Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6 Interactions Between Cells & 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 0 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 & elastin fibers linked to molecules of gel-like ground substance & to plasma membrane integrins  = glycoprotein adhesion molecules that link intracellular & extracellular compartments  Interstitial fluid resides in hydrated gel of ground substance Fig

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 & 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 up a concentration gradient; requires energy & 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 including C0 2 & H 2 0  Diffusion of H 2 0 is called osmosis  Cell membrane is impermeable to charged & most polar compounds  Charged molecules must have an ion channel or transporter to move across membrane Diffusion continued 6-10

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Diffusion continued  Rate of diffusion depends on:  Magnitude of its concentration gradient  Permeability of membrane to it  Temperature  Surface area of membrane 6-11

Osmosis 6-12

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Osmosis  Is net diffusion of H 2 0 across a selectively permeable membrane  H 2 0 diffuses down its concentration gradient  H 2 0 is less concentrated where there are more solutes  Solutes have to be osmotically active  i.e. cannot freely move across membrane Fig

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  H 2 0 diffuses down its concentration gradient until its concentration is equal on both sides of membrane  Some cells have water channels (aquaporins) to facilitate osmosis Osmosis continued Fig

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Is force that would have to be exerted to stop osmosis  Indicates how strongly H 2 0 wants to diffuse  Is proportional to solute concentration Osmotic Pressure Fig

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Molarity & Molality  1 molar solution (1.0M) = 1mole of solute dissolved in 1L of solution  Doesn't specify exact amount of H 2 0  1 molal solution (1.0m) = 1 mole of solute dissolved in 1 kg H 2 0  Osmolality (Osm) is total molality of a solution  E.g. 1.0m of NaCl yields a 2 Osm solution  Because NaCl dissociates into Na + + Cl

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Molarity & Molality  Osmolality (Osm) is total molality of a solution  E.g. 1.0m of NaCl yields a 2 Osm solution  Because NaCl dissociates into Na + & Cl - Fig

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Tonicity  Is effect of a solution (sln) on osmotic movement of H 2 0  Isotonic slns have same osmotic pressure  Hypertonic slns have higher osmotic pressure & 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 6-18

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Effects of tonicity on RBCs Fig 6.11 crenated 6-19

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Regulation of Blood Osmolality  Blood osmolality maintained in narrow range around 300m Osm  If dehydrated, osmoreceptors in hypothalamus stimulate:  ADH release  Which causes kidney to conserve H 2 0  & thirst Fig

Membrane Transport Systems 6-21

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Carrier-Mediated Transport  Molecules too large & polar to diffuse are transported across membrane by protein carriers 6-22

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) Fig

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Facilitated Diffusion Fig 6.14  Is passive transport down concentration gradient by carrier proteins Fig

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

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

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Secondary Active Transport  Uses energy from “downhill” transport of Na + to drive “uphill” movement of another molecule  Also called coupled transport  ATP required to maintain Na + gradient Fig

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 of Na + Secondary Active Transport continued Fig

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 Fig 6.19 Transport Across Epithelial Membranes 6-29

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 6-30

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Bulk Transport  Is way cells move large molecules & particles across plasma membrane  Occurs by endocytosis & exocytosis (Ch 3) Fig

Membrane Potential 6-32

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Membrane Potential  Is difference in charge across membrane  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 & is pumped out Fig

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 reaches its equilibrium potential (E k )  K+ is attracted inside by trapped anions but also driven out by its [gradient]  At K+ equilibrium, electrical & diffusion forces are = & opposite  Inside of cell has a negative charge of about - 90mV Fig

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] inside & outside cell membrane  E x = 61 log [X out ]z = valence of ion X z [X in ] 6-35

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+ Fig

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 Fig

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Resting Membrane Potential (RMP)  Is membrane voltage of cell in unstimulated state  RMP of most cells is -65 to –85 mV  RMP depends on concentrations of ions inside & out  & on permeability of each ion  Affected most by K + because it is most permeable 6-38

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Some Na + diffuses in so RMP is less negative than E K+ Fig 6.25 Resting Membrane Potential (RMP) continued 6-39

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 Fig

Cell Signaling 6-41

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 Fig

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cell Signaling continued  In paracrine signaling, cells secrete regulatory molecules that diffuse to nearby target cells  In synaptic signaling, 1 neuron sends messages to another cell via synapses  In endocrine signaling, cells secrete chemical regulators that move thru blood stream to distant target cells  To respond to a chemical signal, a target cell must have a receptor protein for it 6-43