1. Chapter 13 Diffusion 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
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
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 Leaf In 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
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
Sucrose Solution
Distilled Water
Turgid (fullness) at start
Turgid (fullness) at end
Mass at start (g)
24.1g
24.0g
Mass at end of Activity (g)
29.5g

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. Shrivelled Animal Cells
Red blood cells (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 Cells Pond 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

51. When this happens the cells will shrivel and die and the food will not become contaminated with loads of bacteria and fungi.
These types of preservation can keep food fresh for long periods of time.

52. Food Preservation

53. Ensures more interesting and nutritionally adequate family meals
Food Preservation
Prevents and delay microbial decomposition and damage from mechanical causes, insects and rodents
Ensures more interesting and nutritionally adequate family meals
Ensure a safer food intake

54. Commonly used in meat and fish and less in fruits and vegetables
Salting
Commonly used in meat and fish and less in fruits and vegetables
In some foods, salting is combined with other methods like smoking, fermenting, drying, freezing
Examples: ham, bacon, salted eggs, sausage, tocino

55. Salt as a preservative
Sodium Chloride (NaCl) preserves food by removing water content through osmosis

56. When food is salted, its water content is drawn out, freeing the food or reducing the moisture content which makes the food less susceptible to microorganisms

57. Sugar as a preservative
Draws out water through osmosis
Examples of foods preserved by adding sugar are jams and fruit

58. Examples Fish and Meat (bacon) may be stored in a salty solution
Jams, marmalades and tinned fruits are stored in a sugary solution

59. Learning Check What is meant by plasmolysis?
What kind of a solution would you put plant cells in to cause them to palsmolyse?
How is plasmolysis used to preserve food?
Give an example of some types of food preserved in this way?

60. Exam questions
2005 Ordinary level Q7
2008 Higher level Q14C