1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6 Lecture Outline

2. Chapter 6 Outline Extracellular Environment Diffusion Osmosis Carrier-Mediated Transport The Membrane Potential Cell Signaling 6-2

3. Extracellular Environment 6-3

4. Extracellular Environment Includes all constituents of body outside cells 67% of total body H 2 O is inside cells (=intracellular compartment); 33% is outside cells (=extracellular compartment-ECF) –20% of ECF is blood plasma –80% of ECF is interstitial fluid contained in gel-like matrix 6-4

5. Extracellular Matrix Is a meshwork of collagen and elastin fibers linked to molecules of gel-like ground substance and to plasma membrane integrins –= glycoprotein adhesion molecules that link intracellular and extracellular compartments Interstitial fluid resides in hydrated gel of ground substance 6-5

6. Movement Across Plasma Membrane 6-6

7. Transport Across Plasma Membrane Plasma membrane is selectively permeable-- allows only certain kinds of molecules to pass Many important molecules have transporters and channels –Carrier-mediated transport involves specific protein transporters –Non-carrier mediated transport occurs by diffusion 6-7

8. Transport Across Plasma Membrane continued Passive transport moves compounds down concentration gradient; requires no energy Active transport moves compounds against a concentration gradient; requires energy and transporters 6-8

9. Diffusion Is random motion of molecules –Net movement is from region of high to low concentration 6-9

10. Non-polar compounds readily diffuse thru cell membrane –Also some small molecules such as CO 2 and H 2 O Gas exchange occurs this way Diffusion continued 6-10

11. Cell membranes are impermeable to charged and most polar compounds –Charged molecules must have an ion channel or transporter to move across membrane Diffusion continued 6-11

12. Diffusion continued Rate of diffusion of a compound depends on: –Magnitude of its concentration gradient –Permeability of membrane to it –Temperature –Surface area of membrane 6-12

13. Osmosis 6-13

14. Osmosis Is net diffusion of H 2 O across a selectively permeable membrane –H 2 O diffuses down its concentration gradient –H 2 O is less concentrated where there are more solutes Solutes have to be osmotically active –i.e., cannot freely move across membrane 6-14

15. H 2 O diffuses down its concentration gradient until its concentration is equal on both sides of a membrane Some cells have water channels (aquaporins) to facilitate osmosis Osmosis continued 6-15

16. Is the force that would have to be exerted to stop osmosis –Indicates how strongly H 2 O wants to diffuse Is proportional to solute concentration Osmotic Pressure 6-16

17. Molarity and Molality 1 molar solution (1.0M) = 1 mole of solute dissolved in 1L of solution –Doesn't specify exact amount of H 2 O 1 molal solution (1.0m) = 1 mole of solute dissolved in 1 kg H 2 O 6-17

18. Molarity and Molality continued Osmolality (Osm) is total molality of a solution –e.g., 1.0m of NaCl yields a 2 Osm solution –Because NaCl dissociates into Na + and Cl - 6-18

19. Tonicity Is the effect of a solution on osmotic movement of H 2 O Isotonic solutions have same osmotic pressure Hypertonic solutions have higher osmotic pressure and are osmotically active –Hypotonics have lower osmotic pressure Isosmotic solutions have same osmolality as plasma Hypo-osmotic solutions have lower osmotic pressure than plasma – Hyperosmotics have higher pressure than plasma 6-19

20. 6-20

21. Regulation of Blood Osmolality Blood osmolality is maintained in narrow range around 300mOsm If dehydration occurs, osmoreceptors in hypothalamus stimulate: –ADH release Which causes kidney to conserve H 2 O and thirst 6-21

22. Membrane Transport Systems 6-22

23. Carrier-Mediated Transport Molecules too large and polar to diffuse are transported across membrane by protein carriers 6-23

24. Carrier-Mediated Transport continued Protein carriers exhibit: –Specificity for single molecule –Competition among substrates for transport –Saturation when all carriers are occupied This is called T m (transport maximum) 6-24

25. Facilitated Diffusion Is passive transport down concentration gradient by carrier proteins 6-25

26. An Active Transport Pump Is transport of molecules against a concentration gradient –ATP is required –A carrier protein is required 6-26

27. Na + /K + Pump Uses ATP to move 3 Na + out and 2 K + in –Against their gradients 6-27

28. Secondary Active Transport ATP needed for “uphill” (against the concentration gradient) movement of molecule or ion obtained from “downhill” (with the concentration gradient) transport of Na+ into cell 6-28

