Class Notes 2/15-2/19
Class Notes 2/15-2/19 BIOL 30603
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This 7 page Class Notes was uploaded by Mallory Notetaker on Friday February 12, 2016. The Class Notes belongs to BIOL 30603 at Texas Christian University taught by Dr. Akkaraju, Dr. Misamore, Dr. Chumley, Dr. McGillvray in Spring 2016. Since its upload, it has received 51 views. For similar materials see Molecular, Cellular, and Developmental Biology in Biology at Texas Christian University.
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Date Created: 02/12/16
SS Cell Bio Notes 2/15-2/19 2/15/16 Glycocalyx- protective coating (carbohydrate layer), there is a number of proteins that stick out through this layer so that it can interact with things outside this layer -lots of cells have lectines on the outside and then those can interact with the carbohydrate residues Neutrophils are the type of white blood cell that roll along the sides of capillaries Question: -liposomes are used to study plasma membranes -lipid rafts do exist The reason red blood cell membranes are so resilient is? -due to spectrin cytoskeletal proteins Principles of transmembrane transport a. protein-free artiﬁcial lipid bilayer: poorly permeable -only gases b. cell membrane (has proteins): very permeable Impermeability to ions -need to know that all steroid chemicals are made form cholesterol -water can diffuse through the membrane without any proteins but better through aquaporin -larger uncharged polar molecules can’t go through without proteins -Na+ concentration, most of them are outside the cell -Ca2+ theres a tiny amount in the cytosol, this is important -read the small print under table 12-1 -don’t know numbers just know concepts -lots of negative charge right inside the membrane and lots of positive outside -theres more free hydrogen inside the cell, pka lower -a membranes electrical potential: ability to do work - (-70mV) average resting potential Cells contain two classes of membrane transport proteins: transporters and Channels -Transporters bind tightly to the solute it is going to move (also undergo a conformational change), Channels do not -Channels much weaker interactions with solute and faster solut transfer through aqueous pore -Passive transport: moves through using only a concentration gradient -can be channels or transporters -Active Transport: require an external source of energy (ATP) -transporter -Sometimes the concentration gradient goes against the voltage gradient -then the net overall effect is that it might be less when they go in opposite directions Aquaporins: multipass protein, allow water molecules to vibrate through Passive Transporters move a solute along its electrochemical gradient Pumps Actively transport a solute against its gradient -ATP-driven pump -Light driven pump -couples pump: uses the energy of the electrochemical concentration gradient of one solute to transport another solute against its gradient Na + is in higher concentration outside the cell, we can couple this gradient movement with glucose (moving glucose from outside the cell to inside against its gradient) -glucose gets kicked in and sodium gets kicked in -the epithelial cell gets glucose into the cell -on Luminal side: SGLT1 (needs sodium) -on other side: GLUT1 (doesn’t need sodium) Sodium Potassium pump -nuerons use this the most -moves potassium and sodium against its gradient -needs lots of ATP -3 sodiums for 2 potassiums K+ can move through a membrane using a K+ channel or a Na+/k+ ATPase pump. Which os the following is true? -Channels are faster than pumps 2/17/16 Secondary Active Transport: use free energy stored in electrochemical gradient to transfer solute (example of Na+ glucose transport protein on the lumen of the gut) (couple transporter) -moving glucose against its gradient -common in kidney and intestinal epithelial cells Questions: In the movie showing neutrophils rolling along the inside membrane of a capillary, the following is/are true: a. Neutrophils use lectins to bind the vascular glycocalyx b. involved sugar residues answer both Ca2+ pumps keep the cytosolic Ca2+ concentration low -muscle cell pumps -PMCA pump (plasma membrane calcium): sits out in the membrane, kicks calcium outside the cell using ATP (uniporter) -SERCA pump: (on ER or SR) moves calcium from muscle cell to the SR and use ATP Couple transport proteins -Uniporter: transports a single solute, no coupling -Symporter: two solutes move the same direction together -can use gradient or ATP -example of SGLT 1 (talked about earlier) (couples sodium and glucose from lumen of the gut) -Antiporter: two solutes move in different directions Ion Channels are Ion-selective and Gated -area in the channel called vestibule, where the water and the saturated K+ ions are -when the water is surrounding the K+ ion, it is two big to go through the channel