Chapter 13 Diffusion and Osmosis

1. Chapter 13Diffusion and Osmosis

2. Diffusion & Osmosis Draw a spider diagram about what you know about movement of substances in & out of the cell

3. Syllabus -- Objectives • Define the term: Selectively permeable • Explain the role of selectively permeable membranes. • Define the terms: osmosis & diffusion • Give examples of diffusion and osmosis. • Define the term: turgor. • Explain turgidity in plant cells. • Describe the application of high salt or sugar concentration in food preservation.

4. Selective permeability of membranes All cell membranes (biological membranes) are similar in structure. The cell membrane (plasma membrane) around a cell works the same as membranes around organelles eg. Mitochondria & nucleus.

5. Movement in cells takes place through membranes All the membranes in a cell act in the same way Cell Membranes Mitochondrial membranes Nuclear membranes Chloroplast membranes

6. New Vocabulary Selectively permeable: A selectively permeable membrane allows some but not all substances to pass through. E.g. cell membrane Permeable: A permeable membrane allows all substances through e.g. cell wall is fully permeable. Impermeable: An impermeable membrane does not allow substances to pass through.

7. Biological membranes are semi-permeable. Other membranes e.g. cellophane, visking tubing & dialysis tubing are semi permeable (selectively permeable) These membranes allow small molecules such as water, oxygen and carbon dioxide to pass through freely.

8. They do not allow large molecules e.g. protein, sugars and salts to pass through easily. Cells have specialised mechanisms in their membranes to allow these substances e.g. proteins, sugars & salts through when necessary.

9. Dialysis Dialysis is used on patients whose kidneys do not work properly. The dialysis membrane in the machine is used to filter the persons blood and remove substances e.g. salts, water and urea.

10. Movement Substances move in and out of cells in 2 ways 1. Diffusion Movement of molecules from a region of higher concentration to a region of lower concentration. This method does not require energy. 2. Active Transport Movement of molecules from a less crowded to a more crowded area WITH the use of energy. Molecules are “carried" into or out of the cell using some of the cell's energy.

11. Learning check • What type of movement in cells requires energy? • What type of movement in cells does not require energy? • Diffusion is the movement of substances from an area of ....... concentration to an area of ......... concentration • Name 3 different types of membranes

12. Permeability Membranes can be: Permeable – let everything in and out e.g. cell wall Semi Permeable - let some things in and out e.g. cell membrane Impermeable – let nothing in and out e.g. wall of cork cells

13. Diffusion in everyday life • Smell of perfume • Bread baking • Food colouring in water • Sugar in tea

14. Diffusion in a LeafIn photosynthesis Carbon dioxide gas diffuses into leaf and oxygen gas diffuses out Carbon dioxide Oxygen

15. Learning Check • What type of permeability allows free movement of substances? • What type of permeability allows some movement of substances? • What type of permeability allows no movement of substances? • Can you name a few common examples of diffusion?

16. New Vocabulary

17. Osmosis • Osmosis is the movement of water across a semi-permeable membrane from a region of high water concentration to a region of low water concentration

18. Solvent: A solvent is a liquid that dissolves other substances e.g. water • Solute: A solute is a substance that has been dissolved e.g. sugar & salt • Solution: The mixture of the solvent and solute is a solution. E.g. salty water

19. Osmosis is a special type of diffusion that does not require energy • Osmosis is passive (no energy required)

20. Experiment to Demonstrate Osmosis Results Table T/B pg 134

22. If pure water is separated from salt water by a membrane the following occurs: The water molecules move randomly in both directions The salt molecules cannot move out so more water moves in to the salt solution What you have is water moving from where there is a high concentration of water to where there is a lower concentration of water this is osmosis

23. Learning Check • What is osmosis? • Does osmosis require energy?

24. Osmosis and Animal Cells • Animal cells are only enclosed by a membrane • If an animal cell is surrounded by a solution that is the same concentration as the cytoplasm in the cell water will move in and out at the same rate and the cell will remain intact.

25. Many animals that live in the sea have cytoplasm with a similar concentration to sea water. If this didn’t happen the fish would swell up or shrivel up. • It is the function of our kidneys to make sure the fluid in our bodies that surrounds our cells (plasma) has a stable concentration.

26. Animal cells in a less concentrated solution • If an animal cell is in a solution that is less concentrated than its cytoplasm (i.e. if there is more water outside than inside the cell) the water from outside will move into the cell and may cause it to burst or die.

27. Amoebas (microscopic animal cells)

28. Amoeba Amoeba is a microscopic animal that lives in fresh water or sea water. Amoeba has a special organelle called a contractile vacuole When amoeba absorbs water through its cell membrane it sends it to the contractile vacuole.

29. When the contractile vacuole is full of water it bursts onto the surface of the amoeba like a pimple releasing its water. If amoeba did not have a contractile vacuole it would not be able to get rid of the absorbed water and would swell up and burst.

30. Animal Cells in a more concentrated solution • If animal cells are in a solution that is more concentrated than their cytoplasm (i.e. if there is more water in their cytoplasm than in the solution outside) then the water may leave the animal cell and the cells shrivel up and may die.

31. Red blood cells (animal cells) Shrivelled Animal Cells Normal cells Swollen cells

32. Learning check • What surrounds an animal cell? • How does water move in and out of an animal cell? • What will happen to animal cells in a very dilute (watery) solution? • What will happen to animal cells in a very concentrated solution?

33. Osmosis and Plant Cells • Plant cells have a membrane which is surrounded by a cell wall • Cell walls are fully permeable and will allow all substances in and out of the cell • If a plant cell is surrounded by a less concentrated solution then water from outside will move into the cytoplasm and vacuole of plant cells • This is how roots absorb water by osmosis

34. Water entering roots by osmosis The water outside is less concentrated than the water in the cytoplasm

35. Turgor • When the outside water enters the plant cell the vacuole becomes bigger and the cytoplasm swells • This causes the membrane to be pushed out towards the cell wall • When cells are fully “swelled” like this with the membranes pushing against the cell wall they are described as Turgid.

36. Turgid Cells

37. Plant Cells in solutions of different concentration

38. Turgor pressure • This turgor pressure gives plants their strength • If plants did not have this they would be wilted (dead) • Plants that don’t have wood such as lettuce and house plants rely on turgor pressure for strength

39. Plant cells

40. Plant cells in a more concentrated solution • If plant cells are surrounded by a more concentrated solution (for example if plant cells were surrounded by salt water) the water inside the cell would move out to the more concentrated solution outside and the cell will shrivel up (flacid cell)

41. Turgid plant cell & flacid plant cell

42. Wilted Plant, b) normal plant, c) plant with too much water

44. Learning Check • What surrounds the membrane of a plant cell? • Can this structure control what moves in and out of cells? • How does water enter roots of a plant? • What is meant by a turgid cell? • How could you identify a turgid cell?

45. Plasmolysis • When this happens the cell wall stays intact but the membrane shrivels up away from it • This is called Plasmolysis • Cells in this condition are called plasmolysed cells we can look at this easily by placing a layer of red onion cells in salt water

46. Normal plant cell and plasmolysed plant cell

47. Plasmolysed Plant CellsPond weed (Elodea)

48. Plasmolysed red onion cells

49. Food Preservation Why might you need to preserve foods? Can you name any food preservation methods?

50. Osmosis and Food Preservation • Bacteria and Fungi are the most common causes of food spoilage • Both these cells are enclosed by a membrane • If a food is placed in a sugary or salty solution then any bacteria or fungi present will lose the water in their cells to the more concentrated solution outside