BIOL 213 1st Edition Lecture 9 Outline of Last Lecture I. Cell membranea. Basic overviewII. The cell membrane is a lipid bilayera. Fluid mosaic modelb. Liposome formationIII. Fluidity of the membranea. How lipids move within the membraneb. Things that affect fluidityIV. Membranes are asymmetricala. Each layer of the membrane is unique in its compositionb. How this is accomplishedi. FlipaseV. Membrane proteinsa. Different functionsb. Different associationsc. MobilityVI. Detergentsa. Useful for studying integrated proteinsVII. The cell’s surface is coated with carbohydratesa. Lubricantb. Cell-cell signalingOutline of Current Lecture I. Ion channelsa. Two regionsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.b. Are neither all the way open nor all the way closedc. There are different kinds that are triggered by different stimuliII. Membrane potentiala. Regulated by leakage of K+ ionsIII. Action potentiala. The change in membrane potential as a function of timeb. The processi. The opening and closing of Na+ and K+ ion channelsIV. Synaptic clefts and neurotransmittersa. Excitatory neurotransmittersb. Inhibitory neurotransmittersCurrent LectureI. Ion channelsa. They allow selective, gated, passive transportb. Each channel has two regions:i. Gating region1. This is the region of the protein that opens and closesii. Selective region1. This is the region that only allows specific ions into the channelc. They are either all the way open, or all the way closed, there is never an in between where they’re half way openi. This was discovered in the experiment where a piece of membrane was cut out and put in another solution1. The membrane piece was cut out by a small pipette and submerged in a solution with different ion concentrations2. An electode was submerged in the solution too and turned on to change the charge of the solution3. The channel protein reacted to the change in charge by opening orclosing4. The activity was graphedii. It is apparent from the graph that the channel was either all the way closed, allowing no passage of charge, or all the way open, allowing maximum passage of chargeiii. The protein opens to the same value every timed. The amount of time or probability that an ion channel will be open is increased with the binding of a ligandi. It doesn’t increase the amount the channel is open, only the length of time it’s openii. This allows for more passage of chargeiii. When an ion channel is 100% saturated in activators/ligands, it is considered to be completely active1. Though in reality an ion channel is never always open2. There are always short lengths of time when it is closed, but a majority of the time it is openiv. When there are no activators/ligands bonded to an ion channel, it is considered completely inactive1. Though in reality an ion channel is never always closed2. There are always short lengths of time when it is open, but a majority of the time it is closede. They are gated and can by triggered by specific stimulii. Voltage-gated1. An change in membrane channel opens or closes the channel2. This is seen in action potential3. This kind of channel regulates itselfa. It opens as a result of a change in membrane potential in order to correct the membrane potential to its original chargeb. Once the membrane potential is fixed, the channel closesii. Ligand-gated: extracellular ligand1. A ligand binds to the extracellular side of the channel, causing it toopen or closeiii. Ligand-gated: intracellular ligand1. A ligand binds to the intracellular side of the channel, causing it to open or closeiv. Stress-activated1. The membrane can literally pull open or push closed an ion channel2. An example is when a bacteria cell begins to swell with water, the membrane is stretched and pulls open the channels to allow wateroutII. Membrane potentiala. A difference in ion concentrations on each side of the membrane causes membrane potentialb. The membrane potential is always relative to the intracellular side of the membranei. A negative membrane potential means the intracellular side is more negative than the extracellular sidec. K+ leak channels help to maintain membrane potentiali. K+ concentration is higher on the insideii. The cell allows the leakage of positive K+ ions out of the cell in order to increase the negativeness of the membrane potentialiii. The K+ will flow out of the cell down its concentration gradient, but it won’t flow to equal concentrations because it’s also flowing against its electrochemical gradient (the outside of the cell is positive, and as more K+ flows out, the outside gets more positive)iv. Therefore the concentration gradient and electrochemical gradient balance outv. When the two balance out, there is no net movement of K+vi. This is when the Nernst potential can be measured1. Recall: ΔG = ln [soluteinside]/[soluteoutside] + FEM2. When there is no net movement, ΔG = 03. So 0 = ln [soluteinside]/[soluteoutside] + FEM4. and EM = (RT/zF) ln ([inside]/[outside])5. the Nernst potential of K+ is negative because when the gradients balance out, the membrane potential is negativeIII. Action potential a. Action potential is how nerve cells communicate with each otherb. Nerve cellsi. Basic structure1. The cell body is at one end with dendrites attached to it2. The axon begins at the cell body and can be 1 mm to 1 m+ in length3. It ends by branding into terminal branchesii. Signals are received at the dendrites and sent down the axon and out the terminals to the next nerve cellc. The action potential is a change in membrane potential across the celli. An initial stimulus causes a localized membrane depolarization (more positive)ii. This slight depolarization causes the voltage-gated Na+ channels to open so that Na+ flows into the celliii. As more Na+ flows into the cell, the membrane potential becomes more positiveiv. Eventually, a certain membrane potential value is reached and the Na+ channels become inactivated1. Inactivation is not closed2. There is a little polypeptide tail on the channel that moves inside the channel to plug it up so that no more Na+ ions can go throughv. As the Na + channels are being inactivated, the K+ channels are openingvi. The K+ ions flow out the cell to make the inside more negativevii. This causes a decrease in membrane potential back to its original value – the resting membrane potentiald. As Na+