© Boardworks Ltd 2003 Key Stage 4 Diffusion. © Boardworks Ltd 2003 A slide contains teacher’s notes wherever this icon is displayed - To access these.

© Boardworks Ltd 2003

Key Stage 4

Diffusion


A slide contains teacher’s notes wherever this icon is displayed -

To access these notes go to ‘Notes Page View’ (PowerPoint 97) or ‘Normal View’ (PowerPoint 2000).

Normal ViewNotes Page View

Teacher’s Notes

Flash Files

A flash file has been embedded into the PowerPoint slide wherever this icon is displayed –

These files are not editable.


Have you ever wondered why cooked food can be smelt some distance away?

…or why a glass of water will turn a different colour when concentrated juice is added to it?

How do dissolved substances get across boundaries within the body?


Well the answer to these questions can be found if we look inside the human body. There is a

special scientific process that is controlling these situations and without it, you would not be able to

live your own life!

It has an essential role in the organ systems that control your breathing, circulation, digestion and many other processes.

The scientific process is known as….

DIFFUSION

….or why you can smell peoples aftershave or perfume when they walk past you?


The definition of diffusion is….

"…the spreading of a gas or any substance in solution from a higher to a lower concentration."

What has this got to do with food, perfume, juice and more importantly us!

Well, let's start at the beginning….

All things on Earth are made of particles. These are atoms and molecules.

The particles that make up a substance have varying abilities to naturally move.


ICE LIQUID WATER WATER VAPOUR

Which state, solid liquid or gas allows the particles to move most freely?


Gas particles will move the most freely, followed by liquid particles and in last place are the tightly bound solid particles.

Solid Liquid Gas

The fact that particles can move at all means that they will naturally spread out to fill a given space.

For liquids, remember how they will fill a beaker, whatever the shape

For gases, remember how smoke from a small fire

will fill a room


We can represent this movement by using a simple diagram:

Here is the position of particles before movement.

Particle

Area A Area B

QUESTIONOver time, what will happen to the particles?


Diffusion Animation


They will spread out to fill the space in which they are allowed to move.

We can call this movement diffusion because the particles moved from Area A where they were highly concentrated to area B where they were in

low concentration.

ANSWER


If the particles were part of a solid, the bonds between each particle and the next would be too

strong to allow movement.

Therefore, diffusion is concerned with the movement of gas and dissolved particles.

In other words, gases and dissolved

substances will tend to move from areas

where they are in high concentration, to areas where they are in low

concentration.

Remember, in this case, concentration means the number

of particles of a substance in a particular area.


If we consider the situation where someone could smell cooked food; the chemicals being produced

from the food were highly concentrated around the food. At the same time, the chemicals around the person, who is standing further away, are low in

concentration.

So the chemicals move from high concentration to low concentration.

The person does not have to move. The smell will move toward them.

QuestionDo these particles just keep moving?


The particles move at the same rate in opposite directions. This situation is known as EQUILIBRIUM

They will move this

way

And when the concentration

is equal

Yes! They keep moving in a certain direction until the particle concentration of the place they are moving to is equal to that of the place they are moving from….

ANSWER


…because there is equal movement in both directions, it would appear that overall; nothing was moving

(however, we know better!).

O.K., so we understand diffusion, but what has this got to do with vital life processes?

Well, remember that we rely on certain gases and dissolved substances getting to the cells of

our body every minute of the day.

It is the process of diffusion that is helping us to achieve this.

The most important substances we need are….


FOOD OXYGEN

Once these chemicals are inside our body, they are in the form of either gases or dissolved substances.

Both the dissolved food particles and the oxygen

particles must pass into our blood stream so that they can be transported around the body to the cells.

Each particle diffuses (performs diffusion) at particular sites within the body.


Let's consider them both in detail

Chemical : Oxygen (O2)

Site of Diffusion : Alveoli

Cross-section of an alveolus with arrows showing the movement of oxygen particles from the bronchiole to the end of the air sac.


When we breathe in (inhale), oxygen particles enter our breathing system and eventually find their way to the alveoli. Here, the oxygen particle concentration builds up.

Surrounding the outer lining of the alveoli's are many tiny blood vessels called capillaries. The blood within these vessels is low in oxygen (deoxygenated).


The brief difference in concentration between the inside and the outside of the alveolus is enough for the oxygen

to start diffusing. It passes across the lining of the alveolus and into the blood.