29. Cotransport (symport) is secondary transport in same direction as Na + Countertransport (antiport) moves molecule in opposite direction to Na + Secondary Active Transport continued 6-29

30. Absorption is transport of digestion products across intestinal epithelium into blood Reabsorption transports compounds out of urinary filtrate back into blood Transport Across Epithelial Membranes 6-30

31. Transport Across Epithelial Membranes continued Transcellular transport moves material from 1 side to other of epithelial cells Paracellular transport moves material through tiny spaces between epithelial cells 6-31

32. Transport Across Epithelial Membranes continued Transport between cells is limited by junctional complexes that connect adjacent epithelial cells 6-32

33. Transport Across Epithelial Membranes continued Plasma membranes can join together to form tight junctions In adherens junctions membranes are “glued” together by proteins that pass through both membranes and attach to cytoskeletons In desmosomes proteins “button” two membranes together 6-33

34. Bulk Transport Moves large molecules and particles across plasma membrane Occurs by endocytosis and exocytosis (Ch 3) 6-34

35. Membrane Potential 6-35

36. Membrane Potential Is difference in charge across membranes Results in part from presence of large anions being trapped inside cell –Diffusable cations such as K + are attracted into cell by anions Na + is not permeable and is actively transported out 6-36

37. Equilibrium Potential Describes voltage across cell membrane if only 1 ion could diffuse  If membrane permeable only to K +, it would diffuse until it reaches its equilibrium potential (E k )  K+ is attracted inside by trapped anions but also driven out by its concentration gradient  At K+ equilibrium, electrical and diffusion forces are = and opposite  Inside of cell has a negative charge of about -90mV 6-37

38. Nernst Equation (E x ) Gives membrane voltage needed to counteract concentration forces acting on an ion Value of E x depends on ratio of ion concentrations inside and outside cell membrane E x = 61 log [X out ] z [X in ] Z = valence of the ion 6-38

39. Nernst Equation (E x ) continued E x = 61 log [X out ] z [X in ] For concentrations shown at right: –Calculate E K+ –Calculate E Na+ 6-39

40. Nernst Equation (E x ) continued E K+ = 61 log 5 +1 150 = -90mV E Na+ = 61 log 145 +1 12 = +66mV 6-40

41. Resting Membrane Potential (RMP) Is membrane voltage of cell not producing impulses RMP of most cells is –65 to –85 mV RMP depends on concentrations of ions inside and out –And on permeability of each ion Affected most by K + because it is most permeable 6-41

42. Some Na + diffuses in so RMP is less negative than E K+ Resting Membrane Potential (RMP) continued 6-42

43. Role of Na+/K+ Pumps in RMP Because 3 Na+ are pumped out for every 2 K+ taken in, pump is electrogenic –It adds about –3mV to RMP 6-43

44. Summary of Processes that Affect the Resting Membrane Potential 6-44

45. Cell Signaling 6-45

46. Cell Signaling Is how cells communicate with each other Some use gap junctions thru which signals pass directly from 1 cell to next 6-46

47. Cell Signaling continued To respond to a chemical signal, a target cell must have a receptor protein for it In paracrine signaling, cells secrete regulatory molecules that diffuse to nearby target cells 6-47

48. Cell Signaling continued In synaptic signaling 1 neuron sends neurotransmitter messages to another cell via synapses 6-48

49. Cell Signaling continued In endocrine signaling, cells secrete chemical regulators that move thru blood stream to distant target cells 6-49

50. How Regulatory Molecules Influence Target Cells Nonpolar regulatory molecules pass through plasma membrane, bind to receptors in nucleus, and affect transcription –Examples include steroid and thryroid hormones and nitric oxide 6-50

51. How Regulatory Molecules Influence Target Cells Polar regulatory molecules bind to cell surface receptors –Activated receptors send 2nd messengers into cytoplasm to mediate actions of regulatory molecule 6-51

52. Second Messengers May be ions (e.g. Ca ++ ) or other molecules such as cyclic AMP (cAMP) or G-proteins 6-52

53. G-proteins Are part of 2nd messenger pathway in many cells Contain 3 subunits whose components dissociate when a cell surface receptor is activated –A subunit binds to an ion channel or enzyme, changing their activity 6-53

54. G-proteins continued 6-54