1. Selective barriers to movement
Biological membranes
Selective barriers to movement
© 2016 Paul Billiet ODWS

2. Biomembrane characteristics
Surrounding all living cells and membranous organelles
SELECTIVE PERMEABILITY  
Often carry out ACTIVE TRANSPORT.
© 2016 Paul Billiet ODWS

3. Biomembrane characteristics
ENZYMIC PROPERTIES
A variety of appearances under the electron microscope
Many properties are lost at high temperatures and extremes of pH.
© 2016 Paul Billiet ODWS

4. Biomembrane characteristics
ANTIGENIC properties
May possess BINDING SITES 
Contain both phospholipid, hydrophobic and hydrophilic proteins
Carbohydrate and cholesterol are sometimes present
Membranes are asymmetrical.
© 2016 Paul Billiet ODWS

5. Fatty acids Water © 2016 Paul Billiet ODWS
R.N.Robertson (1983) The lively membranes Cambridge UP
© 2016 Paul Billiet ODWS

6. Phospholipids Hydrophilic Hydrophobic © 2016 Paul Billiet ODWS
R.N.Robertson (1983) The lively membranes Cambridge UP
© 2016 Paul Billiet ODWS

7. Pouring oil on troubled waters
Pliny (AD 23-79)
Benjamin Franklin (1774)
© 2016 Paul Billiet ODWS

8. Bilayers Gorter & Grendel (1925) extracted lipids from red blood cells
showed that lipids could form bilayers as well as monolayers.
SEM erythrocytes
© 2016 Paul Billiet ODWS

9. πr2
Area of the lipids extracted from the blood cells = twice the area of the blood cells from which the lipid was extracted
Conclusion: Cell membranes are made of lipid bilayers.
Oil drop measurement
© 2016 Paul Billiet ODWS

10. Gorter & Grendel’s data
© 2016 Paul Billiet ODWS

11. Phospholipid bilayer Extra cellular fluid Hydrophobic zone Cytoplasm
© 2016 Paul Billiet ODWS
R.N.Robertson (1983) The lively membranes Cambridge UP

12. Danielli-Davson model (1935)
Jim Danielli
© 2016 Paul Billiet ODWS

13. Danielli-Davson model
Proteins could be adsorbed into lipid droplets
Symmetrical arrangement of proteins either side of the membranes more stable
Lipo-protein sandwich model proposed for membranes (with proteins lining pores).
© 2016 Paul Billiet ODWS

14. Danielli-Davson model
Protein
Phospholipid
Phospholipid
Protein
© 2016 Paul Billiet ODWS

15. Seeing is believing Electron microscope invented in 1931
Danielli-Davson model 1935
The ultra-microtome for slicing in 1941
Unit Membrane model Robertson 1967.
Erythrocyte membrane x
© 2016 Paul Billiet ODWS

16. Contradictory evidence
Bio-membranes are asymmetric
Membrane proteins are globular and amphipathic = polar parts towards water and non-polar parts towards lipid
Electron micrograph techniques such as freeze fracturing and sublimation revealed proteins inside lipid bilayer.
Freeze fracturing
© 2016 Paul Billiet ODWS

17. Freeze fracturing Plasma membrane of a plant cell freeze fractured
© 2016 Paul Billiet ODWS

18. The fluid mozaic model (1972)
S.J. Singer
Garth Nicolson
© 2016 Paul Billiet ODWS

19. Inside-Outside Two main categories of membrane proteins:
Peripheral proteins - bound to the surface of the membrane Outside / inside – different (asymetrical)
Integral proteins - permeate the surface of the membrane, they may be exposed on both sides.
© 2016 Paul Billiet ODWS

20. Fluid membranes Membrane lipid layer fluid
Proteins move (float) easily laterally along membrane
Proteins move less easily vertically from side to side.
© 2016 Paul Billiet ODWS

21. The fluid mozaic model Integral protein Peripheral proteins
Non-polar amino acids
Peripheral proteins
Polar amino acids
© 2016 Paul Billiet ODWS
R.N.Robertson (1983) The lively membranes Cambridge UP

22. Cholesterol – in animal cell membranes
© 2016 Paul Billiet ODWS
R.N.Robertson (1983) The lively membranes Cambridge UP

23. Cholesterol thickens membranes
© 2016 Paul Billiet ODWS
R.N.Robertson (1983) The lively membranes Cambridge UP