QuestionThink back to the process of respiration. Which gas is produced during the reaction and carried away from the cells in the blood.

AnswerCarbon Dioxide (CO2)

Therefore, the blood that reaches the alveolus may be deoxygenated, but it does have high levels of CO2

The air we inhale does not contain a large concentration of CO2

Gas Air breathed in Air breathed out

O2 21% 17%

CO2Almost zero 4%

N2 79% 79%


Therefore, we have another situation where a substance is in high concentration in one place and low concentration in another place.

The result of this is that whilst O2 is diffusing into

the blood, CO2 is diffusing

in the opposite direction.

The blood is exchanging one gas for another. We refer to the lining of the alveolus as a…

GAS EXCHANGE SITE


Chemical : Dissolved food particlese.g. GlucoseFatty AcidsGlycerol PhosphateAmino Acids

Site of Diffusion : Villi within the Small Intestine

Once the process of digestion has neared completion, the enzymes have chemically broken down Fats, Proteins and Carbohydrates into useful nutrients. These nutrients are small enough to be able to pass across the lining of the small intestine.

As you will remember from the section on the digestive system, the lining of the small intestine is heavily folded to

create finger like projections called villi.


Small intestineDiagram of magnified villus

Each villus separates the digested food from a network of capillaries.


These capillaries approach the villi, carrying blood that has a low

concentration of digested nutrients.

Again, we have a situation where certain particles are highly

concentrated in one area and in low concentration in another.

Here again, diffusion occurs. The small nutrient particles diffuse across the lining of the small intestine and

enter the blood.


The process of diffusion has resulted in useful substances entering the blood from the

alveoli and the villi.

This blood now carries these useful substances to the cells of the body where they will be

required.

When the blood reaches the cells, it is traveling within capillaries.

You may have noticed that diffusion is only occurring into and

out of one type of blood vessel, namely capillaries….

capillaries

intestine

food particle


Take a look at the cross section of arteries, veins and capillaries to remind yourself why!

Artery Vein Capillary

The arrow shows the route substances would have to take to diffuse and leave the vessel. It is clear that the capillary offers

the least obstruction to diffusion.

Reaching a capillary bed, we yet again find a site of material exchange, powered by the process of diffusion.


The capillary bed sees the exchange of useful and waste substances between the cells and the blood.

Waste

Useful

Cell

BLOOD


From the blood, the dissolved nutrients and the oxygen diffuse into the cell. The cell is probably

running low and needs a new supply.

They will be used to generate energy during respiration

As the cell is a unit of life, it will have been performing respiration prior to the blood arriving with the new nutrients

and oxygen gas. Therefore, it will have been storing up waste materials. These will now be excreted from the cell and given

to the blood for eventual removal.

We have now seen how the process of diffusion is behind the movement of substances within the body.


Does everything diffuse at the same speed?

In every situation we have considered, substances are moving from an area of high concentration to an area of

low concentration, ACROSS something.

This is usually a membrane of a cell.This is only possible if the cell membrane has minute

gaps through which the substances can pass.

Large particles(too big)

Membrane

Small Particles(these can fit through)


Firstly, small particles will diffuse faster than large particles.

But what about two different particles which are the same size. Which one will diffuse fastest?

Well, imagine two football pitches sitting side by side,

separated by a fence with gates in it.

On pitch A, 3 people are playing with footballs. They have plenty of room to themselves.

On pitch B, no one is playing and the gates in the fence

are shut.


If the gates are opened, there is no real rush for the people to move from pitch A to pitch B. They

already have the room they need to move around.

Pitch A Pitch B

It is likely that maybe later, one of the people makes their way over to pitch B.


Now, if the situation was slightly different and pitch A was crowded with people, we would produce a

different outcome.

When the gates are opened, there would be a rush of people from pitch A to pitch B.

This would be due to the players’ desire for more

individual space.

The speed of movement from A to B would be far greater that the speed of movement in the original

situation.


A B A B

In this situation, the difference in the

number of players on each side of the fence

was small

In this situation, the difference in the

number of players on each side of the fence

was large

In living organisms, the same relationship occurs if we consider the movement of substances across boundaries.


To see this in action, let’s replace the word ‘players’ with particles and ‘number of players’ with concentration of particles.

Therefore, the speed of diffusion will depend on the difference in concentration of particles on each side of the membrane (boundary)

Slow movement of players Fast Movement of players

The greater the concentration difference between two areas, the greater the rate of diffusion will occur across the

membrane.

