Diffusion

1. 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 SEM photos of lung alveoli In breathing you exchange carbon dioxide (CO2) and oxygen (O2) between alveoli in the lungs and the blood. This is an example of diffusion.

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

7. Respiration & diffusion

8. Photosynthesis & Diffusion Carbon dioxide diffuses in through the stomata carbon dioxide + water  oxygen + glucose • 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 Key: Water molecule Solute molecule High Water Concentration, few molecules of solute. Dilute solution Water movement Low Water Concentration, many molecules of solute. Concentrated solution In osmosis water moves both ways to balance up the concentrations. The overall (nett) movement is to the area of low water concentration.

11. Osmosis in action Have a look at: http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

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

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

14. 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 byosmosis.

16. Plants- Water moving in Water moves into a plant cell. The cell wall stops the cell from bursting. We say that the cell is turgid. Turgid cell. Vacuole is filling all the space and pushing against cell wall This is useful as it gives plant stems support

17. Cell turgor pressure

18. Plants- Water moving out Flaccid cell. Vacuole is much smaller and taking up less space When water moves out of the plant cell vacuole, by osmosis, it becomes flaccid 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 Low water concentration outside the cell Equal water concentrations inside & out High water concentration outside the cell (ie pure water or dilute solution outside) (ie concentrated solution outside) (ie equal solute concentrations inside & out)

21. Animals- Water moving in If an animal cells are put in pure water (HWC) they take in water by osmosis until they burst. Animal cells have no cell wall to stop the swelling, just a flexible cell membrane. haemolysed normal turgid then burst

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 http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/Osmosis_Animation.htm

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 isthe 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