Crossing Membranes Lecture PDF

Crossing Membranes Lecture 7 BIOL2020 Prof. Nicanor González Molecules need to be transported across membranes Some molecules pass, most do not ● ● Can diffuse freely ○ Small uncharged molecules Can’t diffuse freely ○ Large polar molecules ○ Charged ions. Facilitated diffusion: protein med...

Crossing Membranes Lecture 7 BIOL2020 Prof. Nicanor González Molecules need to be transported across membranes Some molecules pass, most do not ● ● Can diffuse freely ○ Small uncharged molecules Can’t diffuse freely ○ Large polar molecules ○ Charged ions. Facilitated diffusion: protein mediated movement down the gradient ● ● ● It does not require energy (ATP) It only moves molecules from high to low concentrations Glucose is a good example Channels and Carriers are the two major protein types that mediate facilitated diffusion ● ● channels are pores Carrier proteins undergo a conformational change as the solute moves Carrier proteins alternate between two conformational states ● ● they are highly specific to their solute Glucose transporter is an example GLUT3 is shown on the left in the open outward conformation, and GLUT1 is shown on the right in an open inward conformation. Glycolysis → Glycolysis Carrier proteins transport either one or two molecules symporters transport two molecules in the same direction. antiporters move two molecules are transported in opposite directions LacY transports lactose and hydrogen ions into the cell, and GlpT transports phosphate and glycerol-3-phosphate in opposite directions. ● Ion channels ○ specific to certain ions ● Porins ○ not very specific ○ only in mitochondria/chloroplast and bacteria ● Aquaporins ○ specific to H20 Ion channels are transmembrane proteins that allow rapid passage of specific ions 1. Typically gated 2. Things that open the channel are: • voltage, • ligand, • temperature, • pressure https://pdb101.rcsb.org/motm/38 Example: Potassium channels allow potassium ions to pass, but block smaller sodium ions closed open Roderick MacKinnon The Nobel Prize in Chemistry 2003 Protein domains connected to the channel twist the four chains of the channel from the "open" to the "closed" position Outside cell view → Potassium ions Potassium channels have a filter for smaller ions H2O K+ potassium ion (blue) must shed its water molec (red) • Eight oxygen atoms in the channel act as a pe replacement for the water molecules. (also red) • The ions move along oxygen sites. • Sodium ions, on the other hand, are slightly smaller, so they fail to interact with the oxygen atoms Why is passing ions important? Membrane potential is the difference in the concentrations of ions on opposite sides of a membrane. Cell membranes –70 mV to –40 mV. Depolarization if the interior voltage becomes less negative ( –70 mV to –60 mV), Hyperpolarization if the interior voltage becomes more negative ( –70 mV to –80 mV). Cells read the change in voltage and use it as an intracellular signal Ligand-gated ion channels convert a chemical signal into the cell Na+, K+ and Ca2+ Acetylcholine receptor, with the binding site for acetylcholine in red. The membrane is shown schematically in gray. Neuromuscular junction Temperature-gated The 2021 Nobel Prize in Physiology or Medicine is awarded to David Julius and Ardem Patapoutian Transient receptor potential cation channel subfamily V member 1 (TrpV1) Pressure-gated The 2021 Nobel Prize in Physiology or Medicine is awarded to David Julius and Ardem Patapoutian Piezo 1 Voltage-gated sodium channels transmit signals in a wave through the nervous system. Voltage difference is high = channels are tightly closed, Voltage difference is low = channels open and allow sodium ions to pass Small Voltage difference Top view Lateral view V channel Big Voltage difference Amplifiers Voltage-sensing elements Accessory Proteins Channel Restricted to ● Bacteria ● Chloroplasts ● Mitochondria ATP the main energy storage molecule has negative charge ● ● ATP is made in the mitochondria • It can’t pass through the membranes a voltage-dependent anion channel (VDAC) mediates exchange of negatively charged metabolites, most importantly ATP from mitochondria to the cytoplasm. Aquaporins are transmembrane channels that allow rapid passage of water Summary Diffusion ● Simple ● Facilitated ○ Uniporter ○ Symporter ○ Antiporter Carrier proteins GLUT GlpT Gate mechanisms ● Voltage ● Ligands ● Pressure ● Temperature Channels ● Ion channels ● Porins ● Aquaporins