Mechanisms of Membrane Transport II PDF (Keele University, 2022-23)

Mechanisms of membrane transport II PHA10014 Dr David Morgan It’s the Keele difference. 1 Learning outcomes of the lecture To appreciate the principles of active transport To appreciate the principles of coupled transpor...

Mechanisms of membrane transport II PHA10014 Dr David Morgan It’s the Keele difference. 1 Learning outcomes of the lecture To appreciate the principles of active transport To appreciate the principles of coupled transport To understand the mechanism of action potential and electrochemical coupling To understand the basic principles of neurotransmitters 2 3 Suggested reading Essential Cell Biology, 4th Ed. Chapter 12 (pp. 383-418) Alberts et al. Molecular Biology of The Cell, 6th Ed. Chapter 11 (pp. 597-640) 4 Main points of Active Transport Cells exchange molecules with the environment to live and grow Plasma membrane: barrier; interior hydrophobic ➔ blocks passage of water-soluble substances Specialized membrane transport proteins span lipid bilayer Main types: - transporters – Use energy to move specific substrate(s) across membrane (usually up concentration gradient - channels – Allow passive transport of specific substrates across membrane (down concentration gradient 5 Phospholipid Bilayers Have Low Permeability to Solutes and Ions (i) the smaller the molecule and (ii) the fewer its favourable interactions with water: → the more rapidly it diffuses across the lipid bilayer 6 Cell Membrane Permeability Higher than that in Artificial Membranes 7 Transporter complement defines solutes Combined action of all different transporters in the membrane surrounding a compartment defines the solutes inside that compartment 8 Active transport – moving solutes up the concentration gradient concentration gradient 9 Active transport via ATPase pumps It’s the Keele difference. 10 Na+K+ATPase moves Na+ and K+ ions against their concentration gradients Energy derived from hydrolysis of ATP to ADP (Metabolism – cycle 2) P-type ATPase ubiquitous to all cells of the body Essential for maintenance of osmotic balance and resting potential Drives coupled-transport of many solutes (next section) 11 Na+ pump uses ATP to expel Na+ and bring in K+ 12 Na+K+ pump cycle 13 Ion concentrations inside the cell are different from those outside 14 Other important ATPase membrane transporters Calcium ions (Ca2+) are important intracellular signalling molecules – Normally sequestered in intracellular stores by Ca2+ATPase An H+ ATPase is responsible for secretion of HCl into the lumen of the stomach – Critical for the digestion of food 15 NA+K+ATPase CREATES GRADIENTS WHICH CAN DO WORK FOR THE CELL 16 Coupled Transport via Symports and Antiports The energy used by the Na+K+-ATPase is captured in Na+ and K+ gradients The dissipation of these gradients can be used to drive other solutes up their concentration gradients As Na+ flows into a cell it can be used to drive other solutes in or out of the cell This can only happen when Na+ flows into cells via specific transporter proteins (i.e. not ion-channels) Symports carry solutes in the same direction as Na+ Antiports carry solutes in the other direction 17 Active Glucose Transport via Sodium-Glucose Transporters Two Sodium-glucose symports exist which carry glucose into cells SGLT-1 in the intestinal mucosa → glucose absorption SGLT-2 in the kidney (proximal convoluted tubule) → glucose re-absorption 18 Glucose-Na+ symport protein 19  Other Important Co-transporters Serotonin reuptake transporter (SERT) – Serotonin=important neurotransmitter in brain – Uses a Na+ gradient to drive serotonin uptake from synaptic clefts – Inhibited by Serotonin-Specific Reuptake Inhibitors (SSRIs) such as Prozac (Fluoxetine) Na+I+ cotransporter – Drives iodine uptake in the thyroid gland – Important for thyroid hormone production  Other Important Exchangers Na+Ca++exchanger – Ca++ influx into the cytoplasm of cardiac muscle cells drives contraction – This protein is important for removal of Ca++ ions from after muscle contraction Cl-HCO3- cotransporter – Important for exchange of CO2 from blood to the exhaled air in the lungs – Important for thyroid hormone production 22 NA+K+ATPase, K+LEAK CHANNELS AND THE CELL’S RESTING POTENTIAL 23 Ion concentrations inside the cell are different from those outside 24 Concentration gradient and electrochemical gradient combine to form “Resting Potential” A difference in the concentration of ions inside & outside cell → Charge difference across membrane → membrane potential Most cells maintain a potential difference between -20 and -200mV (interior more negative than outside). How? NaK-Pump creates K+ concentration gradient between inside and outside of cell K+ ions can flow down this via passive K-leak channels Cells contain fixed anions – negatively charged organic molecules confined within the cell Creates electrochemical gradient that wants to retain cations (e.g. K + ions) When electrochemical force holding K+ ions in matches concentration gradient pushing K+ ions out there is no net movement of K+ ions This happens at a charge difference across the membrane (“membrane potential”) of ~70 mV Both concentration gradient and membrane potential influence passive transport 26 Na Na + Na + Na + + Na + K+ K+ K+ Na K+ Na + K+ + K+ Na + Na K+ + K+ Na + K+ K+ Na Na+K+ ATPase Na + + Na Na Na + + + K+ Na Na + + K+ K+ Na K+ Na + + Na Na K+ + + K+ K+ K+ K+ Na Na Na K+ + Na + + Na + + 27 Na Na Na + Na + Na Na Na + + Na + + + Na + + Na + Na Na + Na Na + + + Na K+ Na + K+ Na + Na + Na + + Na Na Na + + K+ Leak Channel + Na Na + + Na + K+ K+ K+ K+ K+ Na K+ + K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ K+ 28 Na Na Na Na Na Na + + Na + Na + + + + + Na + Na + Na Na + Na Na K+ + + + Na Na K+ Na + Na + + Na + + Na Na Na + + + Na + + + ++ + + Na + ++ + + + + + + + + + - - - - - - - - -- - - --- - - - - - -- Na +Na + K+ K+ K+ K+ K+ Na K+ +Na K+ Organi K+ K+ + c K+ Organi Anions c K+ K+ K+ K+ Anions K+ K+ K+ K+ 29 Nernst equation 30 Why is the resting potential important? The cell’s ability to fire an action potential relies on the existence of a resting potential (Cycle 2). – The basic signalling properties of neurons are determined by changes in the resting potential. Changes in membrane potential trigger intracellular functions (e.g. secretion of hormones from endocrine cells)