1. IGCSE Science - Biology
Diffusion
IGCSE Science - Biology

2. I need some deodorant!
Close your eyes and as soon as you notice a change in our environment stand up.
I’m going to release something in the room.
Now you have 2 min as a group to explain what just happened!

3. Diffusion
Diffusion is the movement of molecules down a concentration gradient from an area of high concentration to an area of low concentration, until evenly spread out.

4. 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…

5. 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

6. Respiration- in cells
food + oxygen  carbon dioxide + water + energy

7. Respiration & diffusion

8. Photosynthesis & Diffusion
carbon dioxide + water  oxygen + glucose
Carbon dioxide diffuses in through the stomata
Oxygen and water diffuse out of the stomata

9. Osmosis Osmosis is a special case of diffusion.
It is the nett movement of water across a selectively or partially 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 can’t.
We say it is selectively permeable or semi-permeable or partially permeable.

10. Water concentration gradient
In osmosis water moves both ways to balance up the concentrations. The overall (nett) movement is to the area of low water concentration.
Water molecule
Solute molecule
Key:
High Water Concentration,
few molecules of solute.
Dilute solution
Water movement
SQA LO: Hypertonic, isotonic, hypotonic
Low Water Concentration, many molecules of solute.
Concentrated solution

11. Osmosis in action
Have a look at:

12. Why is this not funny if you were a slug??

13. Some real time osmosis videos to set the mood!
Osmosis in Cells
Some real time osmosis videos to set the mood!

14. 5 days ago Today Group Starter Activity
Mr and Mrs Kim went on holiday to Jeju for a few days.
When they came back their favourite plant looked like this!
Explain the changes in terms of water movement
5 days ago Today

15. Why is osmosis important?
Plants rely on the movement of water through them:
Water always moves to areas of lower water concentration.
Root hairs take in water from the soil by osmosis.

16. Plants- Water moving in
We say that the cell is turgid.
Water moves into a plant cell.
The cell wall stops the cell from bursting.
Turgid cell.
Vacuole is filling all the space and pushing against cell wall
SQA LO: Turgid, flaccid,plasmolysed- root hairs, cells to xylem vessel
This is useful as it gives plant stems support 16

17. Cell turgor pressure

18. Plants- Water moving out
When water moves out of the plant cell vacuole, by osmosis, it becomes flaccid
Flaccid cell.
Vacuole is much smaller and taking up less space
If a lot of water leaves the cell, the cytoplasm starts to peel away from the cell wall and we say the cell has undergone plasmolysis.

19. Plasmolysis

20. Osmosis in Plant Cells (ie equal solute concentrations inside & out)
Equal water concentrations inside & out
(ie equal solute concentrations inside & out)
High water concentration outside the cell
(ie pure water or dilute solution outside)
Low water
concentration outside the cell
(ie concentrated solution outside)

21. Animals- Water moving in
Animal cells have no cell wall to stop the swelling, just a flexible cell membrane.
If an animal cells are put in pure water (HWC) they take in water by osmosis until they burst.
Add about contractile vacuole for osmoregulation (joat w/s p9)
In eg Paramecium- features of both animal and plant cells?
haemolysed
normal
turgid then burst 21

22. Animals- Water moving out
If an animal cells are put in a concentrated solution (LWC) they lose water by osmosis until they become shrunken or haemolysed.
haemolysed
normal

23. Quick Quiz- Osmosis
What is a partially permeable membrane?
Which substance moves by osmosis?
What happens to plant cells that take up water by osmosis, and what do you call the cell?
What happens to plant cells that lose water by osmosis, and what do you call the cell?
What does ‘haemolysed’ mean? How does it happen?
A membrane that only lets small molecules through it
Water, H2O
The cells become ‘full’ of water and are called turgid
The cytoplasm comes away from the cell wall and the cells are called plasmolysed.
Animals cells which have lost water become shrunken and are called haemolysed.

24. Osmosis
Two sugar solutions are separated by a semi-permeable membrane like Visking or dialysis tubing.
The larger sugar molecules are too big to pass through the membrane but the water particles can pass.
On both sides of the membrane particles move randomly hitting the membrane and each other.
Dilute sugar solution Concentrated sugar solution

25. Osmosis
Osmosis is like diffusion except there is a membrane in the way.
If a water molecule on either side hits the membrane it will bounce off - if it hits a hole it will go through.
The sugar molecules can’t pass and just bounce off.
Water molecules from both sides pass through the membrane but there is a nett movement from left to right.
Nett movement is down the water gradient.
Dilute sugar solution Concentrated sugar solution

26. Osmosis
Osmosis is the diffusion of molecules from an area of high concentration to an area of lower concentration through a partially permeable membrane.
In terms of water potential we can say that osmosis allows the movement of water particles from areas of high water concentration or “potential” to areas of low water concentration or “potential”, across a semi-permeable membrane.
Nett movement is down the water gradient.
Dilute sugar solution Concentrated sugar solution

27. Now try your osmosis practicals