Chapter 3 Membrane Physiology Sections 3.1-3.3

3.1 Membrane Structure and Composition Plasma membrane Encloses the intracellular contents Selectively permits specific substances to enter or leave the cell Responds to changes in cell’s environment Trilaminar structure under electron microscopy 2

Cell 1 Plasma membranes Intercellular space Cell 2 FIGURE 3-1 Trilaminar appearance of a plasma membrane in an electron micrograph. Depicted are the plasma membranes of two adjacent cells. Note that each membrane appears as two dark layers separated by a light middle layer. Intercellular space Cell 2 Figure 3-1 p71

3.1 Membrane Structure and Composition The plasma membrane is a fluid lipid bilayer embedded with proteins. Phospholipids Most abundant membrane component Head contains charged phosphate group (hydrophilic) Two nonpolar fatty acid tails (hydrophobic) Assemble into lipid bilayer with hydrophobic tails in the center and hydrophilic heads in contact with water Fluid structure as phospholipids are not held together by chemical bonds 4

3.1 Membrane Structure and Composition 5

Head (negatively charged, polar, hydrophilic) Choline Head (negatively charged, polar, hydrophilic) Phosphate Glycerol Tails (uncharged, nonpolar, hydrophobic) Fatty acids FIGURE 3-2 Structure and organization of phospholipid molecules in a lipid bilayer. (a) Phospholipid molecule. (b) In water, phospholipid molecules organize themselves into a lipid bilayer with the polar heads interacting with the polar water molecules at each surface and the nonpolar tails all facing the interior of the bilayer. (c) An exaggerated view of the plasma membrane enclosing a cell, separating the ICF from the ECF. (a) Phospholipid molecule Figure 3-2a p71 6

(b) Organization of phospholipids into a bilayer in water ECF (water) Polar heads (hydrophilic) Nonpolar tails (hydrophobic) Lipid bilayer Polar heads (hydrophilic) FIGURE 3-2 Structure and organization of phospholipid molecules in a lipid bilayer. (a) Phospholipid molecule. (b) In water, phospholipid molecules organize themselves into a lipid bilayer with the polar heads interacting with the polar water molecules at each surface and the nonpolar tails all facing the interior of the bilayer. (c) An exaggerated view of the plasma membrane enclosing a cell, separating the ICF from the ECF. ICF (water) (b) Organization of phospholipids into a bilayer in water Figure 3-2b p71

(c) Separation of ECF and ICF by the lipid bilayer Intracellular fluid FIGURE 3-2 Structure and organization of phospholipid molecules in a lipid bilayer. (a) Phospholipid molecule. (b) In water, phospholipid molecules organize themselves into a lipid bilayer with the polar heads interacting with the polar water molecules at each surface and the nonpolar tails all facing the interior of the bilayer. (c) An exaggerated view of the plasma membrane enclosing a cell, separating the ICF from the ECF. Extracellular fluid (c) Separation of ECF and ICF by the lipid bilayer Figure 3-2c p71

3.1 Membrane Structure and Composition The plasma membrane is a fluid lipid bilayer embedded with proteins. Cholesterol Placed between phospholipids to prevent crystallization of fatty acid chains Helps stabilize phospholipids’ position Provides rigidity, especially in cold temperatures Cold-induced rigidity is countered in some poikilotherms by enriching membrane lipids with polyunsaturated fatty acids 9

3.1 Membrane Structure and Composition Membrane proteins Integral proteins are embedded in the lipid bilayer Have hydrophilic and hydrophobic regions Transmembrane proteins extend through the entire thickness of the membrane Peripheral proteins are found on inner or outer surface of membrane Polar molecules Anchored by weak chemical bonds to polar parts of integral proteins or phospholipids 10

3.1 Membrane Structure and Composition Two models of membrane structure Fluid mosaic model Membrane proteins float freely in a “sea” of lipids Membrane-skeleton fence model Mobility of membrane proteins is restricted by the cytoskeleton 11

3.1 Membrane Structure and Composition Specialized functions of membrane proteins Channels Carriers Receptors Docking-marker acceptors Enzymes Cell-adhesion molecules (CAMs) Self-identity markers 12

