Advanced Cell Bio Chapter 12 Notes!
Advanced Cell Bio Chapter 12 Notes! BCMB 311
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This 9 page Class Notes was uploaded by Izabella Nill Gomez on Tuesday March 1, 2016. The Class Notes belongs to BCMB 311 at University of Tennessee - Knoxville taught by Dr. Barry Bruce, Dr. J. Park in Spring 2016. Since its upload, it has received 21 views.
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Date Created: 03/01/16
Advanced Cell Bio Chapter 12 Notes! The vast majority of the molecules cannot simply diffuse through the plasma membrane like CO2, O2. Transfer depends on membrane transport proteins that span the lipid bilayer, providing private passageways that can access the membrane for selected substances. Transporters shift small organic molecules or inorganic ions from one side to the other by changing shape. Channels form tiny hydrophilic pores across the membrane through which such substances can pass by diffusion. Most only allow inorganic ions to pass through (ion channels). Can create voltage across the membrane. Hydrophilic molecules pass by the bilayer slowly through simple diffusion, can be accelerated by facilitated transport. The smaller the molecule, the more nonpolar, the faster it will diffuse. Many molecules of interest to cells are polar and water soluble. Solutes-- substances that are dissolved in water--unable to cross the bilayer without the aid of membrane transport proteins: 1. Small nonpolar--diffuses readily if small (H2), ethanol, glycerol not so fast). Glucose rarely crosses. 2. Lipid bilayers are highly impermeable to all charged molecules-- inorganic ions--no matter how small. Because cell membranes are impermeable to inorganic ions, living cells are able to maintain internal ion concentrations different from concentrations of ions in media that surrounds them. Most important inorganic ions: Na+, K+, Ca2+, Cl-, H+. Movement of these plays an essential part in biological processes; most striking is ATP production and nerve cell communication. Na+ most plentiful positive charge ion outside the cell, K+ abundant inside. To avoid being torn apart, the number of positive charge inside (outside) must equal the negative charge inside (outside), the same for fluid. Na+=Cl-, K+=other ions (nucleic acids, proteins, etc.). Tiny excesses of charge around the plasma membrane occurs, imbalances create voltage differences across the membrane called membrane potential. When the cell is “unstimulated”, exchange of cations/anions across the membrane are balanced. Resting membrane potential holds steady, but is not zero (for animal cells, can be between -20-- -200 mV depending on the organism). Negatively charged inside more than outside. Allows transport of metabolites and provides cells excitable with means to communicate with neighbors. Membrane transport proteins occur in many forms and are present in all cell membranes. Each provides a private portal across the membrane for a small solute molecule-- ion, sugar, amino, etc. Each membrane has its own characteristic set of transport proteins. 2 classes of membrane transport proteins and transporters and channels--differ in the way they discriminate between solutes, transporting some but not others. Channels discriminate mainly on the basis of size and electric charge: when open, any ion/molecule small enough and has the right charge passes through. Transporter transfers only molecules/ions that fit into specific binding sites on the protein. Transporters and solutes with great specificity, in same way enzyme binds its substrate, and gives requirement selectivity. What controls what molecules pass into the cell/organelle is concentrations of solute on either side of the membrane. The molecule will spontaneously flow downhill from a region of high concentration to a region of low concentration provided a pathway exists --passive because they need no additional driving force. If a solute at a higher concentration outside the cell, and appropriate channel/transport present, solute will move into the cell via passive transport, without using energy of transport protein. To move against the gradient, a transport protein must do work--drives flow uphill by coupling to some other process that provides an input of energy-- active transport--carried out by pumps. For uncharged molecules passing through determined only by concentration gradient. But few electrically charged molecules, additive force--membrane potential exerts force on any molecule that carries electric charge. Cytosolic side of the plasma membrane is usually at a negative potential relative to the extracellular side, so membrane potential bends to pull positively charged solutes into the cell and drive negatively charged ones out. At the same time, a charged solute will also tend to move down concentration gradient--net driving force of the concentration gradient and membrane potential--electrochemical gradient-- determines the direction that each solute will flow across a membrane by passive transport. For some ions, voltage and concentration gradient work in the same direction (steep electrochemical gradient)--Na+. Not so much for K+. Some cells contain aquaporins in the plasma membrane to facilitate the flow of H2O (slow). Total concentration of solute particles inside the cell-- osmolarity--generally exceeds solute concentrations outside. The resulting osmotic gradient tends to pull H2O into the cell--flow from low solute concentration (high H2O concentration) to high solute concentration (low H2O concentration) called osmosis--if it occurs without constraint, it can make the cell swell. Most animal cells have a gel-like cytosol that resists osmotic swelling. Some amoebae eliminate excess H2O using vacuoles that discharge extra H2O outside. Plants prevent swelling with thought cell walls that can stand strong osmotic differences--makes use of turgor pressure to keep cell walls tense and the stem of a plant rigid--if lost, the plant begins to wilt. Transporters are responsible for the movement of most small, water-soluble, organic molecules and some inorganic ions across cell membranes. Each is highly selective, often transporting only one type of molecule. To guide/propel complex traffic into/out, between different membrane- enclosed organelles, each cell membrane contains a characteristic