therefore to go through the channel it needs to be ripped off the K+ ion -selectivity ﬁlter is a smaller area in the channel that only allows certain things through -there are carbonyl oxygens in the selectivity ﬁlter and therefore the cation would love to go through (K+ ion) so this is how it only lets potassium through and not the water -some of these channels have a vestibules on either side, allowing the ion to move in either direction based off the electrochemical gradient In the previous example of aK+ channel, Na+ will not go through the channel. Which of the following is a likely explanation? -Na+ is smaller than K+ and thus does not interact with the selectivity ﬁlter tightly enough -not big enough to allow the water to be pulled away from them and then it won’t go through Channels aren’t open all the time, if they were, we wouldn’t have an electrochemical gradient -Membrane potential is governed by the permeability of a membrane to speciﬁc ions -Na+ may move across due to a concentration gradient but may move back due to an electrochemical gradient -if there is only a sodium channel and not a potassium channel so the K+ offsets the charge and makes the Na+ move back over to balance the overall charge Ion Channels don’t need signals to open and close (it is random) -they spend more time closed -patch clamp method: take off a piece of the membrane with a channel in it -we can view the fact that when we remove any sources of signaling, they will randomly open and close on their own 1. voltage gated -when the charge on the outside is more positive, its closed -if this gets switched its open 2. Ligand-gated -something binds to the receptor causing a conformational change to open the channel 3.ligand-gated -there are two of these because the signal can come from inside the cell or outside the cell 4. Mechanically-gated -example of hair follicles vibrations -closely associated with neurons (auditory nerve ﬁbers) -ﬁlament from one stereocilia is linked to a channel, potassium channel -when the little ﬁlament hairs move one direction against the tectorial structure, the channel will open, when it goes the other way it is close -when potassium ions rush into the ﬁlament, it tilts What do you think would happen if the ﬂuid outside the hair cells in your cochlea suddenly lost all of its K+? -you could not hear -the ﬁlaments release neurotransmitters (acetocholin) sending a signal to the brain -(triggered by potassium rushing into the membrane) calcium voltage gated channel at the other end, calcium rushes in to allow vesicles to fuse and the message isn't relayed -voltage gated channel sense the electrical potential across the membrane (in this case they are sensing the increase of positive charge due to potassium rushing in 2/19/16 Action potentials are mediated by voltage-gated cation channels -All cells have the electrical potential where the inside of the plasma membrane is more negative (the number of the value can differ though). This is because the phsophlipid heads with he negative charge are always located on the inside of the cell. There are ﬂippases that put things like phosphotidyl serine on the inside. -Every cell in your body has to have ion channels to regulate. -Only certain cells can generate action potentials. (nuerons, muscle cells, certain cells in immune system). To generate an action potential, you have to have lots of voltage gated channels for an action potential. Voltage gated sodium channel (know ﬁgure 12-35) -blob of protein is hanging down and the channel is closed. It spends most of its time int he closed position. If the electrical potential across the membrane changes then the sodium channel opens. Sodium rushes in. Sodium channel is only open for a very short time. -Immediately after channel opens, then the protein blob is part of the channel ﬂips up and blocks it —-> the channel is open but inactive (sodium cannot come in) -When electrical potential goes back to its normal negative value, then channel closes. -The speed at which the channel open and closed had an inactive period. The inactive period was even longer than the open period. -The initial increase in membrane potential is very small (15 to 30mV difference). This small difference is what opens the sodium channel. -Originally it was polarized = neg on inside and positive on the outside -But then it depolarizes (pos on inside neg on outside) as positivity charged sodium ions (Na+) come rushing in. -Inactivation starts when there is much more positive inside the cell. During inactivation the channel is still open, but Na+ can not come in. -To repolarize the potassium channels open and allow for K to rush out. Repolarization takes the longest time but this whole process happens in 2 milliseconds -When the membrane changes so channel opens, action potential starts. -you can’t have another action potential until the channel