3.1 Membrane Structure and Composition Membrane carbohydrates Located only on outer surface of membrane Short-chain carbohydrates bound to membrane proteins (glycoproteins) or lipids (glycolipids) Important roles in self-recognition and cell-to-cell interactions 13

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Extracellular fluid Dark line Integral proteins Carbohydrate chain Appearance using an electron microscope Light space Phospholipid molecule Dark line Glycolipid Glycoprotein Receptor protein FIGURE 3-3 Fluid mosaic model of plasma membrane structure. The plasma membrane is composed of a lipid bilayer embedded with proteins. Integral proteins extend through the thickness of the membrane or are partially submerged in the membrane, and peripheral proteins are loosely à ached to the surface of the membrane. Short carbohydrate chains à ach to proteins or lipids on the outer surface only. Lipid bilayer Cholesterol molecule Leak channel protein Gated channel protein Peripheral proteins Cell adhesion molecule (linking microtubule to membrane) Carrier protein Microfilament of cytoskeleton lntracellular fluid Figure 3-3 p72 15

ANIMATION: Cell membranes To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

3.2 Unassisted Membrane Transport The plasma membrane is selectively permeable Permeability across the lipid bilayer depends on: High lipid solubility Small size Force is needed to produce the movement of particles across the membrane Passive forces do not require the cell to expend energy Active forces require cellular energy (ATP) 17

3.2 Unassisted Membrane Transport Diffusion Random collisions and intermingling of molecules as a result of their continuous, thermally induced random motion Net movement of molecules from an area of higher concentration to an area of lower concentration Equilibrium is reached when there is no concentration gradient and no net diffusion 18

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Diffusion from area A to area B FIGURE 3-4 Diffusion. (a) Diffusion down a concentration gradient. (b) Dynamic equilibrium, with no net diffusion occurring. Diffusion from area B to area A Net diffusion (a) Diffusion Figure 3-4a p75 20

Diffusion from area A to area B FIGURE 3-4 Diffusion. (a) Diffusion down a concentration gradient. (b) Dynamic equilibrium, with no net diffusion occurring. Diffusion from area A to area B Diffusion from area B to area A No net diffusion (b) Dynamic equilibrium Figure 3-4b p75 21

3.2 Unassisted Membrane Transport Fick’s law of diffusion The rate at which diffusion occurs depends on: Concentration gradient Permeability Surface area Molecular weight Distance Temperature 22

3.2 Unassisted Membrane Transport The movement of ions across the membrane is affected by their electrical charge. A difference in charge between two adjacent areas produces an electrical gradient. An electrical gradient passively induces ion movement -- conduction Only ions that can permeate the plasma membrane can conduct down this gradient. The simultaneous existence of an electrical gradient and concentration gradient is called an electrochemical gradient. 23

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Positively charged area Negatively charged area – – Cations (positively charged ions) attracted toward negative area Anions (negatively charged ions) attracted toward positive area FIGURE 3-6 Movement along an electrical gradient. Figure 3-6 p77 25

3.2 Unassisted Membrane Transport Osmosis Water moves across a membrane by osmosis, from an area of lower solute concentration to an area of higher solute concentration. Driving force is the water concentration gradient Hydrostatic pressure opposes osmosis Osmotic pressure is the pressure required to stop the osmotic flow Osmotic pressure is proportional to the concentration of nonpenetrating solute 26

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Membrane (permeable to H2O but impermeable to solute) Side 1 Side 2 Osmosis H2O Solute Pure water Lower H2O concentration, higher solute concentration H2O moves from side 1 to side 2 down its concentration gradient Solute unable to move from side 2 to side 1 down its concentration gradient Side 1 Side 2 Hydrostatic (fluid) pressure difference Original level of solutions FIGURE 3-8 Osmosis when pure water is separated from a solution containing a nonpenetrating solute. Osmosis Hydrostatic pressure • Water concentrations not equal • Solute concentrations not equal • Tendency for water to diffuse by osmosis into side 2 is exactly balanced by opposing tendency for hydrostatic pressure difference to push water into side 1 • Osmosis ceases; equilibrium exists Figure 3-8 p79

3.2 Unassisted Membrane Transport Colligative properties of solutes depend solely on the number of dissolved particles in a given volume of solution Osmotic pressure Elevation of boiling point Depression of freezing point Reduction of vapor pressure 29

