Transcript Chapter 7
Chapter 7 Transport of Ions and Small Molecules Across Cell Membranes By Christi Haines Molecule and Ion Movement is Mediated by Selective Membrane Transport Proteins. The phospholipid bilayer is impermeable to most water soluble molecules, ions, and water itself. A model for passive diffusion of small hydrophobic molecules across a pure phospholipid bilayer Membrane Proteins Mediate Transport of Molecules and All Ions across Biomembranes Channel proteins transport water or specific types of ions and hydrophilic molecules down their concentration gradient (facilitated diffusion). ATP Pumps and the Intracellular Ionic Environment Types of Pumps: P-Class Pumps V-Class Pumps F-Class Pumps ABC Superfamily Active transport by ATP-powered pumps Nongated Ion Channels and the Resting Membrane Potential The plasma membrane contains channel proteins that allow the principal ions to move thru them at different rates down their concentration gradients. Voltage (electric) Potential = 70mV Nernst Equation The magnitude of the sodium equilibrium potential is given by the Nernst equation: ENa = 0.059 log10 [Na1]/[Nar] Membrane Potential Depends on Resting K+ Channels The outward flow of K+ ions thru resting K+ channels is the major determinant of the inside negativemembrane potential. Cotransport by Symporters and Antiporters Cotransporters use enery stored in electrochemical gradients of Na+ and H+ Transported molecule and cotransported molecule move in same direction = symport Move in opposite directions = antiport Na+-linked symporters import amino acids and glucose into many cells Movement of Water Osmotic pressure causes water to move across membranes Osmotic pressure is the hydrostatic pressure required to stop the net flow of water across a membrane separating solutions of different compositions. Different Cells have Various Mechanisms for Controlling Cell Volume In a hypotonic solution (concentration of solutes is lower than in the cytosol) cells swell due to the osmotic flow of water inward. In a hypertonic solution (concentration of solutes is higher than in the cytosol) cells shrink as systolic water leaves the cell by osmotic flow. Aquaporins Increase the Water Permeability of Cell Membranes Aquaporins function as water channels The structure of aquaporin, a water channel protein in the erythocyte plasma membrane Transport across epithelia: the intestinal epithelium is highly polarized Proposed model for operation of the two-Na+/one-glucose symporter Transepithelial movement of glucose and amino acids requires multiple transport proteins Voltage Gated Ion Channels and the Propagation of Action Potentials in Nerve Cells Specialized regions of neurons carryout different functions: Cell Body Axons Axon terminals Dendrites Opening and Closing of Voltage Gated Na+ and K+ Channels Generate Action Potentials Voltage Gated Na+ Channels – closed in resting neurons, depolarization caused channels to open allowing Na+ to enter Voltage Gated K+ Channels – repolarization opens K+ channels allowing K+ to leave the cell Myelination Increases the Velocity of Impulse Conduction The presence of a myelin sheath around an axon increases the velocity of impulse conduction to 10-100 meter/second. Neurotransmitters and Receptor and Transport Proteins in Signal Transmission at Synapses Neurotransmitters are packaged in membrane-bound synaptic vessicles in the axon terminus Arrival of an action potential at axon termini in presynaptic cells triggers secretion of neurotransmitters Binding of neurotransmitters by receptors on post-synaptic cells leads to changes in their membrane potential. Neurotransmitters are removed from the cleft after stimulating post-synaptic cells Questions?