The Cell Membrane AP Biology

1. The Cell MembraneAP Biology

2. Helpful links as you go through this chapter… • http://student.ccbcmd.edu/courses/bio141/lecguide/unit3/eustruct/cmeu.html

3. Cell membrane defines the perimeter of the cell • Cell membrane separates living cell from aqueous environment • thin barrier = 8nm thick • Controls traffic in & out of the cell • allows some substances to cross more easily than others

4. Movement across cell membrane • Cell membrane is the boundary between inside & outside… • separates cell from its environment NO! Can it be an impenetrable boundary? OUT waste ammonia salts CO2 H2O products IN food carbohydrates sugars, proteins amino acids lipids salts, O2,H2O OUT IN cell needs materials in & products or waste out

5. Selective Permeability • Barrier allows SOME substances to cross more easily than others • Also called SEMI-Permeable • Key aspect of membrane structure and function • Remember: selectively permeable membranes are important in many organelles as well as the plasma membrane of the cell itself.

6. Membrane Structure • Primary components • Lipids • Foundation molecules • Primarily PHOSPHOLIPIDS • Proteins • Embedded within the lipids of the membrane • Important in carrying out most membrane functions • Remember: It’s the PROTEINS that do all the WORK!

7. 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

8. Why the bilayer? • Water is attracted to hydrophilic heads and NOT to hydrophobic tails • Water pushes tails away and they end up pointed toward each other and AWAY from water. • Results in Bilayer • Animation – • Click HERE

9. Bilayer as Selective Barrier • Because of the close placement and polarity of the phosphate heads, there are some molecules that cannot penetrate and some that can.

10. What CAN penetrate • Phospholipid Bilayer? • Anything NONPOLAR • Lipids • Oxygen • Carbon dioxide • NONPOLAR molecules do • NOT get attracted and • Stuck to polar heads; thus • They pass through. • What CANNOT penetrate • Phospholipid Bilayer? • POLAR / Charged things • Ions (H+, Cl-, Na+) • Water • Glucose • Polar amino acids • Ions get large hydration • Shells in aqueous • Solution which makes them • Too large to fit b/t • Phospholipids • Also, polar/charged molecues • Become attracted and • stuck to polar heads • ______ attracted to nonpolar center • of bilayer

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

12. How, then, can these polar, but necessary molecules pass through the membrane? • PROTEINS • Embedded in the bilayer

13. Permeability to polar molecules? • Membrane becomes semi-permeable via protein channels • specific channels allow specific material across cell membrane • Thus control is also maintained. Not just anybody can get in. inside cell H2O aa sugar salt outside cell NH3

14. Aquaporin – A Protein Channel HI water Conc. • Used to be thought that water could penetrate the phospholipid bilayer unaided • Now we know that a protein channel called aquaporin is found in nearly all cell membranes in large amounts • This is the channel that allows the passage of water. • Remember, water is polar and would NOT be attracted past the polar heads LO water Conc.

15. Cell membrane is more than lipids… • Transmembrane proteins embedded in phospholipid bilayer • create semi-permeabe channels protein channelsin lipid bilyer membrane

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

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

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

19. Protein’s regions 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

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

21. Membrane Proteins do LOTS of different jobs!! “Channel” Outside Plasma membrane Inside Transporter Enzymeactivity Cell surfacereceptor “Antigen” Cell adhesion Cell surface identity marker Attachment to thecytoskeleton

22. Membrane Proteins • Proteins determine membrane’s specific functions • cell membrane & organelle membranes each have unique collections 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, usually across whole membrane • transmembrane protein • ex: transport proteins • channels, permeases (pumps)

23. Cell membrane must be more than lipids… • In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer • FLUID MOSAIC MODEL • Made up of lots of little pieces – like a mosaic • Pieces can slide past each other – like a soap bubble – like a fluid • Click here for an animation

24. Don’t forget Cholesterol • Embedded in membrane b/t phospholipids • Keeps membrane firm yet fluid • Important for cell membrane structure • Even though we often think of cholesterol as being “bad”…

25. 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 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model

26. 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

27. Any Questions??

28. Movement across the Cell Membrane

29. Movement of Molecules Across a Membrane • 2nd Law of Thermodynamics governs biological systems • universe tends towards disorder (entropy) • Remember that molecules are always moving…they don’t have any “brains” to move in a particular direction…they are just moving – randomly. (entropy – no order) • What can affect this random movement? • The size/attractiveness of the openings available for molecules to pass through • “hands” that physically GRAB molecules and FORCE them to go places they might not go just due to random motion.

