1. Mr. Briner Unit 1.3 Membrane structure
ASM
IB DP Biology
Unit 1.3
Membrane structure

2. Cell membranes Structure
The structure of cell membranes make them fluid and dynamic
Maintains its shape and integrity
Does not break
Allows some molecules to move through it
Can form and absorb vesicles

3. 1.3.u1
Phospholipids form bilayers in water due to the amphipathic properties of phospholipid molecules.

4. Cell membranes Structure Phospholipids are amphipathic
Have hydrophobic and hydrophilic regions

5. Cell membranes Structure Hydrophobic fatty acid tails
Repel water and form the middle layer of the membrane.
Hydrophilic phosphate heads
Attract water and form the outer layers of the membrane.

6. Cell membranes Structure
When put in water, phospholipids have emergent properties of organization
Keep the heads ‘wet’ and the tails ‘dry’

7. Cell membranes Structure Can form micelles – lipoprotein complexes!
Can form liposomes – vesicles!

9. Cell membranes Vesicle transport In cells, phospholids form bilayers
Bilayer made of phospholipids and others
Examples:
Cell membrane
Membrane bound organelles (Eukaryotes only)
Nuclear membrane
Mitochondria
Chloroplasts
RER
Golgi apparatus
Vesicles.
Etc…

10. Cell membranes

11. Cell membranes Vesicle transport Membrane structure:
Lipids move laterally (around each other) in a membrane
But flip-flopping across the membrane is rare

12. Cell membranes Vesicle transport Membrane structure:
Unsaturated hydrocarbon tail of phospholipids have kinks
Due to double bonds
Keeps the molecules from packing together too closely
Enhancing membrane fluidity

13. Cell membranes

14. 1.3.u3
Cholesterol is a component of animal cell membranes.

15. 1.3.a1
Cholesterol in mammalian membranes reduces membrane fluidity and permeability to some solutes.

16. Cell membranes Vesicle transport Membrane structure:
Cholesterol reduces membrane fluidity and add stiffness
Reducing phospholipid movement
Also hinders solidification at low temperatures
Bad analogy
Imagine a room full of people wearing fluffy sweaters. Its is crowded, but people can slip past each other. Now sprinkle the crowd with people wearing Velcro suits…

17. Cell membranes Vesicle transport Membrane structure: Cholesterol
Has polar and non-polar regions
So crosses one layer

18. Cell membranes Vesicle transport Membrane structure: Cholesterol
Prevents phospholipids from being to close
Reduces permeability of polar molecules and ions

19. Cell membranes Vesicle transport Membrane have to be:
Fluid enough for cells to move
Fluid enough to let materials cross
Stiff enough to prevent entrance of some materials
Stiff enough to hold cell together

20. Cell membranes Vesicle transport Membrane structure:
Phospholipid molecules can move in the horizontal plane
Phospholipid exchange in the vertical plane does not occur
Cholestoral reduces fluidity

21. Cell membranes
Vesicle transport
Membrane structure:

22. 1.3.u2
Membrane proteins are diverse in terms of structure, position in the membrane and function.

23. Cell membranes Structure Integral proteins: Peripheral proteins:
Embedded in the phospholipid of the membrane
Can be monotopic (penetrating one layer) or polytopic (penetrating both layers)
Peripheral proteins:
Attached to the phospholipid surface
Can be monotopic or just attach to surface

24. Cell membranes Structure Transport: protein channels and pumps
Receptors: for hormones, etc.
Anchorage: cytoskeleton and extracellular matrix
Cell recognition: MHC proteins and antigens
Intracellular joints: junctions between cells
Enzymes: metabolic pathways / electron transport

25. Cell membranes Structure Glycoproteins:
Proteins with markers for recognition
Identify cells as belonging to your body
Receptors for certain types of hormones
Receptors for identifying ‘invaders’ by immune system cells

26. Cell membranes

27. Cell membranes

28. Cell membranes Function of membrane proteins Channel proteins:
Integral proteins that allow movement of large substances from one side of the membrane to the other
Like tunnels through the membrane
Enzymes:
Integral proteins that metabolize reactions
For example, ATP synthetase

29. Cell membranes Function of membrane proteins Receptor proteins:
Proteins with a surface outside of the cell
Can detect external signals
Changing the function of the cell (ie. hormones)
Involved in cell or substance recognition (ie. immune system)
Electron carriers
Proteins involved in the movement of electrons (ie. in photosynthesis)

30. Cell membranes
Function of membrane proteins
Receptor proteins:

31. Cell membranes Structure Membrane proteins Hormone binding sites
Receptors
Immobilized enzymes
Cell adhesion
Cell-to-cell communication
Marker proteins - glycoproteins
Channels for passive transport
Pumps for active transport

32. 1.3.s3
Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicolson model.

33. Cell membranes Structure Models of the cell membrane
Singer-Nicolson fluid mosaic model
Proposed by Singer-Nicholson in 1972
Accepted today
Replaced the Davson-Danielli model

34. Cell membranes Structure Models of the cell membrane
Singer-Nicolson fluid mosaic model

35. Cell membranes Structure Singer-Nicolson fluid mosaic model
Phospholipids in a bilayer
Can move laterally but not flip
Peripheral proteins either inside or outside
Integral proteins can go through the bilayer
Membrane is a fluid mosaic
Bendable and has a mix of phospholipids and proteins
Proteins can move laterally
But not flip

36. Cell membranes Structure Evidence for the S-N fluid mosaic model
Membrane proteins of two cells tagged with red and green fluorescent markers
Cells fused together
Red and green markers mixed
So proteins can move around the membrane!

37. Cell membranes
Structure
Evidence for the S-N fluid mosaic model

38. 1.3.s2
Analysis of evidence from electron microscopy that led to the proposal of the Davson-Danielli model.

39. Cell membranes Structure Davson-Danielli model
Popular before S-N model
Based on electron micrograph of membrane

40. Cell membranes Structure Davson-Danielli model
Proteins usually dark and phospholipids light
Believed proteins covered the whole phospholipid membrane
No proteins in the membrane

41. Cell membranes
Structure
Davson-Danielli model
= wrong!

42. Cell membranes
Structure
Davson-Danielli model
= wrong!

43. Cell membranes Structure Davson-Danielli model
Proteins were found to exist inside the membrane
Photo of membrane interior was uneven, suggesting proteins and lipids mixed there
Demonstrated the D-D model was wrong

44. 1.3.s1
Drawing of the fluid mosaic model.

45. MAJOR SOURCES
Mr. Briner
ASM
Thank you to my favorite sources of information when making these lectures!
John Burrell (Bangkok, TH)
Dave Ferguson (Kobe, JA)
Stephen Taylor (Bandung, IN)
Andrew Allott – Biology for the IB Diploma
C. J.Clegg – Biology for the IB Diploma
Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate
Howard Hugh’s Medical Institute –
Mr. Hoye’s TOK Website –
And all the contributors at