3.2 Unassisted Membrane Transport Tonicity refers to the effect of solute concentration on cell volume Isotonic solution Same concentration of nonpenetrating solutes as in normal cells Cell volume remains constant Hypotonic solution Lower solute concentration than in normal cells Cell volume increases, perhaps to the point of lysis Hypertonic solution Higher solute concentration than in normal cells Cell volume decreases, causing crenation 30

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Intracellular fluid: 300 mOsm nonpenetrating solutes Normal cell volume Intracellular fluid: 300 mOsm nonpenetrating solutes H2O H2O FIGURE 3-9 Tonicity and osmotic water movement. 300 mOsm nonpenetrating solutes 200 mOsm nonpenetrating solutes 400 m Osm nonpenetrating solutes No net movement of water; no change in cell volume. Water diffuses into cells; cells swell. Water diffuses out of cells; cells shrink. (a) Isotonic conditions (b) Hypotonic conditions (c) Hypertonic conditions Figure 3-9 p80 32

3D ANIMATION: Osmosis 33

3.3 Assisted Membrane Transport Phospholipid bilayer is impermeable to: Large, poorly lipid-soluble molecules (proteins, glucose, and amino acids) Small, charged molecules (ions) Mechanisms for transporting these molecules into or out of the cell Channel transport Carrier-mediated transport Vesicular transport 34

3.3 Assisted Membrane Transport Channel transport Transmembrane proteins form narrow channels Highly selective Permit passage of ions or water (aquaporins) Gated channels can be open or closed Leak channels are open at all times Movement through channels is faster than carrier-mediated transport 35

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Outside cell Water molecule Aquaporin Cytosol Lipid bilayer membrane FIGURE 3-10 Aquaporins. (a) Structure of an aquaporin forming a channel in a membrane. The channel is lined with hydrophilic amino acids that attract water molecules in single file. (b) One of the Nobel Prize–winning experiments from Peter Agre’s laboratory. Incubation in hypotonic buffer fails to cause swelling of a control frog oocyte (leȀ ), due to low water permeability. In contrast, an oocyte injected with mammalian aquaporin RNA (right) exhibits high water permeability due to production of aquaporin proteins, and the cell has exploded. Cytosol Figure 3-10 p82 37

FIGURE 3-10 Aquaporins. (a) Structure of an aquaporin forming a channel in a membrane. The channel is lined with hydrophilic amino acids that attract water molecules in single file. (b) One of the Nobel Prize–winning experiments from Peter Agre’s laboratory. Incubation in hypotonic buffer fails to cause swelling of a control frog oocyte (leȀ ), due to low water permeability. In contrast, an oocyte injected with mammalian aquaporin RNA (right) exhibits high water permeability due to production of aquaporin proteins, and the cell has exploded. Figure 3-10 p82 38

3.3 Assisted Membrane Transport Carrier-mediated transport Transmembrane proteins that can undergo reversible changes in shape Binding sites can be exposed to either side of membrane Transport small water-soluble substances Facilitated diffusion or active transport 39

3.3 Assisted Membrane Transport Characteristics of carrier-mediated transport systems Specificity -- each carrier protein is specialized to transport a specific substance Saturation -- limit to the amount of a substance that a carrier can transport in a given time (transport maximum or Tm) Competition -- closely related compounds may compete for the same carrier 40

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concentration gradient (facilitated diffusion) Simple diffusion down concentration gradient Carrier-mediated transport down concentration gradient (facilitated diffusion) Rate of transport of molecule into cell FIGURE 3-12 Comparison of carrier-mediated transport and simple diffusion down a concentration gradient. With simple diffusion of a molecule down its concentration gradient, the rate of transport of the molecule into the cell is directly proportional to the extracellular concentration of the molecule. With carrier-mediated transport of a molecule down its concentration gradient, the rate of transport of the molecule into the cell is directly proportional to the extracellular concentration of the molecule until the carrier is saturated, at which time the rate of transport reaches the transport maximum (Tm). After Tm is reached, the rate of transport levels off despite further increases in the ECF concentration of the molecule. Low High Concentration of transported molecules in ECF Figure 3-12 p83

