger surface to allow substances in even though its volume is small. This means
the space inside the cube will turn white quicker because the acid can enter the
cube quickly.

The larger cube however, has a smaller surface area to volume ratio. This means
that the amount of surface it has compared to its volume is actually smaller
(which sounds odd because it's a bigger cube!) Remember we're talking about a
ratio, so a comparison between the area of the surface of the cube and the amount
of space inside the cube. The large cube will take longer to turn white because the
acid has to interact with all the agar inside the cube (it has a larger volume than
the small cube).
Questions:
1. Billie was investigating diffusion using agar cubes. She looked at how quickly
diffusion took place in different sized cubes. Her results are below but they are
unreliable. Why?
Time taken to turn white
Size of agar cube (cm3)
(minutes)
Small (1 x 1 x 1) 5
Medium (2 x 2 x 2) 10
Large (3 x 3 x 3) 12
Choose one correct answer.
 She used different sized cubes
 She used a stopwatch to time how long the cubes to turn white
 The time went up with every cube
 She only did the investigation once

2. Billie was investigating diffusion using agar cubes. She looked at how quickly
diffusion took place in different sized cubes.
Look at her results.Can you spot the anomalous result?

Size of agar cube (cm3) Time taken to turn white (minutes) Mean time taken to
Trial 1 Trial 2 Trial 3 turn white (minutes)
Small (1 x 1 x 1) 5 5 4
Medium (2 x 2 x 2) 10 9 9

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Large (3 x 3 x 3) 12 2 12

Choose one correct answer.

 12
 9
 5
 2

4. Billie was investigating diffusion using agar cubes. She looked at how quickly
diffusion took place in different sized cubes.
Look at her results.
Can you calculate the mean time for each cube to turn from pink to white?

Size of agar cube (cm3) Time taken to turn white (minutes) Mean time taken to
Trial 1 Trial 2 Trial 3 turn white (minutes)
Small (1 x 1 x 1) 5 5 4
Medium (2 x 2 x2 ) 10 9 9
Large (3 x 3 x 3) 12 2 12
Match:
Small 4.7 minutes
Medium 9.3 minutes
Large 12 minutes

Living organisms need different substances to be able to survive and function.

These substances need to be transported in and out of their cells through
diffusion. During diffusion, particles move from an area of high concentration to
an area of low concentration. We say they move down a concentration
gradient. Particles diffuse until they are evenly spaced apart. Diffusion happens
naturally and doesn't need energy, so we say it's a passive process.

Diffusion in organisms
Organisms usually use different organ systems to carry out the life processes - for
example, humans have a digestive system to help with nutrition and a circulatory
system to help with respiration. These organ systems are made up of groups of
tissues and organs that work together.

However, not all organisms have organ systems or even tissues. Some are only
made up of one cell. We call these unicellular organisms. These unicellular
organisms can carry out the seven life processes, but only have one cell with
which to do so.

In unicellular organisms such as the amoeba or bacteria, diffusion is how the

organism gets its food and oxygen, and how it gets rid of waste products. As it's
only made up of one cell, the particles of food or water, for example, don't have
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far to travel, so diffusion is the simplest way to enter the cell. They don't need
complex transport systems and rely on the cell membrane, which is selective. This
means it won't just let any random particle through - it can monitor which particle
is allowed through it and also how many. Pretty clever, right?!

An example of a unicellular cell is bacteria - you can see what a typical bacterial
cell looks like below:

Another example of a unicellular cell is the amoeba:

Amoebas have a large surface area in comparison to their volume - remember that
they're made up of only one cell. That means it doesn't take long for gases such
as oxygen to diffuse in and carbon dioxide to diffuse out. It has a short diffusion
distance allowing the amoeba to exchange gases easily. This also helps the
amoeba to get its nutrients through diffusion easily too.

In multicellular organisms, surfaces and organ systems are specialised for

exchanging materials. This is to allow molecules to be transported in and out of
cells for the organism’s needs. Diffusion is the main way that substances move
over short distances in organisms.
Questions:.

1. During diffusion, particles move from an area of _____concentration to an area of

_______concentration. They're said to move down a_______ gradient. Diffusion is a
process, which means that no________ is needed.

2. How are unicellular organisms different to multicellular organisms?

Choose two options from below.

