Diffusion
IGCSE Combined Science - Biology
I need some deodorant!
• I´m going to spray some deodorant in the
corner of the room
• As soon as you can smell it stand up
• Now you have 2 min as a group to explain
what just happened!
Fill in the blanks on your sheet as we go through the ppt
Diffusion
Diffusion is the movement of molecules from an
area of high concentration to an area of low
concentration, until evenly spread out.
Why is diffusion important?
All living cells rely on diffusion to live.
They use it for:
 getting raw materials for respiration (dissolved
substances and gases)
 removing waste products (eg. from respiration)
 photosynthesis in plants (raw materials in, waste products out)
Examples…
Respiration- gas exchange
In breathing you exchange carbon dioxide (CO2) and oxygen (O2) between
alveoli in the lungs and the blood.
This is an example of diffusion.
SEM photos of lung alveoli
Respiration- in cells
food + oxygen  carbon dioxide + water + energy
Respiration & diffusion
Photosynthesis
& Diffusion
carbon dioxide + water  oxygen + glucose
 Carbon dioxide
diffuses in through
the stomata
 Oxygen and water
diffuse out of the stomata
Osmosis
Osmosis is a special case of diffusion.
It is the net movement of water across a
selectively permeable membrane from a
high concentration to a low
concentration.
Selectively permeable membrane ???
The cell membrane has very small
holes in it. Small molecules can pass
through, but larger ones cant.
We say it is selectively permeable.
Water concentration gradient
In osmosis water moves both ways to balance up the concentrations. The
overall (net) movement is to the area of low water concentration.
Key:
Water molecule
High Water Concentration,
few molecules of solute.
Dilute solution
Low Water Concentration, many
molecules of solute.
Concentrated solution
Solute molecule
Water movement
Osmosis in action
Have a look at:
http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/animation__
how_osmosis_works.html
Now let´s go outside and act it out!
1.
Nominate some sugar molecules
2.
Everyone else is a water molecule
3.
Draw a dashed line down the middle of the playground with chalk
(your selectively permeable membrane!)
4.
Arrange your water molecules either side of the membrane
5.
Add your sugar molecules to one side and see if equilibrium can
be restored by water moving in the right direction!
6.
What happens if you remove some of the water molecules?
Now try your osmosis problems on
your worksheet