closes. -channel can’t close until it goes through the whole inactive period and returns to where the cell is more neg inside. -can’t open when it is inactive because it IS OPEN, just inactive -Channel will close when it returns back to it’s negative membrane resting potential (-70) -Sodium goes upstream and it goes downstream Na/K ATPase pumps: while sodium is coming both directions, this pump resets the membrane by sending 3 Na out and bring 2 K in. -if it goes to the right, it is opening voltage gated Na channels, but if it goes to the left, the channels are inactive, so the pumps are pumping the sodium back out -Which is why the sodium does not hang out and wait for channels to close so it open them again -When gates close, the impulse from the initial segment has to be what starts the process again -Most Voltage gated sodium channels are near initial segment (start of the axon). These channels sense change in electrical potential and start action potential. Propagation -propagation of the signal goes in 1 direction. It starts in initial segment, and goes one way, it cannot go backwards because channels become inactive as the next channel opens -starts at -70 and as it travels down becomes like -50 -KNOW: the protein blob ﬂips up and closes channel when it is at -70 -we need this process to go as rapidly as possible one way so that it can be reset to be ready for the next signal. Therefore the use of the inactive gates promote 1 way direction. -Channels close after their inactivation period -When sodium channels close, the K channels opens and depolarization starts. -voltage gated K channels are much slower than Na channels -K cannot sense the change in voltage as quickly as Na can -Remember: sodium rushes in and depolarizes the membrane in Both directions. When K channel opens then K leaves depolarizes the membrane, but it can only go in one way because the voltage gated sodium are closed. What happens if this is a resting neuron and you come in right int he middle and zap it with an electrical impulse and depolarize the membrane… Which direction does the action potential go? -Answer: all voltage gated na channels are closed (there are no inactive ones), so voltage na channels in both directions open (impulse goes in both directions) Voltage gated Ca channel -at the end of the neuron -senses electrical potential across the membrane -normally closed -but when the membrane becomes more positive on the inside, the Ca channel opens -Ca comes in (cells do not have very much Ca inside because Ca is a second messenger.. it used for signaling) -Ca stimulates vesicles to twist so they release neurotransmitters into the synapse -Ca is needed to propagate the signal -On the other side of synaptic cleft you have neuron #2 -Nueron #2 has ligand gated Na+ channels -the ligand in the vesicles ﬂood the synapse and bind to these ligand gated Na channels -initiating depolarization of neuron #2 -There are not enough voltage gated Ca channels to allow for an action potential, so how does the electrical change across the membrane get all the way to the initial segment? -Transmitter-gated Ion channels in the in the postsynaptic membrane convert the chemical signal back into an electrical signal -Neurotransmitter can be excitatory or inhibitory Inhibitory: -when neurotransmitter makes the neuron more polarized or more neg on inside -Ex: letting more Cl- in. -the more neg it is inside (meaning its more polarized), means that it is harder to depolarize it enough to open voltage sodium channels. -instead of it being at like -70 it will be at a more polarized state of -100 Excitatory: -makes inside less negative -makes it easier for VG Na channels to open -Little red dots on the picture are synapses onto the postsynaptic cell. -These are all synapses coming from other axons. There are tons of synapses on 1 cell -1 neuron could have 200,000 synapses on them -some may excite it and some may inhibit it Na/K pumps need ATP to drive it -so if you don’t eat breakfast and get ATP, the gradient gets less and less and the action potential starts to slow down. -it gets harder to start an action potential because you have more sodium inside and more K outside. -the pumps need get the gradient back to resting state (neg on inside pos on outside) Optogenetics Took a channel of algae… this is a light dependent channel. -When light hits, the channel opens and allows a ligand to go through it. They forced expression of this light dependent channel in speciﬁc neurons. They could determine functions of that neuron by shining light into he brain. -they force expression of the protein channelrhodopsin and stick a ﬁberoptic cable to the skull of the mouse and they can turn the light on and off and cause the neuron to do something. -they can cause the neurons to have an action potential or not.
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