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Chapter 9 Membrane Transport Molecules in the cell membrane (1) Transport of molecules → Signaling (2) Adhesion to the ECM → Cell recognition Specialized membrane transport proteins are responsible for transferring small water-soluble molecules across cell membranes Impermeable to most water-soluble molecules Transfer a particular type of molecule HCO3-, PO43-,proteins, nucleic acids, metabolites… Simple diffusion The diffusion of water is known as osmosis Osmotic pressure aquaporins Cell use different tactics to avoid osmotic swelling Na+-K+ pump Turgor pressure Stomata open on the underside of a leaf Stomata A Venus flytrap uses electrical signaling to capture its prey Mechanical stimulation on hairs in the central of each leaf Electrical signal Rapid change in turgor pressure The rate at which a molecule diffuses across a synthetic lipid bilayer depends on its size and solubility (1) Smaller molecule (2) Oil solubility (hydrophobic, nonpolar) Rate of diffusuin Small molecules and ions can enter the cell through a transporter or a channel Fit into a binding site on the transporter Size & electric charge Greater rate than transporters Most solutes cross cell membrane by passive or active transport small, uncharged, fat-soluble Facilitated diffusion Some transporters carry a single solute across the membrane (uniports); others couple the uphill transport of one solute across to the downhill transport of another Both passive & active transport Each cell membrane has its own characteristic set of transporter A conformational change in a transporter could mediate the passive transporter of a solute such as glucose uncharged molecule Facilitated transport via transporter Glucose transporter An electrochemical gradient has two components Concentration gradient + Membrane potential Electrochemical gradient (net driving force) Cells drive active transport in three main ways The Na+-K+ pump plays a central role in membrane transport in animal cells X3 X2 Na+-K+ ATPase (Na+-K+ pump) The Na+-K+ pump transporter ions in a cyclic manner Na+-K+ pump (10 ms/cycle) Ouabain inhibits the pump by preventing K+ binding Primary active transport Na-K pump Na-K exchanger Na-K ATPase Na+ outside the cell is like water behind a high dam Potential energy Distribution of Ca2+ in the cell A Ca2+ pump returns Ca2+ to the sarcoplasmic reticulum in a skeletal muscle cell 10 helices Sarco/Endoplasmic reticulum Ca2+ ATPase (SERCA) Secondary active transport The glucose-Na+ symport protein uses the electrochemical Na+ gradient to drive the import of glucose The binding is cooperate, if one of the two solutes is missing, the other will fail to bind o the pump Two types of glucose transporters enable gut epithelial cells to transfer glucose across the gut lining from diet Examples of secondary active transport Na/glucose cotranspoter Na/Ca exchanger Two types of glucose transporters enable gut epithelial cells to transfer glucose across the gut lining Cooperation of primary and secondary active transport systems There are similarities and differences in transporter -mediated solute movement in animal and plant cells fungi, bacteria Plant cells, fungi (including yeast) and bacteria do not have Na+-K+ pumps Na+-glucose symport A K+ channel possesses a selectivity filter that controls which ion it will transport across the membrane Ion channel (1) ion selectivity: a. diameter and shape; b.distribution of the charged amino acids (2) Ion channels are not continuously open: most ion channels are gated A typical ion channel fluctuates between closed and open conformations Hydrophilic pore (1) faster than the transfer rate of transporter (2) cannot couple the ion flow to an energy source to carry out active transport Properties of ion channel Selectivity: (1) Ion size or charge (2) Binding to the protein surface (3) Stabilization of the nonhydrated ion Rate: Simple diffusion Ion channel (Fast) Facilitated transporter Active transporter (Slow) Different types of patch-clamp recording Patch-clamp recording is used to monitor ion channel activity Current can enter or leave the microelectrode only by passing through the channels in the patch of membrane covering its tip Easy to alter the composition of the solution on either side of the membrane Patch-clamp recording is used to monitor ion channel activity The voltage (membrane potential) across the isolated patch of membrane is held constant during the recording Gated