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

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

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

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

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

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

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

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

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

Gorter & Grendel’s data © 2016 Paul Billiet ODWS

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

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

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

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

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 x280 000 © 2016 Paul Billiet ODWS

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

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

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

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

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

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

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

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