Membranes

1. Membranes Structure and function

2. Phospholipids • Lipid micelles • Micelles: vesicles in which polar head groups of lipids face the external aqueous solution and hydrophobic tails collect together inside. (Fig. 4.5a,b)

4. Figure 4.5 Lipid micelles Lipid bilayers Water No water Serine Phosphate Hydrophilic heads interact with water Hydrophobic tails interact with each other Hydrophilic heads interact with water

5. Lipid Bilayer • Amphipathic lipids can form a bilayer structure in an aqueous solution. • Liposomes: vesicles composed of a bilayer enclosing an internal aqueous solution. (Fig. 4.1a,b; 4.6a) • Micelle, bilayer, and liposome formation is spontaneous • Raises questions about property of membranes: permeability? fluidity? Is this selective?

7. The importance of permeability • If certain molecules or ions pass though a lipid bilayer more readily than others, it means that the internal environment of a vesicle can become different from the outside! • This difference in internal and external conditions is vital and a key characteristic of living cells.

8. Figure 4.6a Liposomes: Artificial membrane-bound vesicles Water Water 62.5 nm 62.5 nm

9. Artificial membranes as an experimental system • Planar bilayers: are formed in a hole in a divider between two aqueous solutions. (Fig. 4.6b) • Artificial membranes can be used to study permeability of lipid bilayers.

10. Figure 4.6b Planar bilayers: Artificial membranes Water Water Lipid bilayer

11. Membranes are Composed of Lipids • Artificial lipid structures are used as experimental systems to determine membrane properties. • Selective permeability of the membrane to molecules and ions: polar, charged, and/or large compounds are least likely to cross a bilayer. (Fig. 4.6c, 4.7a,b)

12. Figure 4.6c Artificial membrane experiments How rapidly can different solutes cross the membrane (if at all) when…. 1. Different types of phospholipids are used to make the membrane? Solute (ion or molecule) ? 2. Proteins or other molecules are added to the membrane?

13. Figure 4.7b Summary of relative permeabilities Phospholipid bilayer Hydrophobic molecules O2, CO2, N2 Small, uncharged polar molecules H2O, glycerol Large, uncharged polar molecules Glucose, sucrose H+,Na+,NCO3–, Ca2+,CL-,Mg2+,K+ Ions

14. Unsaturated lipids: double bonds create kinks, prevents close packing of lipid tails, increases fluidity. • Saturated lipids pack more tightly; membrane is less fluid. (Fig. 4.8a,b)

15. Figure 4.8a The angles of carbon bonds C C 120º 109.5º C C Single bonds Double bonds

16. Figure 4.8b Double bonds cause kinks in hydrocarbons. H2C CH2 H2C CH2 H2C CH Kink CH H2C CH2 H2C Unsaturated fatty acid Saturated fatty acid CH2 H2C

17. Figure 4.8c Kinks change the permeability of membranes. Lipid bilayer with no unsaturated fatty acids Low permeability Lipid bilayer with many unsaturated fatty acids High permeability

18. Figure 4.9a Cholesterol: fills spaces between lipid tails, allows closer packing, decreases fluidity. Polar Nonpolar • Temperature effects: higher temperatures increase fluidity and permeability.

19. Temperature and Fluidity • At 25ºC phospholipids are liquid. • Bilayers have consistency of olive oil. • As temps drop, molecules move slower, fluidity decreases… solidify.

20. At room temp, phospholipids have been clocked at 2um/sec. • Can travel the length of a bacteria cell every second. • Phospholipid molecules whiz around each layer while water and small molecules shoot in and out of the membrane.

21. Movement of Substances Across Membranes • Diffusion • Net directed movement of molecules or ions, driven by thermal energy. (Fig. 4.10) • Diffusion is a spontaneous, passive process. • Molecules and ions move downhill along electrochemical gradients. • Facilitated diffusion: assisted by a type of membrane protein called an ionophore. • http://www.biosci.ohiou.edu/introbioslab/Bios170/diffusion/Diffusion.html • http://northonline.sccd.ctc.edu/judylearn/NTR%20150/GUIDE_WEEK_1.htm

22. Figure 4.10 Lipid bilayer DIFFUSION ACROSS A SEMI-PERMEABLE MEMBRANE 1. Start with two different molecules on opposite sides of a semipermeable membrane (a phospholipid bilayer). 2. Molecules diffuse across the membrane - each along its own concentration gradient. 3. Equilibrium is established. Molecules continue to move back and forth across the membrane but at equal rates.

23. Movement of Substances Across Membranes • Osmosis • Diffusion of water across a membrane towards regions of higher solute concentration. (Fig. 4.11)

24. OSMOSIS 1. Start with more solute on one side of the lipid bilayer than the other using molecules that cannot cross the semipermeable membrane. Lipid bilayer 2. Water moves from the region of low concentration of solutes (high concentration of water)to the region of high concentration of solutes (low concentration of water). Osmosis

25. Movement of Substances Across Membranes • Osmosis • Osmosis can cause shrinking or swelling of cells or vesicles: • Water enters vesicle if internal solution is hypertonic to the external solution. (Fig. 4.12) • Water leaves vesicle if internal solution is hypotonic to the external solution.