1. SUB-UNIT 1.2: TRANSPORT ACROSS MEMBRANES

2. Learning Outcomes By the end of the topic, you should be able to -
define diffusion
describe examples of substances which enter and leave the cell by diffusion,
discuss the importance of diffusion to cells.
state that osmosis is a ‘special case’ of diffusion of water
explain osmotic effects in plant and animal cells
discuss the effects of placing plant and animal cells in hypertonic, hypotonic and isotonic solutions.
explain the meaning of the terms plasmolysed, turgid and flaccid.

3. Cells and Diffusion
As the ammonium hydroxide (an alkali) moves through it turns the pH paper blue. This demonstrates that the gas is spreading out from a region of high to low concentration.

4. Importance of diffusion
dissolved food
oxygen
nucleus
cell membrane
cytoplasm
carbon dioxide
Human cheek cell

5. Importance of diffusion
The cell membrane freely allows oxygen to diffuse into the cell.
high
high
high
low
Oxygen moves from a high concentration to a low concentration.
low
high
low
high
low
high
high

6. Importance of diffusion
The cell membrane freely allows carbon dioxide to diffuse out of the cell.
low
Carbon dioxide moves from a high concentration to a low concentration.
low
low
high
high
low
low
high
high
low
low

7. Diffusion in Paramecium
A unicellular animal has only _____ cell. The animal is constantly respiring. This means that it is using up ________ and producing _________ __________.
The concentration of oxygen inside the unicellular animal is _______ than the concentration in the water outside. Oxygen therefore diffuses _____ the cell to allow continued respiration.
one
oxygen
carbon dioxide
less
into

8. Diffusion in Paramecium
Higher concentration of oxygen outside cell
Lower concentration of oxygen
oxygen
CO2
Lower concentration of CO2 outside cell
Higher concentration of CO2

9. Diffusion in Paramecium
At the same time the cell is making CO2. The CO2 concentration becomes greater _______ the cell than in the water _________. As a result CO2 diffuses _____ the cell.
If it wasn’t for diffusion unicellular organisms wouldn’t get any oxygen or be able to get rid of waste CO2.
inside
outside
out of

10. Diffusion in Amoeba Food vacuole containing soluble food
diffusion of food

11. Diffusion in Amoeba
Amoeba get their food by phagocytosis The food becomes enclosed in a food vacuole and digestive enzymes break it down into small soluble products.
The dissolved food then moves by diffusion from a _____ concentration (inside the food vacuole) to a _____ concentration (in the cytoplasm) ______ a concentration gradient.
Therefore diffusion allows all parts of the cell to get food.
high
low
down

12. Importance in Multicellular Organisms
oxygen
carbon dioxide

14. Diffusion in multicellular animals
Blood returning to the lungs from cells has a higher CO2 concentration and lower oxygen concentration than the air in the lungs.
CO2 diffuses from the _____ into the ________ and oxygen diffuses from the ________ into the ________.
Diffusion is also essential for dissolved food (e.g. ___________ and __________ _______ ) moving from the blood to respiring cell.
Urea also moves in the opposite direction by diffusion.
lungs
blood
lungs
blood
glucose
amino acids

15. lung cells High O2 conc O2 Low CO2 conc O2 O2 Low O2 conc CO2
High CO2 conc
cells
CO2

16. Diffusion in Green Plants
Green plants need diffusion to get the raw materials needed for photosynthesis ( ___________ ________ and _________)
CO2 diffuses from the air, through the stomata and into the air spaces in the leaf.
Water moves from the soil into the roots, then enters the xylem and then enters the leaf cells.
carbon dioxide
water

17. Importance of diffusion
Water molecules are also free to move in and out of the cell.
However a cell needs to have some control which substances can enter and leave the cell.
If not, then it could gain substances it doesn’t need or even lose substances it does need due to diffusion.

18. Cell Membrane Cell membrane with pores = SELECTIVELY PERMEABLE
water molecules
small area of cell enlarged
Oxygen molecules
Carbon dioxide molecules
Glucose molecules
Starch/salt molecules

19. Model Cell experiment You will need to set up: Method
a length of visking tubing
a boiling tube
a test tube rack
a filter funnel
a paper towel
a plastic syringe, a dropper
a dimple tray
stopclock
starch and glucose solution
Iodine solution
Benedict’s solution
water bath
Method
1. Fill tubing with starch and glucose solution
2. Wash and dry bag
3. Place in boiling tube
4. Immediately test sample of water for glucose and starch
5. Test every 4 minutes

20. Importance of diffusion
Large molecules like starch cannot pass through the cell membrane.
Molecules of soluble food e.g. glucose can pass through.
The membrane is selectively permeable.

