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Foods Ltd

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 solids the molecules are more or less stationary
2

As a result of their random movements the
molecules become evenly distributed
(a) (b)
Representation of molecules in a gas
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Next slide
4

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
The scent molecules diffuse from a region
where they are concentrated (the flower) to
regions where they are absent or in low
concentration
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Diffusion

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
6

A diffusion gradient
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
Diffusion gradient
7

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
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Scale

The concentration of
oxygen molecules is
greater outside the cell
than inside
So the oxygen molecules
diffuse into the cell
Diffusion of oxygen into a cell
9

Because the cell is using up oxygen, the
concentration of oxygen inside the cell is
always lower then the concentration
outside.
So oxygen continues to diffuse in
The diffusion gradient is maintained
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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
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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
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Diffusion is slow

oxygen
carbon dioxide
maximum
distance
is 0.1 mm
In a single-celled organism (such as Amoeba) the
distance is so small that diffusion is rapid enough
for the cell’s needs
Single-celled organisms 13

Single-celled organisms are rarely more than
1mm in diameter
Bacteria range from 1 - 10 microns (1-10 µ )
( 1 µ is one thousandth of a millimetre)
Diffusion is rapid enough for such small
organisms
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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
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CO2 diffuses
outO2 diffuses in
Section through
worm’s skin
the blood vessels
absorb the O2 and
carry it to the body
0.04mm
Earthworm
diffusion takes place through
the thin skin of the worm
16

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

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
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position of lungs
in thorax
windpipe
lung
diaphragm
heart
human lungs
Human lungs 19

the air passages in the lung
branch into finer and finer tubes
each tube ends up in
a cluster of tiny air
sacs.
Lung Structure 20

blood supply to air sac
air breathed
in and out
diffusion of
oxygen
diffusion of
carbon dioxide O2
CO2
A single air sac 21
0.03 mm
question 4

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.
22

Fish
gill cover
gill cover cut away
gills
gill filaments
23

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
The millions of air sacs in the lungs also vastly
increase the area through which the gases can
diffuse
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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
In darkness, O2 will diffuse in and CO2 will diffuse
out as a result of respiration
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Plants

Leaf
O2 and CO2 diffuse
into the spaces between cells
the ‘veins’
bring water
O2 and CO2
diffuse through
pores in the
epidermis
In a thin leaf, the
diffusion distance
is short
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Question 1
Diffusion can normally take place in
(a) a liquid
(b) a solution
(c) a solid
(d) a gas
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Question 2
A fish breathes
(a) water
(b) oxygen dissolved in water
(c) oxygen in the air
(d) carbon dioxide dissolved in water
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Question 3
Diffusion takes place as a result of
(a) convection currents
(b) air movements
(c) natural movement of molecules
(d) changes in temperature
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Question 4
What is the approximate diameter of
an air sac in the human lung ?
(a) 0.006 mm
(b) 0.06 mm
(c) 0.6 mm
(d) 6.0 mm
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(Slide 21)

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
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Question 6
Which of these would you expect to diffuse
freely through a cell membrane ?
(a) carbon dioxide
(b) water
(c) proteins
(d) oxygen
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Question 7
Through which of these layers would you expect
diffusion to be most rapid ?
(a)
(b)
(c)
(d)
33

Question 8
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
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Answer
Correct
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Answer
Incorrect
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single cell
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’.
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