Chapter 6

1. Chapter 6 Interactions Between Cells and the Extracellular Environment 6-1

2. Extracellular Environment 6-3

3. Extracellular Environment • Includes all constituents of body outside cells • 67% of total body H2O 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- where we want H2O to go. 6-4

4. Body-Fluid Compartments • Body has intracellular and extracellular compartments • Intracellular is inside cells • Extracellular is outside cells • Separated by cell’s outer membrane • Extracellular is composed of blood plasma and interstitial fluid 1-45

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. Diffusion continued • Non-polar compounds readily diffuse thru cell membrane • Also some small molecules such as CO2 and H2O • Gas exchange occurs this way 6-10

11. Diffusion continued • Cell membranes are impermeable to charged and most polar compounds • Charged molecules must have an ion channel or transporter to move across membrane 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 • H2O levels 6-12

13. Osmosis 6-13

14. Osmosis • Is net diffusion of H2O across a selectively permeable membrane • H2O diffuses down its concentration gradient • H2O is less concentrated where there are more solutes • Solutes have to be osmotically active • i.e., cannot freely move across membrane 6-14

15. Osmosis continued • H2O 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 6-15

16. Osmotic Pressure • Is the force that would have to be exerted to stop osmosis • Indicates how strongly H2O wants to diffuse • Is proportional to solute concentration 6-16

17. Tonicity • Is the effect of a solution on osmotic movement of H2O • 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

18. 6-20

19. 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 H2O and thirst 6-21

20. Membrane Transport Systems 6-22

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

22. Active Transport • Is transport of molecules against a concentration gradient • ATP is required 6-26

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

24. Membrane Potential 6-35

25. 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

26. 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 (Ek) • 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

27. 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

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

29. 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

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