1. National 5 Biology Unit 1 – Cell Biology
Section 2 – Transport across the Membrane

2. We will be learning how to:
Name and describe the components of a cell membrane in terms of the fluid mosaic model.
Carry out experiments on the effect of ethanol and temperature on beetroot cells.
Describe diffusion as a method of passive transport down a concentration gradient for substances such as glucose, carbon dioxide and oxygen.
Describe osmosis as a method of passive transport down a concentration gradient for the movement of water through a selectively permeable membrane.
Describe the movement of water into and out of animal cells when they shrink or burst.
Describe the movement of water into and out of plant cells when they become turgid or plasmolysed.
Describe the relationship the relationship between different concentrations of solutions and their effect on cells.
State that active transport requires energy for membrane proteins to move molecules and ions against the concentration gradient.

3. Cell Membrane Structure

4. The Fluid Mosaic Model
The current accepted theory for the structure of the cell membrane is the Fluid mosaic model
This consists of a double layer (bilayer) of phospholipids
And a mosaic of proteins

5. Phospholipids
Phospholipids are the main structural components of membranes
Phospholipids form a bilayer (2 layers)
They each have a hydrophilic (water-loving) head
And two hydrophobic (water-hating) tails
Hydrophilic head
Head
Symbol
HydrophobicTails

6. The Phospholipid Bilayer
The phospholipids line up with the hydrophobic tails pointing inwards, away from the fluids inside and outside the cells
This forms the bilayer

7. How do things get in and out of the cell through the membrane?
The Proteins
How do things get in and out of the cell through the membrane?
There must be holes…
The proteins can be:
Partially embedded in the phospholipid layer
Spanning the phospholipid layer from one side to the other
Have channels forming pores
Are attached to the surface
Made of proteins

8. The proteins have various functions within the cell membrane

9. The Functions of the Proteins
Provide structural support
Contain channels allowing transport of small molecules through the membrane
Act as facilitators for diffusion
Act as carriers for active transport
Act as receptors for hormones
Act as receptors for enzymes which act on the membrane
Serve as antigenic markers (tissue type)

10. Fluid Mosaic Model of the Plasma Membrane
proteins
Pore
(surrounded by proteins)
Hydrophilic head
Hydrophobic tail
Phospholipid bilayer

11. Selectively Permeable Membrane
All cells are enclosed with a cell membrane. A selectively permeable cell membrane is one that allows certain molecules or ions to pass through it by means of active or passive transport.
Some substances cross the membrane freely, some cross with assistance, and others do not cross at all

12. Diffusion and Cell Membrane Structure
You’re going to watch a video about the cell membrane. The answer to each question can be found in the video.
What is the outer layer of an animal cell called?
The lipid molecules have one side that attracts water and one side that repels it – true or false?
What do the proteins in the membranes do? Give one example.
How many layers of lipids does our cell membrane have?
Do the proteins stay still or change positions?

13. Membrane Investigation – Red Cabbage Cells
Cytoplasm
Nucleus
Cell Wall
Vacuole
Cell Membrane
Plant and Animal cells

14. Method
Aim – To predict what will happen when cells are added to acid, alcohol, hot water and cold water.
You will need:
A test tube rack
4 test tubes
A piece of red cabbage
A corer
Alcohol
Acid
Things to think about –
Where is the red pigment concentrated?
Where will the pigment go if the membrane is damaged?

15. Method Measure 6ml of water in tube A Measure 6ml of alcohol in tube B
Label the boiling tubes 1, 2, 3 and 4
Measure 6ml of water in tube A
Measure 6ml of alcohol in tube B
Measure 6ml of acid in tube C
Measure 6ml of water in tube D
Use the corer to get 4 pieces of red cabbage. Put one in each test tube
Put tube 4 in the water bath

16. Investigation
A B C D
water
water
alcohol
acid
70⁰C
30⁰C

17. Results!
This is what you should have seen 1 2 3 4

18. Results
A B C D
water
water
alcohol
acid
70⁰C
30⁰C

19. Results Conclusion In which tubes did bleeding occur?
B (alcohol), C (acid) and D (high temperature)
Explain why bleeding has occurred in these tubes.
Acid , and high temperature (70˚C) destroys proteins in membrane
Alcohol destroys lipid part of membrane
Conclusion

20. Evaluation
What was the purpose of tube A
List what variables you kept constant
How could you make your experiment more reliable?

21. Diffusion (Passive Transport)

22. Start Timer
Think, Pair, Share 4
When the membrane broke down the red cell sap moved into the acid/alcohol/water Why do think this happened? Can you describe how the sap moved in terms of concentration? 2

23. 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.

24. Particles move in the spaces between other particles

25. Diffusion
This is the movement of a substance from an area of high concentration to a low concentration down a concentration gradient
DOES NOT REQUIRE
ENERGY
High concentration
Low concentration

27. Diffusion
Diffusion

28. Selectively Permeable – what do you think?
Substances that can pass through the membrane
Substances that cannot pass through the membrane
Starch amino acids proteins
cells glucose

29. 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

30. Collect sucrose and starch solution A boiling tube Some visking tubing
Boiling tube rack
Filter funnel
Dimple tray
Stopclock
Iodine
Benedicts solution
Goggles
Pippette/droppper
Collect

31. Instructions: Open your visking tubing and tie a knot in one end
Funnel in some sucrose/starch solution
Tie off the other end
Put the visking tubing in your boiling tube
Add water to the boiling tube and pipette some water out immediately to test for starch or sugar
Test every four minutes

32. Selectively Permeable
Substances that can pass through the membrane
Substances that cannot pass through the membrane
Starch
Amino Acids
Proteins
Glucose
Cells
Starch amino acids proteins
cells glucose

33. Diffusion and Cell Membrane Structure
The channels in the proteins will let molecules like sugar/glucose through, but not larger things like starch. Because of this we say the membrane is selectively permeable.
Plant and Animal cells

34. Sites of Diffusion Where do you think diffusion happens in your body?
What way does each substance move?

35. Diffusion Glucose Carbon dioxide Oxygen
Why is diffusion important to cells?
To gain raw materials for respiration and photosynthesis.
To remove waste products

36. Importance of Diffusion in the Body
Where do you think diffusion happens in your body?
What way does each substance move?

