Physical Cell Biology Lecture 10: membranes elasticity and geometry. Hydrophobicity as an entropic effect

Transcription

1 Physical Cell Biology Lecture 10: membranes elasticity and geometry Phillips: Chapter 5, Chapter 11 and Pollard Chapter 13 Hydrophobicity as an entropic effect 1

2 Self-Assembly of Lipid Structures Lipid Self-assembles in water Vesicle Supported Lipid Bilayer Glass Lipid Monolayer Aqueous solution The Balance of Forces in a Lipid Bilayer Electrostatic interactions between headgroups repulsive ~2-11 mn m -1 dipolar interactions repulsive ~1-7 mn m -1 Hydration of lipid headgroups repulsive ~35 mn m -1 Hydrophobic exposure of the hydrocarbon chain to water cohesive ~35 mn m -1 Entropic loss of chain configurations Marsh (1996) BBA 1286, repulsive ~30 mn m -1 Van der Waals Steric (hard core interactions) cohesive ~1-3 mn m -1 per CH 2 repulsive ~10-33 mn m -1 group 2

3 General bio-membrane structure lder Models Pollard Figure 7-1 3

4 More recent models Pollard Figure 7-1 Lipid Bilayer Model Pollard Figure 7-5 4

5 Heterogeneity of the Plasma Membrane Pollard Figure 7-7 Lateral Phase Separation in Model Membranes H N + - P HN H N + - P HN H H HN H H H H H H NH - H H H H H H HN H H H H H N + P - Liquid-ordered phase Translational diffusion coefficient 5x10-9 cm 2 s -1 Cholesterol, Glycolipids, and Sphingomyelin rich High trans/gauche ratio Reduced area per lipid Thicker N P - N P - N P - Liquid-disordered phase Translational diffusion coefficient 10-8 cm 2 s -1 Accommodates unsaturated side chains Low trans/gauche ratio 5

6 Phase Immiscibility in Model Membranes H H H H N+ - H H N+ P - H H H P H NH Vetach (2005) PRL 94, Dynamics of liquid-liquid immiscibility in a giant unilamellar vesicle composed of DPC, DPPC and Cholesterol following a temperature quench Veatch (2003) Biophysical Journal 85, Phase Immiscibility in Model Membranes Vesicle micrographs of GUVs with bulged domains below their miscibility transition temperature. Vesicles are composed of (a) 1:2 DPC/DPPC + 35% Chol, (b) 1:1 DPC/DMPC + 30% Chol, and (c) 1:4 DPC/DPPC + 25% Chol. All scale bars are 20 μm. Sarah L. Veatch, Sarah L. Keller Separation of Liquid Phases in Giant Vesicles of Ternary Mixtures of Phospholipids and Cholesterol Biophysical Journal Volume 85, Issue

7 Spontaneous Curvature arise from Competition Between the Packing Preferences of the Polar Head and the Hydrocarbon Tail Bend toward water Bend toward oil The mechanical properties of membranes depend on their composition and determine overall membrane shape. 7

8 Membrane Deformability Wikimedia commons Membrane Deformability Pollard Figure 7-6 8

9 how to think about deformability of a sheet mathematically Four main modes of membrane deformation: Stretch, bend, shear and thickness change. 9

10 Elastic Parameters of Fluid Bilayer Membranes Material Shear Bending Compression [mn/m] [Joule] [mn/m] Polyethylene (shell of 5 nm thickness) 3x10 2 2x x10 3 Red blood cells 6x10-3 5x x10 3 Evans and Needham J. Phys. Chem. 1987, 91, A simple height function h(x,y) can describe simple membrane bending geometry. 10

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13 Free Energy of Membrane Deformation The equilibrium shapes of red blood cells can be found by minimizing the free energy. 13

14 Free Energy of Membrane Stretching Free Energy of Membrane Bending 14

15 Free Energy of Membrane Dilution 15

16 Integral Membrane Proteins Pollard Figure 7-8 Peripheral Membrane Proteins Pollard Figure

17 Membranes Act as Strong Barriers Membranes Act as Strong Barriers 17

18 Three types of transporters Pollard Figure 8-1 Cellular processes driven by ion gradients Pollard Figure