30. Types of Transport Across a Membrane • Two major types • Passive • Does NOT require energy from the cell • Substances move WITH their concentration gradient • High to Low • Active • DOES require energy from the cell • Substances move AGAINST their concentration gradient • Low to High

31. Note: At equilibrium Molecules are STILL MOVING… Movement just can’t be perceived b/c there is no conc. difference on either side. Diffusion • The movement of any substance… • Due to its random molecular motion… • From an area of HIGH concentration to an area of LOW concentration. • Diffusion • movement from HIGHLOW concentration

32. Simple Diffusion • Move from HIGH to LOW concentration • “passive transport” • no energy needed • Any molecule type can do this diffusion

33. movement of water osmosis Osmosis • Specialized Diffusion • Movement of WATER from an area of HIGH water concentration to an area of LOWER water concentration • Across a membrane

34. HIGH LOW Facilitated Diffusion • Diffusion through protein channels • channels allow specific molecules across cell membrane • Very selective “doors” • no energy needed facilitated = with help diffusion = WITH conc. gradient

35. LOW HIGH Active Transport • Cells may need to move molecules against concentration gradient • From LOW concentration to HIGH • Protein pump attracts the molecules and undergoes shape change which moves the molecule from one side of membrane to other • Conformational change = shape change • “costs” energy = ATP conformationalchange ATP

36. Active Transport - Types of Pumps • Sodium Potassium Pump • Na+/K+ Pump • Classic example • Important in nerve cell and nerve impulse transmission (among others) • Generates voltage difference across the membrane of the neuron • For animations click • Here or Here

37. Cotransport • Membrane proteins couple the transport of one solute to another • A combination of both active transport and facilitated diffusion (passive). • Pump (active) • Transport channel (passive)

38. Cotransport – Example – in plants • Proton Pumps pump H+ against the concentration gradient and OUT of the cell. • Creates a very hi concentration of H+ outside the cell. • ACTIVE transport • That means, if given an opportunity, H+ would flow WITH its concentration gradient back INSIDE the cell.

39. Cotransport – Example – in plants • In plants, sucrose is in HIGH concentration inside the cells, but plants need to get even even MORE sucrose into cells for storage. • Thus, sucrose must move into a plant cell AGAINST its concentration gradient.

40. Cotransport – Example – in plants • “Downhill” (WITH concentration) movement of H+ is coupled with the “uphill” (AGAINST concentration) movement of sucrose into the cell. • Sucrose MUST bind to an H+ ion. “Rides” with the H+ as H+ flows INTO the cell through the cotransport channel WITH it’s concentration gradient. • Remember, the H+ concentration gradient is maintained by the proton pump • The only way that sucrose can enter is if it is bound to H+. Then the very specific cotransport channel will allow both to “diffuse” through. • PUMP to maintain H+ conc. = ACTIVE • Cotransport channel = facilitated diffusion = PASSIVE

41. Getting through cell membrane • Passive Transport • Simple diffusion • diffusion of nonpolar, hydrophobic molecules • lipids • HIGH  LOW concentration gradient • Facilitated diffusion • diffusion of polar, hydrophilic molecules • through a protein channel (“door”) • HIGH  LOW concentration gradient • Active transport • pump against concentration gradient • LOW  HIGH • uses a protein pump • requires ATP ATP

42. Transport summary NONPOLAR cmpds. Lipids; oxygen; Carbon dioxide simplediffusion HI LO facilitateddiffusion LO HI POLAR cmpds. Water; Ions – Na+, Cl-; Glucose, etc. ATP activetransport LO HI REQUIRED

43. How about large molecules? • Moving large molecules into & out of cell • through vesicles & vacuoles • Endocytosis – taking in • phagocytosis = “cellular eating” - solids • pinocytosis = “cellular drinking” - liquids • Exocytosis • Expelling / secreting substances exocytosis

44. Endocytosis Solids fuse with lysosome for digestion phagocytosis Liquids Both are non-specificprocesses pinocytosis Cholesterol is taken Into the cell This way. SPECIFIC receptor-mediated endocytosis LIGAND – substance that fits receptor

45. The Special Case of WaterMovement of water across the cell membrane

46. Osmosis is just diffusion of water • 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

47. 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 moves from a HYPOTONIC (HIGH Concentration of water) solution to a HYPERTONIC (LOW Concentration of water) solution LESS water MORE water LESS solute MORE solute Yellow dot = SOLUTE water Blue = WATER net movement of water

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

49. 1 Managing water balance • Hypotonic • a cell in fresh water • high concentration of water around cell • problem: cell gains water, swells & can burst (lysis) • example: Paramecium • ex: water continually enters Paramecium cell • solution: contractile vacuole • pumps water out of cell • ATP • plant cells • turgid = full • cell wall protects cell from bursting KABOOM! ATP No problem,here – I love it! freshwater

50. Pumping water out • Contractile vacuole in Paramecium ATP