ion channels respond to different types of stimuli (Mechanical force) Voltage sensor: sensitive to changes in the membrane potential Stress-gated ion channels allows us to hear K+ Stereocilia on the organ of Corti in cochlea of the inner ear Stress-gated ion channel Voltage-gated ion channels underlie the leaf-closing response in mimosa The leaf is touched The opening of voltage-gated ion channel Generating an electric impulse Loss of water Leaflets to fold closed suddenly The distribution of ions on either side of the lipid bilayer gives rise to the membrane potential ion channels → membrane potential How the resting potential is generated Neuron Axon Plasma membrane Outside of neuron Na Na Na Na K K Na channel Na Na Na Na K Na Na Na Na-K pump K K Plasma membrane Na Na Na Na Na Na+ is high k+ is low ATP K channel K Na K K Na Na K K Inside of neuron K K K K K K K K+ is high Na+ is low K+ leak channel play a major role in generating the membrane potential across the plasma membrane The K+ leak channel The Nernst equation can be used to calculate the resting potential of a membrane A typical neuron has a cell body, a single axon, and multiple dendrites Up to 100 m/sec Neurons Muscle cells Gland cells How stimuli trigger signals in a living neuron? An electrode can be inserted into the squid giant axon (A) to measure action potentials (B) 100X the diameter of a mammalian axon 10 cm The cytoplasm in an axon can be removed and replaced with an artificial solution of pure ions Na+, K+, Cl-, SO42- The shape of the action potential depends on the concentration of Na+ outside the membrane The Nobel Prize in Physiology or Medicine 1963 The ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane The action potential propagates itself along the axon Action potentials are - self-propagated in a one-way chain reaction along a neuron. - all-or-none events. The frequency of action potentials (but not their strength) changes with the strength of the stimulus. Action potentials are all-or-none events 絕對不反應期 相對不反應期 去極化 再極化 過極化 全有全無律 Ion flows dictate the rise and fall of an action potential Resistant/refractory to stimulation Opening of voltage gated K+ channels An action potential is triggered by a rapid change in membrane potential With voltage-gated ion channels Self-amplifying depolarization of neuron 20 mV Without voltage-gated ion channels A voltage-gated Na+ channel can adopt at least three conformations highly polarized repolarized depolarized depolarized The action potential Na Na Na Na K Additional Na channels open, K channels are closed; interior of cell becomes more positive. Na 50 Membrane potential (mV) 3 Na K 2 Na Sodium Potassium channel channel Action potential 3 0 閥值 4 Na channels close and inactivate; K channels open, and K rushes out; interior of cell is more negative than outside. 5 The K channels close relatively slowly, causing a brief undershoot. 4 2 50 Threshold 1 100 Resting potential 5 1 Time (msec) A stimulus opens some Na channels; if threshold is reached, an action potential is triggered. Na K Outside of neuron Na Na Plasma membrane K 1 Resting state: Voltage-gated Na and K channels are closed; resting potential is maintained by ungated channels (not shown). K Inside of neuron 1 Return to resting state. A action potential can be propagated along the length of an axon (1) Propagation of the action potential along the axon Axon Na Plasma membrane Action potential Axon segment 1 Na Action potential Na K 2 K Na Action potential Na K 3 K Na A action potential can be propagated along the length of an axon (2) Depolarized membrane Neurons transmit chemical signals across synapses Synapse 20 nm (neurotransmitters inside) An electrical signal is converted into a chemical signal at a nerve terminal Electrical signal Chemical signal exocytosis An chemical signal is converted into an electrical signal by transmitter-gated ion channels at a synapse Chemical signal Electrical signal The acetylcholine receptor, present in the plasma membrane of muscle cells, opens when it binds to the neurotransmitter acetylcholine released by a nerve 5 transmembrane subunits Aqueous pore Acetylcholine receptor (transmitter/ligand-gated ion channel) Neuromuscular junction Synapses can be excitatory or inhibitory Acetylcholine Glutamate or Ca2+ GABA Glycine Thousands of synapses form on the cell body and dendrites of a motor neuron in the spinal cord Axon terminals Synapses Cell body Dendrites Actions of excitatory and inhibitory neurotransmitters