Cell Biology: Principles of Cell Function - BIOL2P03 Lecture 7 - 2024 PDF

Cell Biology: Principles of Cell Function BIOL2P03 - Winter 2024 Lecture 7 - Membrane Transport Chapters 11 & 12: Membrane Transport and Excitable Cells Testable Material: Slides 4-18 Chapters 11 & 12: ATP-Powered Pumps Ion channels open doorways into cells based on instructive signals Chap...

Cell Biology: Principles of Cell Function BIOL2P03 - Winter 2024 Lecture 7 - Membrane Transport Chapters 11 & 12: Membrane Transport and Excitable Cells Testable Material: Slides 4-18 Chapters 11 & 12: ATP-Powered Pumps Ion channels open doorways into cells based on instructive signals Chapters 11 & 12: Ion Distribution Ion channels are transmembrane proteins that create pores in the plasma membrane that are permeable to select ions Present in all eukaryotic cells Chapters 11 & 12: Ion Channels Single Subunit Tetrameric Channel Chapters 11 & 12: Ion Channels 100’s of ion channels have been identified Typically selective for a particular ion (e.g. K+) or group of ions (e.g. cations such as Na+, K+ and Ca2+) Broadly categorized into: ▫ Non-gated ion channels ▫ Ligand-gated ion channels ▫ Mechanosensitive ion channels ▫ Voltage-gated ion channels Chapters 11 & 12: Ion Channels (Non-gated) Chapters 11 & 12: Ion Channels Chapters 11 & 12: Ion Channels & Membrane Potential All eukaryotic cells contain ion channels -ve At rest, there is a subset of non-gated ion channels that allow K+ to freely pass across the membrane V ▫ Also known as leak channels ▫ K+ will leave cell, making the inside more negative compared to the outside +ve http://www.bem.fi/book/03/03.htm  Chapters 11 & 12: K+ (Potassium) Ions As K+ leaves the cell, the inside of the cell becomes more negative than the outside -ve This separation of charge is known as a potential difference (V or voltage) V The potential difference across a cell membrane is known as the membrane potential +ve http://www.bem.fi/book/03/03.htm  Chapters 11 & 12: K+ (Potassium) Ions The membrane potential Concentration creates an electrical gradient -ve For K+ to move out of the cell, down its concentration gradient it must go against V the electrical gradient Electrical The more K+ that leaves the +ve cell, the greater the electrical gradient becomes http://www.bem.fi/book/03/03.htm Chapters 11 & 12: K+ (Potassium) Ions Eventually the movement of K+ across the membrane reaches an equilibrium ▫ At this point, the electrical gradient is equal and opposite to the concentration gradient for K+ For K+, this equilibrium is reached at a membrane potential of approximately -90 mV ▫ Depends on temperature and concentration gradient and is predicted by the Nernst equation (non-testable): ▫ This is true when only K+ is present Chapters 11 & 12: Ion Channels & Membrane Potential Leak channels are largely Concentration responsible for setting the resting membrane -ve potential ▫ Resting membrane potential refers to the membrane potential of a V Electrical cell in the absence of any stimulus ▫ Typically -70 mV ▫ True for all animal cells +ve http://www.bem.fi/book/03/03.htm Excitable Cells Chapters 11 & 12: Na+ (Sodium) Ions Na+ (sodium) is another very important ion that can move across the plasma membrane through transporters, pumps and ion channels Na+ is more concentrated outside of the cell than inside of the cell ▫ Na+ will enter the cell down its concentration gradient when allowed ▫ Makes the membrane potential more positive ▫ Equilibrium potential of approximately +60 mV Na+ -ve +ve Extracellular Intracellular http://cellularlifeprocesses.weebly.com/transport. Chapters 11 & 12: Ca2+ (Calcium) Ions Ca2+ (Calcium) Ions are very important signalling molecules ▫ Involved in neuronal communication, exocytosis, gene expression, muscle contraction, etc. Similar to Na+, Ca2+ is more concentrated outside of the cell than inside Will enter the cell down its concentration gradient when allowed Makes the membrane potential more positive Chapters 11 & 12: Excitable Cells Excitable cells are specialized cells that possess gated ion channels that open in response to specific stimuli, which include: ▫ Changes in voltage ▫ Stretching of the membrane ▫ Presence of extracellular or intracellular signalling molecules (ligands) Ion fluxes rapidly alter the membrane potential ▫ Depolarization = membrane potential becoming more positive ▫ Hyperpolarization = membrane potential becoming more negative Examples of excitable cells include neurons, muscle cells and endocrine cells Chapters 11 & 12: Excitable Cells Voltage-gated Ion Channels Sensitive to changes in membrane potential ▫ Voltage sensor changes the conformation of the channel to allow ions to pass Typically in response to depolarization Voltage sensor  1 9 end