Fundamentals of Human Physiology Lecture Notes Spring 2021 PDF

Fundamentals of Human Physiology Spring 2021 Session 1 – Cell Physiology Physiology of the Cell Membrane Patricio E. Mujica, Ph.D. Department of Natural Sciences, School of Health and Natural Sciences–Mercy College Department of Pharmacology, Physiology and Neuroscience, Rutgers–NJMS pmujicaurzua@me...

Fundamentals of Human Physiology Spring 2021 Session 1 – Cell Physiology Physiology of the Cell Membrane Patricio E. Mujica, Ph.D. Department of Natural Sciences, School of Health and Natural Sciences–Mercy College Department of Pharmacology, Physiology and Neuroscience, Rutgers–NJMS [email protected] Cells are the smallest functional units capable of carrying out all of life’s processes Cells are surrounded by the plasma membrane that separates the cell’s internal and external environment The internal (intracellular) environment: cytoplasm – – Cytosol, a colloidal (gel-like) matrix of water and proteins Organelles, carry out specific functions necessary for normal cellular/organ function The external (extracellular) environment – access to nutrients, gases and molecules necessary for cell survival, growth, and reproduction Cells and the Fluid Compartments Body Fluid Compartments Extracellular fluid, ECF (outside the cells) Blood Interstitial fluid plasma surrounds is the extracellular most cells. fluid inside blood vessels. The intracellular and extracellular compartments are the fluid compartments of the body The intracellular compartment accounts for 2/3 of the total body fluids The extracellular fluid compartment is 1/3 of the total body fluid compartments Cells (intracellular fluid, ICF) Fat cell: 50–150 µm Ovum: 100 µm – further subdivided into interstitial fluid (the fluid that bathes the cells) the plasma (of the blood) Red blood cell: 7.5 µm White blood cell: 15 µm Smooth muscle cell: 15–200 µm long Cells subdivide into intracellular compartments The chemical composition and the physical state of these fluid compartments is not identical Darrow-Yannet Diagram Explains changes in the body fluid compartments (i.e. osmolarity and volume) with excessive salt intake, sweating, dehydration, etc. The cell membrane is a semi-permeable lipid bilayer with many functions The cell (plasma) membrane functions to: 1. Physically separate the body fluid compartments 2. Regulate the exchange of nutrients, ions and hormones with its environment 3. Communicate between the cells and it’s external environment 4. Structurally support the cell (in conjunction with many cytoskeletal proteins) Structure of the Cell Membrane Membrane proteins are critical to the transport of nutrients, ions and molecules Two main classes of membrane proteins – Peripheral Proteins: temporarily associated with lipid bilayer or with integral membrane proteins – Integral proteins: permanently bound to the lipid bilayer These proteins – Provide mechanical linkage for other proteins on either side of the membranes (i.e. cytoskeletal proteins, extracellular matrix proteins) – Mediate the selective movement of ions and small molecules from one side of the membrane to the other: transport proteins Transport proteins come in two main forms: carrier proteins and channel proteins – Sense a ligand on one side of the membrane and transmit a signal to the other side (i.e. membrane receptor proteins) Membrane receptor proteins serve as connection between the cell's internal and external environments. Structure and Function of Membrane Proteins MEMBRANE PROTEINS can be categorized according to Structure Integral proteins Function Peripheral proteins Membrane transporters Structural proteins Membrane enzymes activate are found in Carrier proteins Channel proteins change conformation form Open channels Mechanically gated channel Membrane receptors are active in are active in Cell junctions Gated channels Voltage-gated channel Receptormediated endocytosis Cytoskeleton Metabolism Chemically gated channel Signal transfer open and close Figure 5.8 Map of membrane proteins Membrane Transport Proteins Types of Transport Proteins Ion Channels (Passive diffusion) Mechanically-gated (Mechanosensitive) Voltage-gated Chemically-gated (Chemosensitive/ Ligand-gated) Transporters/ Carrier Proteins Antiporters (Primary and Secondary Active Transport/ Ion