Membrane Structure and Function

1. Membrane Structure and Function Chapter 5, pt 1

2. Plasma Membrane - the Cell Boundary • Controls what goes in and out of the cell • Semipermeable - allows some things to cross more easily than others

3. Membrane Structure Phospholipids – main lipid in membranes Amphipathic molecule – has a hydrophilic region & a hydrophobic region

4. Membranes are fluid • Held together by hydrophobic interactions • Most lipids and some proteins can move laterally • http://www.dnatube.com/video/360/Fluid-Mosaic-Model

5. Evidence for drifting of membrane proteins RESULTS Membrane proteins Mixed proteinsafter 1 hour Mouse cell Human cell Hybrid cell

6. Viscous Fluid Unsaturated hydrocarbontails Saturated hydrocarbon tails (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol within the animal cell membrane

7. Fluid Mosaic model of membrane structure

8. What’s part of the membrane • Phopholipids • Cholesterol • Proteins: • Integral proteins – embedded in lipid bilayer • Peripheral proteins – not embedded • Carbohydrate chains on proteins or lipids – found on outside face of membrane • Oligosaccharides (15 or fewer sugars)

9. Integral protein Hydrophobic region is in the membrane Hydrophilic regions are exposed on either side of membrane

10. Signaling molecule • Functions of Proteins in Membrane Receptor Enzymes ATP Signal transduction (a) Transport (c) Signal transduction (b) Enzymatic activity Glyco-protein (f) Attachment to the cytoskeleton and extracellular matrix (ECM) (e) Intercellular joining (d) Cell-cell recognition

11. Traffic across membranes • What needs to go in and out of a cell?

12. Membrane structure results in selective permeability • Small hydrophobic molecules – easily go through plasma membrane • Ions, polar molecules do not • (water, glucose) • Transport proteins aid in moving these molecules across membrane

13. Passive Transport -Energy is not required -Substance moves down concentration gradient

14. Osmosis • Passive transport of water • Hypertonic solution – more solute • Hypotonic solution – less solute • Isotonic solution – same amt of solute

15. Osmosis animation • http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/animation_-_osmosis.html • - What is plasmolysis? • - What is turgor pressure?

16. Facilitated Diffusion • Transport proteins help move polar molecules or ions across membrane • Channels – corridor for specific molecules or ions • Gated channels – open with a stimulus - physical, electrical, chemical • Many change shape (conformation) in process

17. Facilitated diffusion • http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

18. Aquaporins • Water channel proteins • (2003 Nobel prize – co winner Pete Agre for work on aquaporins) Move massive amounts of water molecules across the membrane – 3 billion per second Water molecules move through in a single file

19. Active Transport • Requires energy (in form of ATP) • Moves substances against the concentration gradient • From low concentration to high concentration

20. Sodium Potassium Pump • http://highered.mcgraw-hill.com/sites/9834092339/student_view0/chapter38/sodium-potassium_exchange_pump.html

22. Voltage across membranes • Some ion pumps generate voltage across membranes – difference in charge across membrane • An ion diffuses down electrochemical gradient

23. Cotransport

24. Endocytosis, Exocytosis • Transport of large molecules Animation: • http://www.youtube.com/watch?v=4gLtk8Yc1Zc • Amoeba eating paramecium: • http://www.youtube.com/watch?v=pvOz4V699gk

25. Types of Endocytosis

26. Receptor mediated endocytosis • Coated pits have receptors for specific molecule • Allow cells to get large amts of specific materials that may be in lower concentrations in the environment • Humans – cholesterol travels in LDLs – attach to receptors in pits • LDLs act as ligands – bind to LDL receptors in cell membrane • If LDL receptors are defective, leads to hypercholesterolemia (build up of LDL and cholesterol in the blood)

27. Receptor mediated endocytosis – LDL animation • http://www.dnatube.com/video/5092/Receptor-Mediated-Endocytosis-of-LDL-Cholesterol

28. Practice problems – use the terms hypertonic, hypotonic, isotonic as appropriate • If you soak your hands in dishwater, your hands absorb the water and swell into wrinkles. This is because your skin cells are to the dishwater. • Why does lettuce wilt in a vinaigrette salad dressing? • Why is seawater dangerous to drink? • Why will a marine fish die in freshwater?