CHAPTER. Movement Across Plasma Membrane. Chapter 6 Outline. Diffusion Osmosis. Membrane Potential Cell Signaling

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1 CHAPTER 6 Interaction Between Cells and the Extracellular Environment Chapter 6 Outline Extracellular Environment Diffusion Osmosis Carrier-Mediated Carrier Mediated Transport Membrane Potential Cell Signaling The Movement Across Plasma Membrane 1

2 Diffusion Is random motion of molecules Net movement is from region of high to low concentration Diffusion Non-polar compounds readily diffuse thru cell membrane 2

3 Rate of Diffusion Rate of diffusion of a compound depends on: Magnitude of its concentration gradient Permeabilityy of membrane to it Temperature Surface area of membrane Size of molecule 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 Facilitate diffusion Active transport Non-carrier mediated transport occurs by diffusion through the membrane (simple diffusion) 3

4 Non-carrier mediated transport Carrier-mediated transport Transport Across Plasma Membrane Active transport moves compounds against a concentration gradient; requires energy and transporters Osmosis 4

5 Osmosis 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 that cannot pass through the membrane are said to be osmotically active Some cells have water channels (aquaporins) to facilitate osmosis Both sides are equal at 270 g/l of sucrose (360g/L + 180g/L)/2 = 270 g/l What will take place, osmosis or diffusion? Osmotic Pressure 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 5

6 Mole and Molarity Glucose molecular weight MW = 180 Sucrose (glucose + fructose) molecular weight MW = 342 ( ) All compounds equal to its molecular weight in grams contain the same number of molecules X = Avogadro s number = 1 mole 1 mole = mw of a compound in grams 180 grams of glucose contains 1 mole (180 g/1 mole) 180 grams of glucose contains 6.02 X molecules glucose 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 (molality - 1.0m) = 1 mole of solute dissolved with 1 kg H 2 O 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 - Molarity and Molality Increase Osm = Increase Osmotic Pressure 6

7 Measurement of Osmolality Freezing point of a solution depends on the concentration of the solution. A 1 molal solution depresses the freezing point of water by 1.86 deg C. 1 m of glucose freezes at 1.86 C 1 m of NaCl freezes at 3.72 C (Osm) (-1.86) = freezing point Plasma freezes at 0.56 C (Osm) ( -1.86) = C -0.56/-1.86 = 0.3 Osm = 300 mosm Tonicity 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 solutions have lower osmotic pressure 7

8 What is effect of immersing an animal cell in a hypertonic or hypotonic solution? Regulation of Blood Osmolality Blood osmolality is maintained in narrow range around 300 mosm If dehydration occurs, osmoreceptors in hypothalamus stimulate: ADH release Which causes kidney to conserve H 2 O and thirst 8

9 Membrane Transport Systems Carrier-Mediated Transport Molecules too large and polar to diffuse are transported across membrane by protein carriers Facilitated Diffusion Passive transport down concentration gradient by carrier proteins 9

10 Carrier-Mediated Transport 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) An Active Transport Pump transport of molecules against a concentration gradient ATP is required A carrier protein is required Na + /K + Pump Uses ATP to move 3 Na + out and 2 K + in Against their gradients 10

11 Secondary Active Transport Cotransport (symport) - transport of molecules in same direction as Na +. Common way to transport glucose. Countertransport (antiport) transport of molecule in the opposite direction to Na + Transport Across Epithelial Membranes 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 Transcellular transport movement of molecules through the cytoplasm of the epithelial cells Paracellular transport movement across the tiny gaps between cells 11

12 Transport Across Epithelial Membranes Paracellular transport is limited by junctional complexes that connect adjacent epithelial cells Transport Across Epithelial Membranes Paracellular transport is limited by Junctional Complexes Bulk Transport Moves large molecules and particles across plasma membrane Occurs by endocytosis and exocytosis (Ch 3) 12

13 Cell Signaling Cell Signaling How cells communicate with each other To respond to a chemical signal, a target cell must have a receptor protein for it 13

14 Cell Signaling- Paracrine Cells secrete regulatory molecules that diffuse to nearby target cells Cell Signaling - Synaptic Neuron sends neurotransmitter messages to another cell via synapses Cell Signaling - Endocrine Endocrine signaling, cells secrete chemical regulators that move thru blood stream to distant target cells 14

15 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 Polar regulatory molecules bind to cell surface receptors Activated receptors send 2nd messengers into cytoplasm to mediate actions of regulatory molecule Second Messengers May be ions (e.g. Ca ++ ) or other molecules such as cyclic AMP (camp) or G-proteins 15

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