The Cell Membrane

1. The Cell Membrane

2. Phospholipids Phosphate “attracted to water” • Phosphate head • hydrophilic • Fatty acid tails • hydrophobic • Arranged as a bilayer Fatty acid “repelled by water” Aaaah, one of thosestructure–function examples

3. Arranged as a Phospholipidbilayer • Serves as a cellular barrier / border sugar H2O salt polar hydrophilic heads nonpolar hydrophobic tails impermeable to polar molecules polar hydrophilic heads lipids waste

4. Cell membrane defines cell • Cell membrane separates cell from aqueous environment • Thin = 8nm thick • Controls transport in & out of the cell • Some substances cross more easily than others • hydrophobic (nonpolar) vs. hydrophilic (polar) • Small vs. big.

5. Cell membrane must be more than lipids… • In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipidbilayer It’s like a fluid…It’s like a mosaic… It’s the Fluid Mosaic Model!

6. Permeability to polar molecules? • Membrane becomes semi-permeable via protein channels • specific channels allow specific material across cell membrane inside cell H2O aa sugar salt outside cell NH3

7. Cell membrane is more than lipids… • Transmembrane proteins embedded in phospholipidbilayer • create semi-permeable channels lipid bilayer membrane protein channelsin lipid bilyer membrane

8. Why areproteins the perfect molecule to build structures in the cell membrane?

9. Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic

10. Classes of amino acids What do these amino acids have in common? I like thepolar onesthe best! polar & hydrophilic

11. Protein domains anchor molecule Polar areas of protein • Within membrane • nonpolar amino acids • hydrophobic • anchors protein into membrane • On outer surfaces of membrane in fluid • polar amino acids • hydrophilic • extend into extracellular fluid & into cytosol Nonpolar areas of protein

12. Porin monomer H+ Retinal chromophore b-pleated sheets NH2 Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH Cytoplasm H+ H+ Examples aquaporin = water channel in bacteria H2O H+ proton pump channel in photosynthetic bacteria function through conformational change = protein changes shape H2O

13. Many Functions of Membrane Proteins Outside Plasma membrane Inside “Channel” Transporter Enzymeactivity Cell surfacereceptor “Antigen” Cell adhesion Cell surface identity marker Attachment to thecytoskeleton

14. Membrane Proteins • Proteins determine membrane’s specific functions • Every membrane in a cell has a unique collection of proteins • Classes of membrane proteins: • peripheral proteins • loosely bound to surface of membrane • ex: cell surface identity marker (antigens) • integral proteins • penetrate lipid bilayer, across whole membrane • “transmembrane”protein • ex: transport proteins • channels, permeases (pumps)

15. Glycoprotein Glycolipid Transmembrane proteins Peripheral protein Filaments ofcytoskeleton Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Extracellular fluid Phospholipids Cholesterol Cytoplasm

16. Membrane carbohydrates • Play a key role in cell-cell recognition • ability of a cell to distinguish one cell from another • antigens • important in organ & tissue development • basis for rejection of foreign cells by immune system

17. Cell Membranes are not static!! • Watch this

18. Any Questions??

19. Movement across the Cell Membrane(Ch. 7)

20. Diffusion • 2nd Law of Thermodynamicsgoverns biological systems • universe tends towards disorder (entropy) • Diffusion • movement from HIGHLOW concentration

21. Simple Diffusion • Move from HIGH to LOW concentration • “passive transport” • no energy needed movement of water diffusion osmosis

22. HIGH LOW Facilitated Diffusion • Diffusion through protein channels • channels move specific molecules across cell membrane • no energy needed facilitated = with help open channel = fast transport “The Bouncer”

23. LOW HIGH Active Transport • Cells may need to move molecules against concentration gradient • conformational shape change transports solute from one side of membrane to other • protein “pump” • “costs” energy = ATP conformationalchange ATP “The Doorman”

24. Getting through cell membrane • Passive Transport • Simple diffusion • nonpolar, hydrophobic molecules • HIGH LOW concentration gradient • Facilitated transport • polar, hydrophilic molecules • through a protein channel • HIGH LOW concentration gradient • Active transport • againstconcentration gradient • LOW HIGH • uses a protein pump (requires ATP) ATP

25. Transport summary simplediffusion facilitateddiffusion ATP activetransport

26. What about large molecules? • Moving large molecules into & out of cell • through vesicles & vacuoles • endocytosis • phagocytosis = “cellular eating” • pinocytosis = “cellular drinking” • exocytosis exocytosis

27. Endocytosis fuse with lysosome for digestion phagocytosis non-specificprocess pinocytosis triggered bymolecular signal receptor-mediated endocytosis

28. About Osmosis • Water is very important to life, so we talk about water separately • Diffusion of water from HIGH concentration of water to LOW concentration of water • across a semi-permeable membrane

29. hypotonic hypertonic Concentration of water • Direction of osmosis is determined by comparing total solute concentrations • Hypertonic - more solute, less water • Hypotonic - less solute, more water • Isotonic - equal solute, equal water water net movement of water

30. Managing water balance • Cell survival depends on balancing water uptake & loss freshwater balanced saltwater

31. 1 Managing water balance • Hypotonic • a cell in fresh water • high concentration of water around cell • problem: cell gains water, swells & can burst • example: Paramecium • solution: contractile vacuole • pumps water out of cell • Uses ATP • plant cells • turgid = full • cell wall protects from bursting KABOOM! No problem,here ATP freshwater

32. Pumping water out • Contractile vacuole in Paramecium ATP

33. 2 Managing water balance I’m shrinking,I’m shrinking! • Hypertonic • a cell in salt water • low concentration of water around cell • problem: cell loses water & can die • example: shellfish • solution: take up water or pump out salt • plant cells • plasmolysis= wilt • can recover I willsurvive! saltwater

34. 3 Managing water balance • Isotonic • animal cell immersed inmild salt solution • no difference in concentration of water between cell & environment • problem: none • no net movement of water • cell in equilibrium • volume of cell is stable • example:blood cells in blood plasma • slightly salty IV solution in hospital That’sperfect! I couldbe better… balanced

35. Aquaporins 1991 | 2003 • Water moves rapidly into & out of cells • evidence that there were water channels • protein channels allowing flow of water across cell membrane Peter Agre John Hopkins Roderick MacKinnon Rockefeller

36. Do you understand Osmosis… .05 M .03 M Cell (compared to beaker)  hypertonic or hypotonic Beaker (compared to cell)  hypertonic or hypotonic Which way does the water flow?  in or out of cell

37. Any Questions??