Transmembrane Transport of Ions & Small Molecules

Transmembrane Transport of Ions & Small Molecules  Biomembranes are Selectively Permeable Lipid bilayer structure results in selective permeability: – Meaning only certain types of substances can move through Figure 11-1 Mechanisms of Transmembrane...

Transmembrane Transport of Ions & Small Molecules  Biomembranes are Selectively Permeable Lipid bilayer structure results in selective permeability: – Meaning only certain types of substances can move through Figure 11-1 Mechanisms of Transmembrane Transport Simple (Passive) Diffusion Facilitated Diffusion Primary Active Transport Secondary Active Transport (Cotransport)  Simple Diffusion Diffusion refers to the movement of permeable substances across a membrane, from a region of higher concentration to one of lesser concentration. There is no expenditure of energy. Diffusion occurs spontaneously because entropy increases as a substance moves from high conc. to low conc. – ΔG assumes a negative value since the products have more entropy and less free energy ΔG = Gproducts - Greactants ΔG = ΔH – TΔS H-enthalpy S-entropy What Determines Diffusion Rate Across a Membrane? Concentration Size of the gradient across diffusing particles membrane Electric potential across membrane Hydrophobicity (for charged particles)  Hydrophobicity Hydrophobicity measures the degree to which a substance is hydrophobic. – measured by the substance’s partition coefficient (K) – K is the equilibrium constant for it’s partition between oil & water = [in oil] / [in H2O] – the higher the K for a substance, the more lipid-soluble it is and the faster it’s rate of movement across a bilayer. What influences a substance’s hydrophobicity? – proportion of uncharged regions in a substance Example: a long chain fatty acid is more hydrophobic than a shorter chained one Which are More Hydrophobic? OR OR  How do Impermeable Substances Move Through Membranes? Require transport proteins: pumps, channels and transporters These protein-mediated processes may or may not need energy Multiple transport proteins may function together (fig 11-3)  Transport Protein Mediated Processes Facilitated Diffusion – substance moves from region of higher conc to lower conc with no expenditure of energy – typically involves use of channel proteins or uniport transporters – E.g., glucose can enter cells via facilitated diffusion Primary Active Transport – substance moves from a region of lower to one of higher concentration with an expenditure of energy – requires use of proteins called pumps, which are ATPases Secondary Active Transport (Cotransport) – energetically unfavorable transport of a target substance against its conc gradient is coupled to the energetically favorable movement of an ion down its electrochemical gradient – can be symport or antiport cotransport  Membrane Transport Proteins #1: ATP powered pumps primary active transport substance moves from a region of lower to one of higher concentration expenditure of energy requires use of proteins called pumps, which are ATPases Figure 11-3a  Membrane Transport Proteins #2: Ion channels these proteins transport ions, water and small hydrophilic molecules Channels may be gated or non- gated (ungated): Non-gated channels are always open, as such they are often called “leaky” channels. Gated channels are closed until opened by a stimulus Figure 11-3b Membrane Transport Proteins #3: Transporters Figure 11-3c  Comparing Simple Diffusion & Uniport Transport Uniport transport, a form of facilitated diffusion, involves the transport of small hydrophilic molecules through a membrane. Uniport transport differs from simple diffusion: – Rate of uniport transport is higher – K is irrelevant (molecule never enters the hydrophobic core of the phospholipid bilayer) – There is a maximal Vmax – Transport is reversible (direction changes depends on the concentration gradient – Transport is specific  Glucose Uniport Transporters (GLUT) Humans encode 12 glucose uniporters (GLUT 1-12) they are isoforms and are tissue-specific – Eg) GLUT 1 is expressed in erythrocytes and GLUT2 in liver cells Osmotic pressure causes