exchanger) Symporters (Secondary Active Transport) Uniporters (Primary Active Transport/ Facilitated Diffusion) Ion Channels and the Cell Membrane Ion channels – Membrane-spanning proteins – Highly selective allow passage of specific ions and repel other ions – Control the flow of ion movement (106–107 ions/sec) Mediate the generation of electrical currents across the membrane – Named according to the ion that passes through them the agent or mechanism that causes them to open Mechanically-gated channels: open when the membrane that surrounds them is stretched  important in somatosensation Voltage-gated channels: open in response to a change in the membrane (electrical) potential Chemically (or ligand)-gated channels: open when they bind specific molecules: neurotransmitters or hormones on their extracellular surface intracellular messengers on their intracellular region Functional Architecture of the Voltage-Gated Ion Channel Mechanically-gated and Voltage-gated channels Mechanically-gated channels open when physical forces cause the channel gates to open – Ion movement leads to changes in the membrane potential Are nonselective cation channels – allow a variety of large cations, small metabolites and electrolytes to pass through Diverse class of structurally unrelated channels that have a variety of functions Important for sensory perception – Somatosensation: touch – Hearing – Proprioception: joint movement and position, muscle force and stretch – Monitoring blood pressure Voltage-gated (VG) ion channels are stimulated to open by changes in the electrical state of the cell VG Na+ channels – Responsible for the upstroke (depolarization) of the action potential VG K+ channels – Responsible for the bringing the cell to its resting membrane potential and the maintenance of that resting value VG Ca2+ channels – Increase the intracellular calcium levels necessary for muscle contraction and for calcium signaling VG Cl– channels – Are poorly understood, but are known to have a role in kidney regulation Chemically-gated ion channels generate electrical signals in response to specific chemicals Chemically (aka ligand)-gated channels – Control the flow of ions and generate electrical signals from the binding of a hormone or neurotransmitter to the channels – There are many types of ligand-gated channels Nicotinic acetylcholine (Ach) receptor (nAChR) channels Postsynaptic receptors channels in CNS: Glycine, GABA and serotonin channels ATP-sensitive (purinergic) or P2X receptors Inositol triphosphate (IP3) and ryanodine receptors Pharmacological agonists and antagonists can activate or block the ligand gated channels, respectively – Agonists: cause the channel to open, produce ion movement and electrical activity in the cell – Antagonists: block the opening of the channel and the production of electrical activity Carrier Proteins / Membrane Transporters Carrier Proteins/Transporters: – move ions and polar organic molecules (glucose, amino acids, hormones, etc.) across the membranes, – Can be divided into three categories: Uniporters Cotransporters – Symporters and – Antiporters (exchangers) Figure 5.10 ESSENTIALS – Membrane Transporters Uniporters Uniporters facilitate the diffusion of one molecule across the membrane The molecule concentration is important to its transport – Ex: glucose transporters (GLUT) facilitate the movement of glucose along its concentration gradient Symporters – Symporters (cotransporters): transport two or more molecules across the plasma membrane in the same direction – Examples: Na/glucose cotransporter (SGLT): transports glucose from the lumen into kidney and intestine epithelia Na-K-2Cl Transporter (NKCC): transports sodium, potassium, and chloride into the renal tubule – the diuretic medication Lasix blocks this transporter Table 5.8 Examples of Secondary Active Transporters Antiporters Antiporters (exchangers) transport two or more molecules across the membrane in opposite directions – Sodium-Potassium ATPase: an important regulator of intracellular sodium and potassium concentrations (will be discussed in detail with primary active transport) – Sodium-calcium exchanger (NCX): an important regulator of intracellular calcium concentration reversible transporter that is important to cardiac physiology – Cl/HCO3– exchanger (anion