37. Importance of Diffusion
glucose
oxygen
nucleus
cell membrane
cytoplasm
carbon dioxide
Human cheek cell

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

39. Water molecules are also free to move in and out of the cell.
So what’s next?
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.

40. Diffusion: true or false?
Boardworks Science for Scotland: S1 and S2
Competition
Diffusion: true or false?

41. 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.
blood
lungs
blood
lungs
glucose
amino acids

42. 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

43. Osmosis

44. Osmosis

45. Osmosis in Potato Samples
Draw a diagram of your apparatus before and after the experiment
Label all parts e.g. solutions, samples & mass
Write an explanation for your results:
- Pure water
- Salt solution

46. Explain why the sucrose molecules do not diffuse across the membrane.
In what direction do the water molecules diffuse?
Explain how a selectively permeable membrane works.

47. 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

48. 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

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

50. 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

51. Osmosis
Osmosis

52. Osmosis
Expt: potato cylinder
Osmosis

53. Three bits of potato are put into different concentrations of solution
Three bits of potato are put into different concentrations of solution. Inside the potato cells there is a concentration of around 2%. What happens to each?
Discuss with class variables, why did you blot the cylinders before weighed them
Distilled water
2% salt solution
10% salt solution

54. High and low concentrations
Boardworks Science for Scotland: S1 and S2
Competition
High and low concentrations
During osmosis, water molecules diffuse from pure water or dilute solution to more concentrated solutions.
Dilute solutions have a high concentration of water molecules.
Concentrated solutions have a low concentration of water molecules.
pure water
dilute solution
concentrated
solution

55. Boardworks Science for Scotland: S1 and S2 Competition
Predicting osmosis

56. Model Cell – Gummy Bears!
Collect 2 gummy bears and two beakers
Put a few centimetres of water in one beaker
Put a few centimetres of salt solution in the other beaker
Put a gummy bear in each
Label your beakers with your intials and the solution used

58. 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
(90% water)
(99% water) 4
10% starch solution
1% starch solution
(100% water)
(99.5% water) 5
distilled water
0.5% sucrose solution
(100% water) 6
animal cell
distilled water

59. Boardworks Science for Scotland: S1 and S2 Competition
Osmosis and cells
Plant and animal cells are surrounded by a partially-permeable plasma membrane. This allows water and other small molecules to diffuse across.
Plant cells additionally have a strong cell wall surrounding the membrane which offers support and protection.
plasma
membrane
cell wall
red blood cell
plant cell

60. Animal cells in water

61. Osmosis and animal cells
Boardworks Science for Scotland: S1 and S2
Competition
Osmosis and animal cells

62. Plant cells in water
turgid
plasmolysed

63. Osmosis and plant cells
Boardworks Science for Scotland: S1 and S2
Competition
Osmosis and plant cells

64. 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

65. The animal cell does not.
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.

66. 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

67. Now I can….. 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
Explain the meaning of the terms plasmolysed and turgid.

68. Active Transport

69. We will be learning… To state what is meant by active transport
To describe the process of active transport
To give examples of substances which move by active transport
To state what factors limit the rate of active transport

70. What is a Concentration Gradient?
The difference in concentration between two areas
Which is the steepest gradient?
Which particles move up a gradient (to an area of high concentration)?

71. Diffusion
Which way will the particles move?

72. Moving against a concentration gradient
ENERGY
What do think is required to move the molecules against the concentration gradient?
This is active transport

73. Active Transport
The movement of molecules from a region of low concentration to a region of high concentration against a concentration gradient

74. Active Transport
Inside of cell Outside of cell
ATP
ADP P

75. Active Transport
molecules ow
igh
oncentration
Low conc.
High conc.

76. Protein molecules act as carrier molecules.
                            
These proteins recognise specific substances and transfer them across the plasma membrane.

77. Sodium Potassium Pump
Sodium moves out of the cell.
So where must the sodium ion concentration be higher?

78. An example of active transport
Sodium and potassium ion movement in nerve cells.
Potassium ions move into the cell.
So where must the potassium ion concentration be higher?

79. Active transport carriers are often called ‘pumps’ .
They often work by swapping one substance for another.

80. Factors affecting active transport
Can you think of any?
Remember! Active transport needs energy from respiration (the burning of food in the presence of oxygen inside a cell).
Glucose concentration
Oxygen concentration
Temperature

81. Factors affecting active transport
Describe and explain the results shown

82. Summary
Make a list or table of similarities and differences between active transport and diffusion.

83. Now I can….. State what is meant by active transport
Describe the process of active transport
Give examples of substances which move by active transport
State what factors limit the rate of active transport

84. Passive Transport

86. Active Transport - Sodium pump