Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6 Interactions Between Cells and the Extracellular Environment 6-1

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6 Outline  Extracellular Environment  Movement Across Plasma Membrane  Osmosis  Membrane Transport Systems  Membrane Potential  Cell Signaling 6-2

Extracellular Environment 6-3

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Movement Across Plasma Membrane 6-6

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Diffusion  Is random motion of molecules  Net movement is from region of high to low concentration 6-9

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Osmosis 6-13

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 6-20

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Membrane Transport Systems 6-22

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Carrier-Mediated Transport  Molecules too large and polar to diffuse are transported across membrane by protein carriers 6-23

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Facilitated Diffusion  Is passive transport down concentration gradient by carrier proteins 6-25

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Active Transport  Is transport of molecules against a concentration gradient  ATP is required 6-26

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Na + /K + Pump  Uses ATP to move 3 Na + out and 2 K + in  Against their gradients 6-27

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Secondary Active Transport  Requires ATP to first move Na + uphill to create a gradient  Secondary active transport then uses energy from “downhill” movement of Na + to drive “uphill” transport of another molecule 6-28

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Transport Across Epithelial Membranes continued  Transport between cells is limited by junctional complexes that connect adjacent epithelial cells 6-32

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bulk Transport  Moves large molecules and particles across plasma membrane  Occurs by endocytosis and exocytosis (Ch 3) 6-34

Membrane Potential 6-35

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 = valence of ion X z [X in ] 6-38

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Nernst Equation (E x ) continued  E K+ = 61 log = -90mV  E Na+ = 61 log = +60mV 6-40

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Some Na + diffuses in so RMP is less negative than E K+ Resting Membrane Potential (RMP) continued 6-42

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Summary of Processes that Affect the Resting Membrane Potential 6-44

Cell Signaling 6-45

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cell Signaling continued  In synaptic signaling 1 neuron sends neurotransmitter messages to another cell via synapses 6-48

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cell Signaling continued  In endocrine signaling, cells secrete chemical regulators that move thru blood stream to distant target cells 6-49

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Second Messengers  May be ions (e.g. Ca ++ ) or other molecules such as cyclic AMP (cAMP) or G-proteins 6-52

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. G-proteins continued 6-54