exchanger) helps maintains acid-base balance Membrane Transport Processes Types of Membrane Transport Simple Diffusion (No carrier protein/channel necessary) Passive Diffusion (Ion channels) Facilitated Diffusion (Uniporters) Primary Active Transport (Pumps) Secondary Active Transport (Symporters and Antiporters) Osmosis (Aquaporins) Vesicle-mediated Transport Epithelial Transport Membrane Transport Processes Selective Permeability of the Plasma Membrane The plasma membrane is selectively permeable because it permits some molecules to pass through (lipophilic) and others are excluded from passing through (hydrophilic) If a substance can cross the membrane (by itself), it is considered permeable – These substances are able to freely diffuse across the membrane – Two properties influence membrane permeability: lipid solubility and molecular size – Stokes-Einstein Equation (Diffusion coefficient): describes the relationship between molecular radius and the diffusion coefficient – The Permeability (P) equation describes the relationship between the diffusion coefficient and the partition coefficient If a substance is unable to cross the membrane, it is an impermeable molecule Many types of membrane transport can occur within the cells of the body to transport the impermeable molecules into the cell – Ions and small molecules are transported rapidly by processes that do not require energy (passive transport) or by processes that require energy (active transport) – Exocytosis and endocytosis allow the transport of large molecules into and out of the cell Membrane Permeability of Various Substances Membrane permeability is high for lipid-soluble molecules and low for ions and polar molecules Impermeable polar molecules and ions require the aid of membrane transport proteins in order to cross the membrane Simple Diffusion and Fick’s Law of Diffusion Simple diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration Allows the passage of small uncharged (nonpolar) molecules (CO2, O2, NH3, NO, etc.) directly through the membrane Simple diffusion requires no external energy (ATP) for the permeable molecules to cross the membrane  it’s given by the random thermal motion of particles in solution, which depends on the kinetic energy of the particles themselves Extracellular fluid Membrane surface area Lipid solubility Molecular size Concentration outside cell Concentration gradient Composition of lipid layer Intracellular fluid Concentration inside cell Factors affecting rate of diffusion through a cell membrane: Lipid solubility Molecular size Concentration gradient Membrane surface area Composition of lipid layer Simple Diffusion and Fick’s Law of Diffusion (J) (A) (ΔC) (P) Fick’s Law of Diffusion: – the diffusion rate, or flux (J) of a particular substance, is directly proportional to: the surface area available for diffusion (A) the concentration gradient (ΔC) of the substance and the membrane’s permeability (P) to the substance – describes the net movement of these uncharged molecules through the membrane J = P A (CA–CB) = P A (ΔC) J = net diffusion of a solute A = total area available for diffusion P = permeability coefficient Cx = concentration of molecules on side/compartment x Selective Permeability of the Plasma Membrane The Stokes-Einstein Equation describes the inverse relationship between molecular radius and the diffusion coefficient (D) D (diffusion coefficient) = kBT 6πrη kB = Boltzmann’s Constant T = absolute temperature r = molecular radius η = viscosity of the medium – ↑radius ↓diffusion coefficient – ↓radius ↑diffusion coefficient – Large proteins (i.e. albumin) have a low membrane permeability The Permeability (P) equation describes the relationship between the diffusion coefficient and the partition coefficient on the permeability of the plasma membrane P (Permeability coefficient) = KD Δx K = Partition Coefficient D = Diffusion coefficient Δx = membrane thickness Factors that can affect Simple Diffusion Table 5.6 Rules for Diffusion of Uncharged Molecules The magnitude of the concentration gradient (ΔC) – The greater the difference in concentration, the faster the rate of diffusion The permeability (P) of the membrane to a substance – More permeability (of a substance) = more diffusion – If a substance is impermeable = no diffusion The surface area (A) of the membrane across which diffusion is taking place – More surface area = greater diffusion The membrane thickness (Δx) through which the diffusion must take place – The greater the distance, the slower rate of diffusion Facilitated Diffusion Facilitated diffusion uses carrier proteins to transport substances that are impermeable and unable to cross the plasma membrane – Carrier proteins (Uniporters) facilitate solute movement by binding molecules on one side of the membrane (i.e. extracellular side) and releasing the molecule on the other side (i.e. intracellular side) – At high concentrations the carrier becomes saturated, reaching their transport maximum (Vmax), despite increases in concentration Facilitated Diffusion Facilitated diffusion uses carrier proteins to transport substances that are impermeable and unable to cross the plasma membrane Active Transport Mechanisms Active transport is a form of carrier-mediated transport that requires energy expenditure to transfer substances “uphill” against their concentration gradient Two types of active transport can occur within a cell – Primary active transport: moves substances against their concentration gradients at the direct expense of ATP consumption – Secondary active transport: use the gradient from primary active transport (i.e. Na-K ATPase) to move another molecule against its gradient Does not require additional energy, it depends on the metabolic energy of the primary active transport mechanism Antiporters and symporters couple secondary active transport in the cells with the movement of sodium or hydrogen ions with their gradients, while the movement of other molecules are against their gradients Primary Active Transport Primary active transporters can be uniporters or antiporters These transporters are also called as pumps and are referred to as ATPases Create and maintain ionic gradients, moving ions against the gradient at the expense of ATP Can be classified as P-type, Ftype and V-type ion pumps, depending on their mechanism of activation Table 5.7 Primary Active Transporters Primary Active Transport: Na-K ATPase 3 Na+ molecules are pumped out of the cell and 2 K+ are pumped in at the expense of one ATP molecule Normal Ion Concentrations extracellular (interstitial fluid) [Na+] 145 mM intracellular [Na+] 12 mM [K+] 4.5 mM [K+] 155mM [Ca+] 1.8 mM [Ca+] 0.1µM [Cl-] 132 mM [Cl-] 4 mM Because more sodium ions move out, this pump is electrogenic (generates a small outward current) Figure 5.14 The sodium-potassium pump, Na+-K+-ATPase Digitalis and ouabain are drugs which block the Na-K Pump activity The Na-K ATPase is a P-Type ATPase Pump P-type pumps have two conformational states and are phosphorylated during the reaction cycle There are three important classes of P-type ATPases Na-K Pump: the most abundant pump in eukaryotic cells Calcium Pumps: – SarcoplasmicEndoplasmic Reticulum Ca-ATPase (SERCA) pump – Plasma membrane Calcium ATPase (PMCA) pump H-K Pump Calcium Pumps: SERCA and PMCA SR Calcium ATPase (SERCA) Pump Plasma membrane calcium pump – Pumps calcium from the cytoplasm (PMCA) into the sarcoplasmic reticulum – Pumps calcium from the cytoplasm – Plays a critical role in the re-uptake of into the extracellular space calcium after cardiac muscle – Is predominantly found in noncontraction excitable cells (liver cells, etc.) F-type ATPases: the F0-F1 H+ ATPase V-type ATPases are proton pumps V-ATPases couple ATP hydrolysis to pumping of protons V-ATPases acidify the endosomal network, lysosomes and the Golgi network. Also present in plasma membrane in acid-secreting cells Osteoclasts intercalated cells of the distal tubule (kidney) Bladder Nucleus Lysosome ER Golgi Endosomal Network Multi-drug Resistance and ABC Transporters ATP-binding cassette (ABC) transporters – Consume ATP while transporting molecules across the membranes – Are involved in tumor resistance, cystic fibrosis, bacterial multidrug resistance, and a range of other inherited human diseases – The multidrug resistance(MDR) pump Is highly expressed in bacteria and cancer cells Increases the excretion of a drug a patient is taking, resulting is a loss of potency of the medication Many body molecules and drugs (yellow balls) encounter multidrug-resistance pumps (blue) after passing through a cell membrane © LINDA S. NYE