Chapter 5 Membrane Dynamics

Chapter 5 Membrane Dynamics - Ch 5: Membrane Dynamics Ch...

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Unformatted text preview: Ch 5: Membrane Dynamics Ch Cell membrane structures and functions – – – Membranes form fluid body compartments Membranes as barriers and gatekeepers How products move across membranes • i.e., methods of transport – Distribution of water and solutes in cells & Distribution the body the – Chemical and electrical imbalances – Membrane permeability and changes Law of Mass Balance • Most simply, ins = Most outs outs • Homeostasis is Homeostasis not the same as equilibrium equilibrium – E.g., membrane E.g., potentials potentials Membrane – 2 Meanings! Meanings! • Epithelial membranes vs. • Cell membranes and Membranes Cell around organelles around Thickness ~ 8nm Cell Membrane Structure: Fluid Mosaic Model Cell PLs Cholesterol Proteins: peripheral (associated) or integral Membrane Structure: Protein to Lipid Ratio varies from cell type to cell type Ratio for cells with high metabolic activity? Membrane Proteins Membrane Integral ( Associated Associated Membrane-spanning or intrinsic (peripheral or ) extrinsic) extrinsic) • Can span membrane Can several times several • Either move around Either or are kept in place by cytoskeleton proteins cytoskeleton Allows for cell • Loosely bound to Loosely membrane membrane • Enzymes and Enzymes structural proteins structural Other Phospholipid Behaviors in H2O: Other • Phospholipid bilayer • Micelle – Role in digestion and Role absorption of fats in GI tract absorption • Liposome – Larger, bilayer, hollow center Larger, with aqueous core with Clinical relevance? Movement across Membrane Movement Membrane permeability varies for Membrane different molecules & cell types molecules cell Two movement categories: • Passive and Passive • Active depends on?? Passive Transport Passive = Diffusion (Def?) – 3 types: Def?) types: 1. simple diffusion 1. 2. osmosis 3. facilitated diffusion (= mediated transport) (= facilitated Active Transport Always protein-mediated – 3 types: co-transport co-transport vesicular transport vesicular receptor mediated transport receptor Membrane Spanning Protein Fig 5-5 Cytoskeleton Proteins anchor membrane proteins Diffusion Process (Passive) •Uses energy of concentration Uses gradient gradient •Net movement until state of Net equilibrium reached (no more conc. gradient) •Direct correlation to temperature Direct (why?) (why?) •Indirect correlation to molecule Indirect size size •Slower with increasing distance •Lipophilic molecules can difuse Lipophilic through the phospholipid bilayer through Fig 5-5 Distance – Time Relationship Time for diffusion to progress to given distance ~ to distance squared diffusion over 100 µ m takes 5 sec. diffusion over 200 µ m takes ?? diffusion over 400 µ m takes ?? diffusion over 800 µ m takes ?? Diffusion effective only over short distances! Fick’s law of Diffusion (p 135) rate of = diffusion surface area x conc. gradient membrane resistance x membrane thickness depends on size and lipid-solubility of molecule and composition of lipid bilayer Membrane Proteins Fig 5-7 Protein-Mediated Transport • More selective – Active or Passive • Membrane Proteins – – – – Structural Enzymes Receptors Transporters (allows Specificity, Transporters Competition, Saturation p 145) Competition, • Channel • Gated Transporters Transporters Cell Membrane Regulates Exchange with Environment Many molecules use transporters to cross cell membrane. Why? Examples ? Why? Examples Two categories of transporter proteins Two 1. Channel proteins (rapid but not as selective – for small molecules only, e.g., water and ions) for 2. Carrier proteins (slower but very selective – also works for large molecules) also 1. Channel Proteins 1. • For small molecules such For as ?? as • Aquaporin; plus > 100 ion Aquaporin; channels channels • Selectivity based on size & Selectivity charge of molecule charge • All have gate region – Open – Gated Open Channels vs. Gated Channels Gated Gates