1. Diffusion © Centura Foods Ltd
“Ah! Bisto” The ‘Bisto Kids’ featured on advertising for ‘Bisto’ gravy powder from the 1920s - 90s. After you have studied this presentation you may be able to decide whether this is a good example of diffusion.
© Centura
Foods Ltd

2. In solids the molecules are more or less stationary2
All substances are made up of sub-microscopic particles called molecules
In gases (like air) the molecules can move freely
In liquids (like water) the molecules can also move
In solutions (e.g sugar dissolved in water) the molecules of the dissolved substance can move.
In solids the molecules are more or less stationary

3. Representation of molecules in a gas3
Representation of molecules in a gas
(a)
(b)
As a result of their random movements the
molecules become evenly distributed

4. 4
Next slide

5. Diffusion As a result of this random movement, the5
As a result of this random movement, the
molecules of a gas become evenly dispersed
This movement is called DIFFUSION
The scent of a hyacinth diffuses throughout
a room
In an area as large as a room, there will be air currents and convection currents which will play a greater part than diffusion
The scent molecules diffuse from a region
where they are concentrated (the flower) to
regions where they are absent or in low concentration

6. One of the ways substances enter and leave cells is by diffusion6
One of the ways substances enter and leave cells
is by diffusion
If a substance is more concentrated outside a cell
than inside, the molecules will tend to diffuse
into the cell
If a substance is more concentrated inside the
cell than outside, the molecules will tend to
diffuse out of the cell

7. Diffusion gradient The molecules are more densely packed on7
The molecules are more densely packed on
the left and so they tend to diffuse into the
space on the right. This is a diffusion gradient
The bigger the difference in concentration, the greater will be the diffusion gradient.
A diffusion gradient

8. Scale The scale of the following drawings is greatly distorted.8
The scale of the following drawings is greatly
distorted.
Even if the cells were as large as they appear
on the screen, the molecules would still be
invisible particles

9. Diffusion of oxygen into a cell9
The concentration of
oxygen molecules is
greater outside the cell
than inside
So the oxygen molecules
diffuse into the cell
The concentration of carbon dioxide is greater inside the cell than outside. Carbon dioxide molecules will diffuse out of the cell.

10. Because the cell is using up oxygen, the 10
Because the cell is using up oxygen, the
concentration of oxygen inside the cell is
always lower then the concentration
outside.
The diffusion gradient is maintained
The oxygen is used in respiration. Carbon dioxide is produced by respiration. The concentration of carbon dioxide inside the cell increases, so carbon dioxide diffuses out of the cell.
So oxygen continues to diffuse in

11. If all kinds of substance could diffuse into a cell 11
If all kinds of substance could diffuse into a cell
there would be a danger that poisonous
substances could diffuse in and kill the cell
If all the substances in a cell could diffuse out
the cell would lose essential substances
(e.g. glucose) needed to keep the cell alive
In fact, although the cell membrane does allow
some substances (e.g. oxygen and carbon
dioxide) to diffuse freely, it controls the exit and
entry of nearly all other substances

12. Diffusion is slow For living processes it is effective only over12
Diffusion is slow
For living processes it is effective only over
short distances
The distance from a cell membrane to the
centre of the cell may be 0.1mm or less
Diffusion is rapid enough to keep a cell
supplied with oxygen and food

13. Single-celled organisms13
In a single-celled organism (such as Amoeba) the
distance is so small that diffusion is rapid enough
for the cell’s needs
maximum
distance
is 0.1 mm
oxygen
Compared with its bulk, a cell like this has a huge surface area through which diffusion can occur
carbon dioxide

14. Single-celled organisms are rarely more than 1mm in diameter14
Single-celled organisms are rarely more than
1mm in diameter
Bacteria range from microns (1-10 µ )
( 1 µ is one thousandth of a millimetre)
Diffusion is rapid enough for such small
organisms

15. Inside the bodies of large animals, diffusion of15
Inside the bodies of large animals, diffusion of
oxygen into their cells is rapid enough
For transport across the whole body, diffusion
would be much too slow
Large organisms have evolved transport
systems (e.g. blood circulatory systems)
that carry oxygen from outside the body to
the cells inside

16. Earthworm diffusion takes place through the thin skin of the worm16
diffusion takes place through the thin skin of the worm
CO2 diffuses
out
O2 diffuses in
Section through
worm’s skin
0.04mm
the blood vessels
absorb the O2 and
carry it to the body

17. Question Did you notice anything in particular about the17
Did you notice anything in particular about the
earthworm’s ‘skin’ as seen in the section ?
Have another look.
How might this affect diffusion ?