21. The Cell Membrane

22. Structure of cell membrane

23. Structure of cell membrane
Fluid Mosaic model – Composed of protein and lipid molecules
Surface proteins – give support
- carry molecules across membrane
- enzymes to speed up reactions
Channel proteins – allow small molecules to pass through membrane
Phospoholipid bilayer – molecules can move and swap places

24. Relative water concentrations
Water concentration - compares concentration of solutions
Higher solute concentration
Lower solute concentration
Lower water concentration
Higher water concentration
hypotonic
Solution A is _________________ to solution B
Solution B is _________________ to solution A
hypertonic

25. More concentrations More concentrations hypotonic
Higher solute concentration
Higher water concentration
Lower water concentration
Lower solute concentration
hypotonic
Solution X is _________________ to solution Y
Solution Y is _________________ to solution X
hypertonic

26. hypertonic = less water
Lower solute concentration
Higher solute concentration
Higher water concentration
Lower water concentration
HYPOTONIC
HYPERTONIC
hypotonic = more water
hypertonic = less water

27. Osmosis is a special kind of diffusion, where:
Water molecules move from a region of high water concentration to a region of low water concentration across a selectively permeable membrane.
Let’s take a look

28. High water concentration Low water concentration
Selectively permeable membrane
Sucrose molecules
High water concentration
Low water concentration
Water molecules
Inside the cell
Outside the cell

29. Selectively permeable membrane
Sucrose molecules
Water molecules are small enough to pass across the selectively permeable membrane
Water molecules
Inside the cell
Outside the cell

30. Water molecules passing into the cell
Selectively permeable membrane
Water molecules passing into the cell
Inside the cell
Outside the cell

31. Selectively permeable membrane
Sucrose molecules
Osmosis continues until the water molecules are evenly spread inside and outside of the cell
Water molecules
Inside the cell
Outside the cell

32. SELECTIVELY PERMEABLE MEMBRANE
Solute molecules
are too large to
pass through the
pores of the
membrane
Net movement
of small water
molecules
PURE
WATER
SOLUTION
(SOLVENT IS WATER)

33. Osmosis Diffusion of water through cell membranes
Diffuses from an area of high water concentration [HWC] to an area of low water concentration [LWC]
The difference in concentration between the areas is called the concentration gradient

34. Osmosis Set up test tubes shown Weigh dried cylinders
Cylinder of potato
water
Salt solution
Weigh dried cylinders
Leave for 30 minutes
Dry then re-weigh

35. Molecular Model of Osmosis
Model cell A
0.5M sucrose solution
0.1 M sucrose solution

36. Molecular Model of Osmosis
Model cell B
0.5M sucrose solution
Model cell B
0.5M sucrose solution

37. Molecular Model of Osmosis
Model Cell C
0.5M sucrose solution
1.0M sucrose solution

38. Direction of Osmosis? 1 High water concentration
Low water concentration
(100% water) 2
10% sucrose solution (90% water)
distilled water
(90% water)
(80% water) 3
10% sucrose solution
20% sucrose solution
(100% water)
(99.5% water) 4
distilled water
0.5% sucrose solution
(90% water)
(99% water) 5
10% starch solution
1% starch solution
(100% water) 6
animal cell
distilled water

39. Animal cells in water

40. Plant cells in water
turgid
plasmolysed

41. Animal Cells Plant Cells In distilled water, an animal cell will gain
water & burst
In strong salt/sugar solution, an animal cell will lose water & shrink
Animal Cells
In distilled water, plant cells gain water
& become swollen (turgid)
Plant Cells
In strong salt/sugar solution, plant cells lose
water, the cytoplasm & vacuole shrink in from the
cell wall – cell becomes plasmolysed

42. The animal cell does not.
So why does the animal cell
burst in distilled water
when the plant cell does not?
The plant cell has a rigid cell wall that helps the plant cell keep its shape.
The animal cell does not.

43. Unicellular animals Contractile vacuole with canals to collect water
When water passes in, vacuole expands then bursts when full
When one vacuole is emptying, the other is filling
This is osmoregulation

44. Osmosis and Energy Production
The world's first test plant to harness osmotic power, a new emission-free source of energy, opened recently in Norway.
Near Oslo, the facility will exploit the energy produced when fresh water meets seawater. 44

45. Fresh water and salt water is guided into two separate chambers, divided by an selectively permeable membrane.
The fresh water moves by osmosis towards the seawater. The flow puts pressure on the seawater side, and that pressure can be used to drive a turbine, producing electricity.

46. Problems……..
The plant at Oslo produces 2 kilowatts of electricity – enough to run a coffee machine!!
Very Expensive to build
But
Huge potential with further research.

47. Active Transport

48. low concentration high concentration energy from respiration
ACTIVE TRANSPORT
low
concentration
high
concentration
PLASMA MEMBRANE
…..releases them
on the other side
Carrier protein takes
up particles on one
side of the plasma
membrane and ……….
energy
from respiration
low
concentration
gradient
high

49. It takes energy to move molecules against their concentration gradient.

50. Active Transport is….. The movement of molecules against a
concentration gradient, this means from a low
concentration to a high concentration.
Energy is needed for active transport.
Carrier proteins pump molecules e.g. into the cell in order to maintain a high concentration inside the cell or out of the cell in order to maintain a low concentration in the cell.

51. Active transport in High conc. of A inside cell
Low conc. of A outside cell
Plasma membrane
protein
Active transport in

52. Animation: How the Sodium Potassium Pump Works
Nerve Cells: Sodium ions are actively transported out of the cell and potassium ions transported into the cell.

53. Cells in our thyroid gland actively uptake iodine
Cells in our thyroid gland actively uptake iodine. They use it to make hormones.

54. More oxygen Less oxygen More respiration Less respiration
More energy produced
Less energy produced
More active transport
of molecules
Less active transport
of molecules

55. Hyperlink: Summary of Diffusion, Osmosis, Active Transport
Membrane Transport