closed most of the time = pores Have gates, but gates are open Have most of the time. most Also referred to as “leak Also channels”. channels”. Chemically gated channels (controlled by messenger molecule or ligand) molecule Voltage gated channels (controlled by electrical state of cell) cell) Mechanically gated channels (controlled by physical state of cell: temp.; stretching of cell membrane etc.) membrane 2. Carrier Proteins 2. • Never form direct connection Never between ECF and ICF – 2 gates! between • Bind molecules and change Bind conformation conformation • Used for small organic molecules Used (such as?) (such • Ions may use channels or carriers • Rel. slow (1,000 to 1 Mio / sec) Cotransport Cotransport Symport • Molecules are carried Molecules in same direction in • Examples: Glucose Examples: and Na+ Antiport • Molecules are carried Molecules in opposite direction in • Examples: Na+/K+ pump pump Facilitated Diffusion (as a form of carrier mediated transport) carrier Some characteristics same as simple diffusion Some but also: but • specificity • competition • saturation Figs 5-18/20 Active Transport Active • Movement from low conc. to Movement high conc. high • ATP needed • Creates state of Creates disequilibrium disequilibrium • 1o (direct) active transport – ATPases or “pumps” (uniport (uniport and antiport)– examples? • 2o (indirect) active transport (indirect) – Symport and antiport 1o (Direct) Active Transport • ATP energy directly fuels transport • Most important example: Na+/K+ pump = sodium-potassium ATPase (uses up to 30% of cell’s ATP) ATPase (uses Fig 5-16 • Establishes Na+ conc. Establishes gradient ⇒ Epot. can be harnessed for other cell functions functions ICF: high [K+], low [Na+] ECF: high [Na+], low [K+] Mechanism of the Na+/K+-ATPase start Fig 5-17 2o (Indirect) Active Transport • Indirect ATP use: uses Epot. Indirect uses stored in concentration gradient (of Na+ and K+) gradient • Coupling of Ekin of one molecule with movement of another molecule another • Example: Na+ / Glucose symporter symporter – other examples • 2 mechanisms for Glucose mechanisms transport transport Body Fluid Compartments Body EC fluid IC fluid Exchange much more selective; Why ? Interstitial fluid plasm a Relatively free exchange Fig 5-13 Body Fluid Compartments: Critical Thinking Question ECF ICF What properties should a molecule have to be used as marker for one of the fluid compartments? Do total H2O; total EC and plasma. Then, how do you figure out ICF and interstitial fluid? Competitio n and Saturation Fig 5-18 Glucose and fructose use same transport protein Saturation of carrier mediated transport: Fig 20 Table 5-4 Vesicular Transport Vesicular Movement of macromolecules across cell Movement macromolecules membrane: membrane: 1. Phagocytosis (specialized cells only) 2. Endocytosis – – – Pinocytosis Receptor mediated endocytosis (Caveolae) Potocytosis 1. Exocytosis 1. Phagocytosis 1. • Requires energy Requires • Cell engulfs particle into vesicle via pseudopodia Cell formation formation • E.g.: some WBCs engulf bacteria engulf • Vesicles formed are much larger than those formed Vesicles by endocytosis by • Phagosome fuses with lysosomes ⇒ ? (see Fig. 5-23) (see Phagosome 2. Endocytosis 2. • Requires energy Requires • No pseudopodia - Membrane surface indents • Smaller vesicles • Nonselective: Pinocytosis for fluids & dissolved substances substances • Selective: – Receptor Mediated Endocytosis via clathrin-coated pits via Example: LDL cholesterol and Familial Hypercholesterolemia – Podocytosis via caveolae Fig 5-24 Receptor Mediated Endocytosis and Membrane Recycling Fig 5-28 3. Exocytosis 3. Intracellular vesicle fuses with membrane → Requires energy (ATP) and Ca2+ Examples: large lipophobic molecule secretion; receptor insertion; waste removal Movement through Epithelia: Transepithelial transport Uses combination of active and passive transport Molecule must Molecule cross two phospholipid bilayers Apical and basolateral cell membranes have different Apical proteins: proteins Na+- glucose