Rule

Remember that this will occur until a state of equilibrium is reached.


We call this difference in concentrations, a concentration gradient.

Inside

Outside

OutIn

Concentration Gradient

Out

Inside

OutsideConcentration Gradient

In

The steeper the gradient, the greater the

difference in concentration. We can represent this using a diagram.


You can probably imagine that it is in the interest of the body to try to maintain a high concentration difference

between two sides of a membrane.

Especially, if the body is trying to absorb a useful chemical or remove a toxic one.

By having a large concentration gradient, diffusion would occur more quickly and the body would have its demands

satisfied.


Here is one site employing a maintained concentration gradient….

Capillaries within Villi

The lumen of the small intestine will be highly concentrated in dissolved food. It is essential that a continuous supply of this

material diffuse across the lining of the villus and into the blood capillary.

This is because…

If the blood were stationary within the capillaries, this continuous process would stop.


Small Intestine B

LOOD

Food particles

…Initially the concentration gradient would

be high with food particles diffusing in to

the blood.

But, if the blood just received these particles and remained stationary, a state of equilibrium would then occur.

Small Intestine B

LOOD

Food particles


The blood and the small intestine would have equal concentrations of dissolved food. Once this situation was

reached, no extra dissolved food particles would move into the blood!

In fact, some diffusion would occur in the opposite direction.

Useful materials would be LEAVING the blood and heading back into the small intestine.

(Remember the body needs a constant supply of dissolved

food so that cellular respiration can occur)


Therefore to overcome this, the blood is constantly moving through the capillaries, bringing new blood to the villi and

taking blood

Blood carrying dissolved food away from the small intestine

Dissolved food

(now containing dissolved food) away.

Small Intestine Small Intestine

Concentration Gradient

Blood


The concentration gradient remains large and ensures that the blood constantly receives dissolved food.

Although this fast moving blood makes sure that dissolved

substances are diffusing into the blood in high enough quantities to satisfy the bodies needs, there is another

process at work, also doing its bit to get these nutrients into the blood.

ACTIVE TRANSPORT

We must now consider……

The cell membrane of the cells that line the villi of the small intestine can use ENERGY to actively move dissolved

substances across from one side to the other.


With this extra process, the rate of diffusion will increase. Therefore active transport can speed up

the diffusion of substances across membranes.

Small Intestine

Diffusion

Active Transport

Blood


This means that Active transport

can be used to move substances in both directions: with and against the concentration

gradient.

High Concentration

Low Concentration

High Concentration Low Concentration

Diffusion down concentration

gradient

However, because Active transport is powered by

energy, it does not have to move substances in the same direction as diffusion.

Active transport


Where does the energy for active

transport come from?

As you can see, the production and use of this energy is linked in a cycle. This is one way the body helps itself perform important

functions.

The energy for active transport comes from respiration

Aerobic Respiration requires a supply of glucose and oxygen

These two substances are carried in the blood to cells.

They have to move from the blood to the cells of the body.

The quicker they move, the quicker the cell can perform respiration

The faster the cell performs respiration, the faster it generates energy


Are there common structural features shared by all exchange sites ?

Let’s finally consider… what makes a good exchange site? Is it possible to look at the shape of a living structure and establish whether it would make an effective site for exchanging materials?

Well, let’s look at the two exchange sites that we have been learning about.

Alveoli Villi


Alveoli Villi

Small Intestine Lining

Both sites share a common function. They are there to enable substances to pass into and out of the blood as quickly as possible.

In response to this, they both have enormous surface areas.

The small intestine has an enormously folded lining. These folds have further microscopic folds, increasing the number of possible exchange

sites.

These microvilli are lined with cells. The process of diffusion occurs across these cell’s membranes.

Diagram of microvilli


The lungs consist of millions of individual alveoli. Each alveolus is a small air sac, covered with blood

vessels.

Diagram of the cross section of many alveoli

Therefore, in humans, the overall surface area of the lungs is enormously increased by the presence of alveoli and that

of the small intestine by the folded villi.


Diffusion

Let’s get moving!Let’s get moving!

This activity will look at the movement of substances into and out of the cells.

It is essential that you pay close attention to the diagram of the cell on the next page. The questions are based on this diagram.

Good luck!Good luck!


