Transport across the cell membrane - PDF

Transport across the cell membrane; osmosis, diffusion, passive and active transport Spring 2025 Dr. Maximilian Lyon MWF, 12:00-12:50 pm ISE 211 Housekeeping Office hour and SI schedule: Sunday: Katie 6-7 pm in Ritter 323 Monday: Dr. Lyon 10-11 am in Macelwan...

Transport across the cell membrane; osmosis, diffusion, passive and active transport Spring 2025 Dr. Maximilian Lyon MWF, 12:00-12:50 pm ISE 211 Housekeeping Office hour and SI schedule: Sunday: Katie 6-7 pm in Ritter 323 Monday: Dr. Lyon 10-11 am in Macelwane 100; Shikara 4:45-5:45 pm in Xavier G14 Sign into iClicker for attendance! Learning Objectives 1. Explain why, based on the structure and properties of phospholipids and of membrane proteins, biological membranes are semipermeable. 2. Differentiate active and passive transport mechanisms. 3. Identify different types of plasma membrane transporters in a model of a cell. 4. Determine the net movement of water across a membrane due to osmosis. Going with the flow Diffusion – random movement of particles Leads to things being more spread out Solutes (dissolved things) move from areas of high concentration to areas of low concentration At equilibrium things are randomly distributed  No way in, no way out Phospholipid bilayers block diffusion of most substances Passive – no energy needed Nonpolar or very small, simple uncharged molecules can pass through 1 Na+ moves as fast as: 1,000 glucose, 1,000,000,000 H2O, or 100,000,000,000 O2 molecules The other part of the mosaic Plasma membranes also contain lots of proteins Some use it as surface for reactions Some transmit matter or information across the membrane Osmosis – “diffusion of water” Energy: no Proteins : sometimes Movement of water across a selectively-permeable membrane Moves from low solute to high solute until concentration reaches equilibrium Doesn’t use energy, can be facilitated by aquaporins (“water hole” proteins) Pop goes the cell Direction depends on total amount of dissolved “stuff” Tonicity describes relative solute concentration on either side of a membrane  Energy: no Simple diffusion Proteins : no Movement of small, uncharged particles directly through a membrane Net movement from high concentration side to low concentration side (down the concentration gradient) At equilibrium movement in = movement out  Energy: no Facilitated diffusion Proteins : yes Movement of normally impermeable (large/polar) particles across a membrane Still move from high to low concentration Requires protein channel or carrier Both increase “effective permeability” Channel proteins Act as a tunnel (pore) through a membrane, usually always open Specific size, shape, and amino acids filter for specific materials Some have gates that need to be “unlocked” and opened Kim DM, Nimigean CM. Voltage-Gated Potassium Channels: A Structural Examination of Selectivity and Gating. Cold Spring Harb Perspect Biol. 2016 May 2;8(5):a029231. doi: 10.1101/cshperspect.a029231. PMID: 27141052; PMCID: PMC4852806. https://en.wikipedia.org/wiki/Aquaporin Carrier proteins Act as a “revolving door” through the membrane Specific size, shape, and amino acids filter for specific materials Binding their target causes a shape change  Energy: yes Energy time! Proteins : yes Moving particles from low concentration to high concentration (up the gradient) requires work/energy done by proteins in active transport Energy can come from one of two sources: Primary: Breaks down ATP to release energy Secondary: Uses the pre-existing concentration gradient of another substance Individual proteins can be cotransporters, moving two substances in the same direction (symporters) or opposite directions (antiporters) Active transport is how concentration gradients form inside the body  This will come back so many times Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. Carrier Proteins and Active Membrane Na+/K+ antiporter (Na+/K+ ATPase) Transport. Available from: https://www.ncbi.nlm.nih.gov/books/NBK26896/ Breaks ATP (adenine triphosphate) into ADP (adenine diphosphate) and Pi (inorganic phosphate) Transports 3 Na+ ions out, 2 K+ ions in, both against their concentration gradients Na+/K+ antiporter uses 1/3 of the energy in cells (2/3 in neurons)! Cellular windmills Uses flow of one particle down it’s concentration gradient to drive another up its concentration gradient Na+/glucose cotransporter Binds Na+ and glucose on the same side of the membrane Na+ moves down it’s gradient, pulls glucose up it’s gradient Symporter (both move the same direction) The initial gradient has to be made by a primary active transporter using ATP  Why it matters Cystic fibrosis is the most common genetic disease in humans of Northern European descent Caused by mutations to CFTR, a Cl- ion channel Cl- builds up in the cell, drives osmosis in Mucus outside of the cell gets thick and stiff Different causes, same effect https://www.frontiersin.org/articles/10.3389/fphar.2016.00275/full Make that “effects” For next class: Complete the reading quiz – due Monday at 12:00 pm Read Chapters 2.3, 2.4, and 5.1 Follow my highlights: https://macmillan.vitalsource.com/home/subscribe/maximilian.lyon%40slu.edu In-class group assignment Complete your sheet as a group of 2-4, 1-2 groups per table The worksheet is available on Canvas Raise your hand if you have any questions Turn in sheet with the printed full name of all participants who are here before you leave! If you are not in class today, submit by email by Monday at 12 pm Connect to iClicker for attendance today