18. Humans obtain their oxygen by diffusion18
Humans obtain their oxygen by diffusion
But not through the skin
Although the skin is well supplied with blood
vessels, there are too many layers of cells for
diffusion to be fast enough
Humans have lungs and it is in these lungs
that diffusion occurs

19. Human lungs windpipe lung diaphragm heart position of lungs in thorax19
windpipe
lung
The lungs are not hollow, but spongy. They are made up of thousands of tiny air pockets
diaphragm
heart
position of lungs
in thorax
human lungs

20. Lung Structure each tube ends up in a cluster of tiny air sacs.20
The windpipe (trachea) divides into two smaller passes (bronchi) which continue to divide repeatedly into smaller and smaller branches until they end up in tiny, thin-walled air pockets (air sacs)..
each tube ends up in
a cluster of tiny air
sacs.
the air passages in the lung
branch into finer and finer tubes

21. A single air sac blood supply to air sac diffusion of oxygen21
blood supply to air sac
diffusion of
oxygen
air breathed
in and out
diffusion of
carbon dioxide O2
0.03 mm
The air is carried to the air sacs by the breathing movements. Oxygen then diffuses across the air sac, though the single cell layer lining the air sac, and into the blood vessels.
Carbon dioxide diffuses in the opposite direction.
CO2
question 4

22. In mammals, birds, reptiles and amphibia, 22
In mammals, birds, reptiles and amphibia,
oxygen and carbon dioxide are exchanged by
diffusion in the lungs
In fish, this exchange of gases takes place by
diffusion through the gills
The oxygen dissolved in the water diffuses into
the blood vessels in the gills.

23. Fish gill cover gills gill filaments gill cover cut away 23
Water is taken in through the mouth, passed over the gills and expelled from the gill cover.
gill filaments
gill cover cut away

24. Diffusion takes place through the surface of the gills24
Diffusion takes place through the surface of the gills
Diffusion is a slow process
The branching gill filaments offer a big surface area through which oxygen and carbon dioxide
can diffuse
In this way, the total diffusion through the gills is
greatly increased
It is said that if all the air sacs were opened out and spread flat they would occupy an area the size of a football pitch.
The millions of air sacs in the lungs also vastly
increase the area through which the gases can
diffuse

25. Plants Plants have no special organs for breathing25
Plants have no special organs for breathing
They have to rely on diffusion for their supplies
of oxygen and carbon dioxide
There are pores in the leaves and stems through
which the gases diffuse
In daylight, CO2 (for photosynthesis) will be
diffusing in and O2 will be diffusing out
The plant is respiring all the time, using up oxygen and producing carbon dioxide.
In daylight, photosynthesis will use up the carbon dioxide as fast as it is produced by respiration, so there will be no outward diffusion of carbon dioxide.
Similarly, the oxygen produced by photosynthesis is used up by respiration.
In darkness, O2 will diffuse in and CO2 will diffuse
out as a result of respiration

26. Leaf the ‘veins’ bring water In a thin leaf, the diffusion distance26
O2 and CO2
diffuse through
pores in the
epidermis
the ‘veins’
bring water
The leaf needs carbon dioxide for photosynthesis and oxygen for respiration.
These gases diffuse through pores in the leaf’s epidermis and enter the air spaces between cells. They then diffuse through the cell wall into the cytoplasm.
In a thin leaf, the
diffusion distance
is short
O2 and CO2 diffuse
into the spaces between cells

27. Question 1 Diffusion can normally take place in (a) a liquid27
Diffusion can normally take place in
(a) a liquid
(b) a solution
(c) a solid
(d) a gas

28. Question 2 A fish breathes (a) water (b) oxygen dissolved in water28
A fish breathes
(a) water
(b) oxygen dissolved in water
(c) oxygen in the air
(d) carbon dioxide dissolved in water

29. Question 3 Diffusion takes place as a result of29
Question 3
Diffusion takes place as a result of
(a) convection currents
(b) air movements
(c) natural movement of molecules
(d) changes in temperature

30. Question 4 What is the approximate diameter of30
Question 4
What is the approximate diameter of
an air sac in the human lung ?
(Slide 21)
(a) mm
(b) 0.06 mm
(c) 0.6 mm
(d) 6.0 mm

31. 31
Question 5
In a plant leaf, CO2 diffuses (a) into the air space between cells, (b) into the cytoplasm, (c) through the cell wall, (d) through a pore in the epidermis.
The correct sequence is
(a) a,b,c,d
(b) c, b, d, a
(c) d, a, c, b
(d) d, c, a, b

32. Question 6 Which of these would you expect to diffuse32
Question 6
Which of these would you expect to diffuse
freely through a cell membrane ?
(a) carbon dioxide
(b) water
(c) proteins
(d) oxygen

33. 33
Question 7
Through which of these layers would you expect diffusion to be most rapid ?
(a)
(b)
(c)
(d)

34. Question 8 The cells inside an earthworm receive oxygen34
The cells inside an earthworm receive oxygen
via (a) diffusion into the blood vessels, (b) transport
by the blood, (c) diffusion out of the blood into the
cells, (d) diffusion through the epidermis.
Which of the following is the correct sequence?
(a) b, c, d, a
(b) a, b, c, d
(c) d, a, b, c
(d) d, b, a, c

35. 35
Answer
Correct

36. 36
Answer
Incorrect

37. single cell The earthworm’s ‘skin’ is only one cell thick.37
The earthworm’s ‘skin’ is only one cell thick.
To reach a blood vessel, the oxygen has to diffuse
over a very short distance and so is rapid enough to
meet the earthworm’s needs.
The single cell layer is an epidermis rather than
a ‘skin’.
single cell