Look closely at this cell. It is surrounded by tissue fluid. We can see various chemicals inside and outside the

cell.

KeyKey

Water

Oxygen

Glucose

Carbon Dioxide


1. Draw and label the picture of the cell.

2. What process does the cell use to release energy from food?

3. Which 2 substances are needed for this process to happen?

4. What are the waste products of this process?

Water

Oxygen

Glucose

Carbon Dioxide

It is very important that you count the number of each chemical very carefully.


5. If there was a large concentration difference between the inside and outside of the cell, do you think that diffusion would occur faster or slower?

6. Count the number of particles of each substance inside and outside the cell. Put your results in the following table.

Substance Concentration Concentration

inside Outside

Carbon Dioxide

Oxygen

Glucose

Water


7. For each of the four substances, draw an arrow across the table to indicate which way it will move.

e.g.

Substance Concentration Concentration

inside outside

or

Make the thickness of the arrow represent how fast the substance would diffuse I.e. a fat arrow represents a greater rate of diffusion.

8. Define the term diffusion.


DiffusionDiffusion

Multiple choice questions


1. Which of the following is the proper definition of the term diffusion?

A The spreading of any substance from where it is in lower concentration to where it is in higher concentration.

B The spreading of a gas or any substance in solution from where it is in lower concentration to where it is in higher concentration.

C The spreading of any substance from where it is in higher concentration to where it is in lower concentration.

D The spreading of a gas or any substance in solution from where it is in higher concentration to where it is in lower concentration.


2. Choose the correct statement from the following (only one is correct).

A Oxygen diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the blood into the air because it is at a higher concentration in the blood.

B Oxygen diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the air into the blood because it is at a higher concentration in the air.

C Oxygen diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the blood into the air because it is at a lower concentration in the blood.

D Oxygen diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the air into the blood because it is at a higher concentration in the blood.


3. Choose the correct statement from the following (only one is correct).

A Carbon dioxide diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the blood into the air because it is at higher concentration in the blood.

B Carbon dioxide diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the air into the blood because it is at higher concentration in the air.

C Carbon dioxide diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the blood into the air because it is at lower concentration in the blood.

D Carbon dioxide diffuses across the exchange surface of the lungs (alveolar/capillary walls) from the air into the blood because it is at higher concentration in the blood.


4. A chip is weighed and then placed in a very concentrated salt solution for 24 hours. After this period of time the chip is re-weighed and it is discovered that the chip weighs less. The reason for this is:

A Water has entered the chip, moving from an area of higher concentration to an area of lower concentration.

B Water has moved out of the chip, moving from an area of higher concentration to an area of lower concentration.

C Water has moved out of the chip, moving from an area of lower concentration to an area of higher concentration.

D Water has entered the chip, moving from an area of lower concentration to an area of higher concentration.


5. Look at the diagram below. Choose the correct statement to explain what is happening.

A B

membrane

A The rate of diffusion of particles with be greater in A because the difference in concentration is smaller.

B The rate of diffusion of particles with be greater in B because the difference in concentration is smaller.

C The rate of diffusion of particles with be greater in A because the difference in concentration is greater.

D The rate of diffusion of particles with be greater in B because the difference in concentration is greater.


6. The surface area of the lungs as an exchange surface is increased by the possession of…

A millions of glandular cells.

B millions of villi.

C millions of alveoli.

D millions of blood cells.


7. Gas and solute exchange surfaces (e.g. fish gills, human lung wall and small intestine wall, placenta of a pregnant woman) maximise their effectiveness by…

A having a minimal surface area, being permeable to the particular gases/solutes, being thin and maximising the concentration difference.

B having a massive surface area, being permeable to the particular gases/solutes, being thin and minimising the concentration difference.

C having a massive surface area, being permeable to the particular gases/solutes, being thick and maximising the concentration difference.

D having a massive surface area, being permeable to the particular gases/solutes, being thin and maximising the concentration difference.


8. Active transport is the…

A movement of a substance across a cell membrane against the direction in which it would tend to diffuse and the process releases energy.

B movement of a substance across a cell membrane against the direction in which it would tend to diffuse and the process uses energy.

C movement of a substance across a cell membrane in the same direction in which it would tend to diffuse and the process releases energy.

© Boardworks Ltd 2003 Key Stage 4 Diffusion. © Boardworks Ltd 2003 A slide contains teacher’s notes wherever this icon is displayed - To access these.

Documents