A Unicellular organisms are made up of more than one cell whereas

multicellular organisms are made up of only one cell.
B Unicellular organisms are made up of only one cell whereas multicellular
organisms are made up of more than one cell.
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C A unicellular organism uses diffusion to exchange substances through its cell
membrane whereas multicellular organisms have specialised structures for
exchanging materials.
D A unicellular organism doesn't use diffusion to exchange substances through
its cell membrane while multicellular organisms have specialised structures for
exchanging materials.
Choose all correct answers.
 A
 B
 C
 D
3. Unicellular organisms use diffusion to exchange substances
with the environment. What sort of substances do
unicellular organisms exchange?
Choose all correct answers.
 Waste products
 Food molecules
 Oxygen
 Sunlight
4. Give two examples of unicellular organisms.Choose all correct answers.
 Amoeba
 Lungs
 Intestines
 Bacteria
5. Diffusion is a process that doesn't need energy, which is really useful for
unicellular organisms.What's the word used to describe this?
Diffusion is a __________________process, which means that it doesn't need energy
6. Amoebas are unicellular organisms that use diffusion to exchange substances.
Why are amoebas good at diffusion?
Choose one correct answer.
 Amoebas have a small surface area
 Amoebas have a large surface area
 Amoebas don't have a surface area

7. How are unicellular organisms well suited for diffusion?

Unicellular organisms are well suited for diffusion because they have a long / shor
t diffusion distance from the outside of the cell to the inside of the cell allowing dif
fusion to happen quickly / slowly.
8. Amoebas are found living in water.
Imagine that one amoeba is using 50 molecules of oxygen to respire. Surrounding
the amoeba there are 100 molecules of oxygen dissolved in the water.
Which way will the oxygen diffuse? Choose one correct answer.
 The oxygen won't diffuse at all - it will stay as it is
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  From the water into the amoeba
 From the amoeba into the water
9. Why do multicellular organisms have specialised systems to get the substances
they need?
Because multicellular organisms are made up of many________ , they need help to
exchange materials as simple __________won't always work.
10. X organisms don't have complex transport systems. They have a Y membrane
which is selective. This means it will only let selective substances diffuse Z.
10. X organisms don't have complex transport systems. They have a Y membrane
which is selective. This means it will only let selective substances diffuse Z.

Diffusion is the spreading out of particles. It can occur across membranes, into
and out of cells and between different regions. Some examples of where diffusion
takes place almost constantly is in the lungs, in a leaf, in the root hair cells of a
plant and in liver cells.

It is the movement of particles down a concentration gradient, from a high

concentration to a low concentration.

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Diffusion takes place in liquids and gases very readily. It doesn't take place in
solids however. This is because the particles in a solid do not move very much -
they just vibrate on the spot, so particles have no space to wriggle their way
through a solid to diffuse.

Can you speed up diffusion?

The rate of diffusion can be affected by temperature. The higher the temperature,
the more kinetic energy the particles will have, so they will move faster and mix
more quickly. Similarly, the lower the temperature, the less kinetic energy the
particles will have so diffusion will happen much slower.

Questions:

1. The picture above shows a small drop of dye placed into a sealed box.

Can you predict how the picture might have changed after it has fully
diffused? Choose one correct answer.

2.
The picture shows diffusion taking place in the lungs, where
oxygen moves from air sacs called alveoli into the blood vessels
around them.
Oxygen diffuses from the alveoli, where there is a
_____concentration of oxygen, to the blood. The oxygen

moves ________a concentration gradient

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 3. The picture shows diffusion taking place in the
lungs, where carbon dioxide moves from the blood
vessels through to small air sacs called alveoli.
What does this tell you about the
concentration of carbon dioxide in the
blood at this time?
Choose one correct answer.
 There is a low concentration of carbon dioxide
in the blood vessels
 There is a high concentration of carbon dioxide
in the blood vessels
 The concentration of carbon dioxide in the
blood vessels is the same as in the alveoli

4. Red blood cells carry oxygen around the body to be used in various cellular and
bodily processes.Oxygen moves into red blood cells in the lungs.
In the lungs, what must the concentration of oxygen in the red blood cells be,
for oxygen to move in by diffusion? ____________________

5. Diffusion is an important process for living things.

In which of the following substances would diffusion not be able to take
place?
Choose all correct answers.
 Glass
 Milk
 Nitrogen
 Copper
 Chlorine
6. Can you sort the statements about diffusion depending on whether they are true
or false?Choose the correct answer in each row.
 Diffusion can happen in solids
 Diffusion happens faster in warm liquids than cold
 Smells travel by diffusion
 Diffusion is the movement of particles from low to high concentration
 Diffusion will not happen in cold gases
7. Complete the sentence to describe why diffusion would happen faster through
carbon dioxide than water.
Substances would diffuse through carbon dioxide than through water. This is because the
particles in carbon dioxide are _________spread out and move________ than the
particles in water.

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8. What would happen to the speed at which diffusion happens if you lowered the
temperature of a liquid or gas?
Diffusion would happen _____________as the particles would have____________
energy
9. What would happen to the speed at which diffusion happens if you raised the
temperature of a liquid or gas?
Diffusion would happen __________as the particles would have___________ energy
10. William is making a cup of tea but he is late for an appointment.
What could he do to speed up the time taken for the tea to diffuse out of the
tea bag? Choose all correct answers.
 Use slightly colder water
 Leave it still
 Use the hottest water possible
 Stir
 Add sugar

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