transporter on apical membrane Na+/K+-ATPase only on basolateral membrane Fig 5-26 Transepithelial Transport of Glucose apical 1. Na+/Glucose symporter only found on apical side 2. Na+/K+-ATPase only found on basolateral side basolateral 3. Facilitated diffusion Concept check: Apply Ouabain to either side of cell, what happens? Transcytosis • Endocytosis → vesicular transport → exocytosis Endocytosis • Moves large proteins intact • Examples: Examples: – Absorption of maternal antibodies from antibodies breast milk – Movement of proteins Movement across capillary endothelium endothelium Distribution of Solutes in Body Depends on • selective permeability of cell membrane • transport mechanisms available Water is in osmotic equilibrium (free movement Water across membranes) across Ions and most solutes are in chemical Ions disequilibrium (e.g., Na-K ATPase Pump) disequilibrium Electrical disequilibrium between ECF and Electrical ICF ICF Fig 5-33 Distribution of Solutes in Body Fluid Compartments Compare to Fig 5-33 Osmosis Compare to Fig. 5-29 Movement of water down its concentration gradient. Opposes movement Osmotic of water pressure across membrane Water moves freely in body until osmotic equilibrium is reached Molarity vs. Osmolarity In chemistry: • Mole / L • Avogadro’s # / Avogadro’s L In Physiology I mportant is not #of Important molecules / L but molecules #of particles / L: osmol/L or of OsM OsM Why? Osmolarity takes into account dissociation (solubility) of molecules in solution Osmolality = OsM/Kg of sol’n Convert Molarity to Osmolarity Osmolarity = # of particles / L of solution • 1 M glucose = 1 OsM glucose • 1 M NaCl = 2 OsM NaCl • 1 M MgCl2 = 3 OsM MgCl2 • Osmolarity of human body ~ 300 mOsM • Compare isosmotic, hyperosmotic, hyposmotic (p 156) Tonicity • Physiological term describing how cell Physiological volume changes if cell placed in the solution solution • Always comparative. Has no units. – Isotonic sol’n = No change in cell – Hypertonic sol’n = cell shrinks – Hypotonic = cell expands • Depends not just on osmolarity but on Depends nature of solutes and permeability of membrane membrane Penetrating vs. Nonpenetrating Solutes • Penetrating solute: can enter cell (glucose, Penetrating urea) urea) • Nonpenetrating solutes: cannot enter cell Nonpenetrating (sucrose, NaCl*) (sucrose, • Determine relative conc. of nonpenetrating Determine solutes in solution and in cell to determine tonicity. tonicity. – Water will move to dilute nonpenetrating solutes – Penetrating solutes will distribute to equilibrium Fig 5-30 Osmolarity and Tonicity Comparison A is isosmotic to B Compare to Fig 5-35 A is hypotonic to B IV Fluid Therapy 2 different purposes: – – Get fluid into dehydrated cells or Keep fluid in extra-cellular compartment Electrical Disequilibrium and Resting Membrane Potential (pp.156-163) will be covered at the beginning of Ch 8 Which of the following is a way for solutes in a aqueous solution to move from an area of high solute concentration to an area of low solute concentration? A. Facilitated diffusion B. Osmosis C. Active transport D. A and B E. None of these Which of the following defines the term specificity? A. movement of molecules by the use of vesicles B. the energy required to move molecules C. a group of carrier proteins operating at their maximum rate D. carrier transport of a group of closely related molecules E. none of these Water will always move from ___________ situations to _______ situations. A. Hyperosmotic, hyposmotic B. Hyposmotic, hyperosmotic C. Hyposmotic, isosmotic D. Hyperosmotic, isosmotic Which of the following pairs of molecular characteristics favors diffusion through the cell membrane? A. Large, polar B. Large, non-polar C. Small, polar D. Small, non-polar Which of the following is a way for solutes in a aqueous solution to move from an area of high solute concentration to an area of low solute concentration? A. Facilitated diffusion B. Osmosis C. Active transport D. A and B E. None of these ...
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