Secondary Active Transport: SGLT Transporter Na+ binds to carrier. Intracellular fluid Lumen of intestine or kidney Na+ SGLT protein [Na+] high Glu [Na+] low [glucose] high Na+ binding creates a high-affinity site for glucose. Na+ Glu Lumen Glucose binding changes carrier conformation so that binding sites now face the ICF. ICF Na+ Gl u Secondary active transport: uses the gradient from primary active transport (i.e. Na-K ATPase) to move another molecule against its gradient The SGLT is a symporter that uses the energy gradient from the Na-K ATPase to move sodium and glucose into the cell [glucose] low Lumen Na+ is released into cytosol, where [Na+] is low. Release changes glucose-binding site to low affinity. Glucose is released. ICF Na+ Glu [Na+] low [glucose] high Lumen ICF Other Secondary Active Transporters Aquaporins and Osmosis Water can move rapidly into and out of the cells, but has a low partition coefficient, which results in a low permeability to water Because of the low permeability many cells produce aquaporins (AQPs) or water channels to allow water to cross the membrane much faster Anti-diuretic hormone (ADH) in the kidneys insert AQPs to increase water flow from the tubular fluid (forming urine) back into the blood, resulting in water retention This passive movement of water is known as osmotic flow Osmosis Osmosis is the movement of water through a selectively permeable membrane, from an area of high H2O concentration (low solute concentration) to an area of low H2O concentration (high solute concentration) Osmolarity is the number of osmotically active particles in the solution (Units: Osmoles/Liter – Osm/L) Osmolarity = gC g = number of particles per mole in solution C = molar concentration Calculate the osmolarity for the following solutions: (A) 1 mM Glucose solution Osmolarity = g C = 1 1 mM = 1 mOsm/L Glucose (B) 1mM NaCl solution Osmolarity = g C = 2 1 mM = 2 mOsm/L NaCl Osmosis and Osmotic Pressure Osmotic pressure is the mechanical pressure needed to produce a flow of water equal and opposite to the osmotic flow produced by a water concentration gradient – i.e. the pressure needed to “neutralize” the natural osmotic movement of water Osmotic pressure depends on: – the concentration of osmotically active particles – whether or not the solutes can cross the membrane Van’t Hoff Equation and Osmotic Pressure The van’t Hoff Equation is used to estimate the osmotic pressure (π) between two solutions π = σgCRT g = number of particles/mol in solution C = concentration (mol/L, M or mM) R = Ideal gas constant (0.082 L-atm/mol-K) T = absolute temperature (°K) σ = reflection coefficient When σ = 1.0, the membrane is impermeable to the solute, σ is 1.0 – – When σ = 0, the membrane is freely permeable to the solute, σ is 0, and the solute will diffuse across the membrane down its concentration gradient until the solute concentrations of the two solutions are equal – – the effective osmotic pressure will be maximal and will cause maximal water flow. Examples: albumin In this case there is no driving force for osmotic water flow Example: Urea (or nearly 0) When σ = a value between 0 and 1, the solutes are neither impermeable (σ = 1) nor freely permeable (σ = 0) across membranes – when σ is between 0 and 1, the calculated effective osmotic pressure will be less than its maximal possible value, but greater than zero Osmolarity, Osmolality and Tonicity Osmolarity is the number of moles of solute per liter of solution Osmolality is the number of moles of solute per kg of solvent – Is determined by the total concentrations of all the solutes present in the solution – Can be measured by the changes it produces in the freezing point Tonicity a measure of the osmotic pressure gradient between two solutions separated by a semipermeable membrane Refers to the response of cells to the extracellular solution in which they are immersed Hypertonic solutions have a greater osmolarity than the cytosol of the cell Isotonic solutions have the same osmolarity as the cytosol of the cell Hypotonic solutions have lower osmolarity than the cytosol of the cell Tonicity and cell volume Comparing Osmolarities between Different Solutions The Relationship between Osmolarity and Tonicity OSMOLARITY TONICITY Hypotonic Isotonic Hypertonic Hyposmotic Isosmotic Hyperosmotic