H2O to move across membranes Osmotic pressure Is the hydrostatic pressure required to stop the net flow of water across membrane separating solutions of different water concentrations Figure 11-7: Aquaporin expressing frog oocytes burst in hypotonic solution Oocytes at the top microinjected with mRNA encoding aquaporin Aquaporins increase the water permeability of cell membranes Figure 11-8: Structure of Aquaporin 4 Classes of ATP Powered Pumps Figure 11-10: Model of Ca2+ ATPase Structure of Catalytic a-subunit of Muscle ATPase Figure 11-12: Na+/K+ ATPase in PM Effects of V-class H+ pumps on H+ conc. & electric potential gradients V-class pump only V-class pump w/ Cl- channel  ATP-powered ion pumps generate & maintain ion gradients across membranes  ABC Superfamily About 50 ABC superfamily pumps are known in mammals They transport lipids, sugars, and other molecules. E.g, MDR1 and CFTR – MDR1 transports lipid-soluble substances out of cells – CFTR transport Cl- Also found in bacteria – import nutrients from environment  Flippase Mechanism Used by ABC superfamily pumps that transport lipid soluble substances  Gated & Ungated Channels Gated channels are closed until opened by a stimulus: – Chemical (ligand) gated – Voltage gated Non-gated (ungated) channels are always open, as such they are often called “leaky” channels. – help to maintain a membrane resting potential – Major nongated channels are: Na+ K+ Cl- Ca2+ – K+ is the most important ion in maintaining the value of the membrane potential.  Figure 11-18: Generation of a transmembrane electric potential (voltage) depends on selective movement of ions across semi-permeable membrane Structure of Resting Bacterial K+ Channel Figure 11-25  Cotransport Examples include: – Na+/H+ antiporter – Cl-/HCO3- antiporter – 2Na+/glucose symporter – 3Na+/Ca+2 antiporter Figure 11-26, 8th edition Figure 11-26: 2 Na+/1 Leucine Symporter  Co-transporters and pH The activity of membrane transport proteins that regulate the cytosolic pH of mammalian cells changes with pH. For example, the Na+/H+ and Na+HCO3-/Cl- antiporters that act to increase cytosolic pH, are activated when cytosolic pH decreases. In contrast, the Cl-/HCO3- antiporter is activated at high pH, and it leads to a reduction of cytosolic pH.  Transport Across Epithelial Membranes: Transcellular Glucose Transport from Small Intestine Lumen The Na+/K+ ATPase inside the basolateral surface membrane generates Na+ and K+ conc gradients. The outward movement of K+ ions through nongated K+ channels generates an inside- negative membrane potential across the plasma membrane. The Na+ conc gradient and the membrane potential are used to drive the uptake of glucose from the intestinal lumen by the 2 Na+/1 glucose symporter located on the apical surface membrane  Rehydration Therapy States of intense dehydration caused by disease, chemotherapy, or physical stress can be treated by the ingestion of water, sugar and salt to stimulate the Na+/glucose symporter. Why? This principle is the basis for the ingredients of Gatorade and similar drinks. Transport Across Epithelial Membranes: Acidification of Stomach Lumen by Parietal Cells  Proton Pump Inhibitors Your stomach produces acid to help break down food so it is easier to digest. – In certain circumstances, this acid can irritate the lining of your stomach and duodenum causing indigestion and even ulceration and bleeding. The proton pump inhibitors work by completely blocking the production of stomach acid. They do this by inhibiting (shutting down) a system in the stomach known as the proton pump. full name for this system is 'hydrogen-potassium adenosine triphosphate enzyme system'.  Ouabain, A Cardiac Glycoside Endogenous ouabain is found in mammals – acts as a steroid hormone – may be implicated in hypertension medicinal value is as a heart stimulant. Mechanism: Ouabain blocks the Na+/K+ pump of muscle. – Consequently: Na+ gradient is destroyed, cytosolic Ca2+ levels remain high, muscle contracts more strongly – Ouabain directly causes cytosolic Na+ levels to increase, Na+/Ca2+ antiporter does not function as efficiently, cytosolic Ca2+ levels increase