IGCSE diffusion in cells

1. Diffusion

2. States of Matter
 You will remember from science that matter
exists in different forms depending on the
amount of energy the atoms have
 We only need to know about liquids, solids
and gases
 When molecules are in a liquid or in a gas they
move and get mixed together
 This is important in living organisms
 Let’s look at the States of matter:

3. This is what they look like in a container:

4.  Kinetic energy is the energy of movement
 Remember that in the solid the molecules only
vibrate – they have low kinetic energy
 In a liquid the molecules have more kinetic
energy and kind of roll over each other
 In a gas the molecules have a lot of kinetic
energy and move randomly in any direction
 If you have internet click on this animation
which shows the different states. You will
need to look at water and increase the pressure
on the one axis

5. If we opened a bottle of a type of gas what would happen? The blue is the air
particles:

6. Using a diffusion tube
 Litmus paper tells us what the pH of
something is; that is if it is acidic or alkali
 For the experiment with ammonia we will use
red litmus paper
 It should change to blue when ammonia is
present (because it is alkali)

7. This is the experiment we will set up

8. The results:
Cotton wool soaked in
ammonia
Litmus paper
Direction of diffusion of ammonia gas

9. Potassium permanganate molecules mix with the water molecules
The purple colour slowly spreads throughout the water: this is
diffusion

12. Diffusion in Living Organisms
 Living organisms rely on diffusion to move
substances in and out of cells and organs
 This only works when there is a diffusion
gradient: when you look at the examples think
about how the diffusion gradient is kept as
steep as possible

13. Diffusion in the lungs
 we need oxygen in our
bodies for respiration
 This oxygen is used in cells
to make energy by
respiration
 Carbon dioxide is a product
of respiration that we must
get rid of!
 We take oxygen in and
expel carbon dioxide in the
lungs

14. The lungs are a system of branching tubes that end in many small structures called alveoli
These are surrounded by blood vessels
Look at how gases move in/out of alveoli
How do you think we keep the oxygen content in the alveoli high even though oxygen is moving
into the blood constantly?

15. Gaseous Exchange in a Leaf
 Leaves need carbon dioxide for photosynthesis
 The carbon dioxide has to go into the palisade
cells at the top of the leaf
 Oxygen is a product of photosynthesis; this
must move out of the palisade cells
 Let’s look at a leaf in section first

16. Can you see the Palisade cells? The green dots are chloroplasts where
photosynthesis takes place. They are also found in spongy mesophyll cells
Note the stomata which are openings through which gases can pass

17. This picture shows how oxygen and carbon dioxide move in and out of the
leaf
To get in and out of cells they must diffuse across the membrane
The cells have large air spaces to allow gases to move around

18. Diffusion in the small intestine
 When you eat your food is
digested, or broken down, into
small molecules
 These can now diffuse out of the
gut and into the blood – this is
called absorption
 This happens in a place called
the ileum in the small intestine
 To increase the surface area for
absorption the ileum has tiny
projections called villi:

19. This is what villi look like
Look at the scale bar to get an idea of how small they are - there are 1000µm in 1mm

20. This is what a villus looks like inside
Note the thin layer of cells around it – this makes the distance for diffusion shorter
Also see there is a capillary with blood flowing through to take up digested food and move
it away
The lacteal also absorbs some digested food – we will learn more about all of tis later this year