3.3 Assisted Membrane Transport Facilitated diffusion Passive carrier-mediated transport from high to low concentration Does not require energy Example: Glucose transport into cells 43

3.3 Assisted Membrane Transport Facilitated diffusion Molecule to be transported attaches on binding site on protein carrier Carrier protein changes conformation, exposing bound molecule to the other side of the membrane (lower concentration side) Bound molecule detaches from the carrier Carrier returns to its original conformation (binding site on higher concentration side) 44

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1 4 2 3 Carrier protein takes conformation in which solute binding site is exposed to region of higher concentration. 1 Concentration gradient ECF Solute molecule to be transported (High) Carrier protein Plasma membrane Binding site (Low) ICF Direction of transport Transported solute is released and carrier protein returns to conformation in step 1. 4 Solute molecule binds to carrier protein. 2 FIGURE 3-11 Model for facilitated diffusion, a passive form of carrier-mediated transport. Carrier protein changes conformation so that binding site is exposed to region of lower concentration. 3 Figure 3-11 p82

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ANIMATION: Passive transport To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

3.3 Assisted Membrane Transport Active transport Carrier-mediated transport that moves a substance against its concentration gradient Requires energy Primary active transport Energy is directly required ATP is split to power the transport process Secondary active transport ATP is not used directly Carrier uses energy stored in the form of an ion concentration gradient built by primary active transport 49

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1 5 2 4 3 Concentration gradient ECF Carrier protein splits ATP into ADP plus phosphate. Phosphate group binds to carrier, increasing affinity of its binding site for ion. 1 Plasma membrane Binding site Carrier protein ICF Ion to be transported When binding site is free, carrier reverts to its original shape. 5 Ion to be transported binds to carrier on low-concentration side. 2 FIGURE 3-13 Model for active transport. The energy of ATP is required in the phosphorylation–dephosphorylation cycle of the carrier to transport the molecule uphill from a region of low concentration to a region of high concentration. Carrier releases ion to side of higher concentration. Phosphate group is also released. 4 In response to ion binding, carrier changes conformation so that binding site is exposed to opposite side of membrane.The change in shape also reduces affinity of site for ion. 3 Direction of transport Figure 3-13 p85

ANIMATION: Active Transport To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

3.3 Assisted Membrane Transport Na+-K+ ATPase pump Pumps 3 Na+ out of cell for every 2 K+ in Splits ATP for energy Phosphorylation induces change in shape of transport protein Maintains Na+ and K+ concentration gradients across the plasma membrane Helps regulate cell volume 53

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3.3 Assisted Membrane Transport Secondary active transport Simultaneous transport of a nutrient molecule and an ion across the plasma membrane by a cotransport protein Nutrient molecule is transported against its concentration gradient Driven by simultaneous transport of an ion along its concentration gradient Example: Cotransport of glucose and Na+ across the luminal membrane of intestinal epithelial cells 55

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1 2 3 Lumen of intestine Na+ high Glucose low Na+ high Glucose high Na+ low Na+ low Glucose high Na- high Epithelial cell lining small intestine Glucose low Na+ low Glucose high FIGURE 3-15 Symport of glucose. Glucose is transported across intestinal and kidney cells against its concentration gradient by means of secondary active transport mediated by the sodium and glucose cotransporter (SGLT) at the cells’ luminal membrane. Na+–K+ pump (active) GLUT (passive) Na- high Glucose low Blood vessel Primary Active Transport establishes Na+ concentration gradient from lumen to cell, which drives Secondary Active Transport creating glucose concentration gradient from cell to blood used for Facilitated Diffusion Na+–K+ pump uses energy to drive Na+ uphill out of cell. 1 SGLT uses Na+ concentration gradient to simultaneously move Na+ downhill and glucose uphill from lumen into cell. 2 GLUT passively moves glucose downhill out of cell into blood. 3 Figure 3-15 p88

3.3 Assisted Membrane Transport Vesicular transport Transport between ICF and ECF of large particles wrapped in membrane-bound vesicles Endocytosis -- incorporates outside substances into cell Exocytosis -- releases substances into the ECF The rate of endocytosis and exocytosis must be balanced to maintain a constant membrane surface area and cell volume Caveolae may play a role in transport of substances and cell signaling 58