set of different transporters appropriate to that particular membrane. Ex: plasma membrane has transporters that import amino acids, sugars, nucleotides; lysosome contains H+ transporter to acidify the lysosome and others that have move digestion into the cytosol; the mitochondria has pyruvate imported to use as fuel for ATP generation. Glucose transporter mediates passive transport. The protein crosses the plasma membrane 12 times, can have different conformations, switches randomly between them. In one, a transporter exposes binding sites for glucose to the exterior of the cell; in another, to the interior. Because glucose is uncharged, the chemical component of electrochemical gradient (E.G.) is zero--determined by the concentration gradient alone. Transporters are selective; use only D-glucose because it cannot use L for glycolysis--cells can’t rely on passive transport alone--active transport is essential to maintain the right ionic composition of cells and to import cells at a lower concentration outside the cell than inside--depend on transmembrane pumps, which can carry out active transport in 3 main ways--ATP-driven hydrolyzes ATP to drive uphill transport, coupled pumps link uphill transport of one solute to the downhill of another, light-driven pumps are found mainly in bacteria and use energy from sunlight to drive uphill transport. Different forms of active transport are often linked. ATP driven Na+ pump drives Na+ out of the cell against E.G., then, it is brought back in down E.G. Influx of Na+ provides energy for the transport of other substances into the cell against the E.G. gradient. ATP-driven Na+ pump bas a central role in active transport of small molecules across the plasma membrane of animal cells. Plants, fungi, bacteria use ATP-driven H+ pumps analogously. ATP- driven Na+ pump plays a central part in energy economy og animal cells that typically accounts for 50% or more of ATP consumption. Pump uses energy derived from ATP hydrolysis to transport Na+ out of the cell as it carries K+ in. Pump is known as Na+-K+ ATPase of Na+-K+ pump. Energy from ATP hydrolysis involves a series of proton conformational changes that drive Na+/K+ ion exchange. As part of the process, P group from ATP gets transferred to pump itself. Ion transport involves reaction cycle with each step dependent on the last. Toxin ciabain prevents the pump from binding extracellular K+. Process takes 10 milliseconds . Avoids useless ATP hydrolysis. Na+ functions like a bilge pump in a leaky ship, expelling Na+ constantly entering the cell. This way, the pump keeps Na+ in the cytoplasm at about 10-30 times lower than in the extracellular fluid and K+ 10-30 times higher. Steep concentration gradient of Na+ across the plasma membrane acts with the membrane potential to create a large Na+ E.G., which pulls Na+ back into the cell. High concentrations of Na+ outside the cell on the uphill side of the E.G. is like a large volume of water building behind a dam; large store of energy. Ca2+ is kept at a low concentration in the cytosol occupied with concentration in the extracellular fluid, but less plentiful than Na+ both inside/outside of cells. Ca2+ can bind tightly to a variety of protons in the cell, altering activities. Influx of Ca2+ into the cytosol through Ca2+ channels is used by different cells as a trigger to cell processes (ex: inside concentration, nerve cell communication). If there is a lower concentration of free Ca2+, more sensitive the cell is to increase in the cytosol of Ca2+. Huge concentration difference is achieved by ATP-driven Ca2+ pumps in the plasma membrane and ER membrane. Ca2+ pumps work like Na+, main difference is that Ca2+ returns to its original conformation without the requirement for binding and transporting an ion. Ca2+ and Na+ are similar with a common evolutionary origin. Coupled pumps couple movement of one inorganic ion to that of another movement of inorganic ion, small molecule, etc. If the pump moves both solute in the same direction across the membrane, it is called symport. If it moves in opposite directions called antiport. Transporter that ferries only one type of solute is called a uniport (ex: glucose transporter). Symports that make use of the inward flow of Na+ down a steep E.G. have important role of driving import of other solutes into animal cells. Ex: epithelial cells that line the gut pump glucose from gut lumen to gut epithelium and ultimately to blood). If the cells had only passive glucose import, would release glucose into the gut after fasting as freely as they take in after a feat. Gut epithelials also have glucose-Na+ symport, which allows to take up glucose from the lumen even when the concentration is higher in the cytosol than the lumen. Because Na+ is pumped in, glucose dragged into the cell with it. Both molecules must be present for couple transport to occur. Cells need other transporters to take the glucose out--at apical domain, have Na+-glucose symport--at lateral domain, have passive glucose uniports--which release glucose down the concentration gradient. Na+-H+ exchanger (antiport) in the plasma membrane of animal cells uses downhill influx of Na+ to pump H+ out of the cell--one of the main devices used to control pH in the cytosol. Plants, bacteria, fungi (yeasts) don’t have Na+ pumps in the plasma membrane--rely on H+. Gradient created by H+ pumps in the plasma membrane that pump H+ out, setting up an electrochemical proton gradient across the gradient and creating an acidic pH. In some photosynthetic bacteria, H+ is created by light-driven H+ pumps (ex: beta-rhodopsin). In others, use ATP hydrolysis to pump H+ out of the cell. Turbine-like pumps in lysosomes/vacuoles actively transport H+ from cytosol to organelles, helping keep the pH of the cytosol neutral and the pH of the organelle acidic--crucial to function. Channels allow small H2O soluble molecules to cross from one side of the membrane to the other through hydrophilic channels--through transmembrane pores to allow passive movement of small H2O solubles in/out of the cell. A few channels form large, aqueous pores--ex: proteins that form gap junctions between 2 adjacent cells--porins that form pores in the outer membrane of the mitochondria and some bacteria. If directly connected to the cytosol, would be disastrously leaky--most are narrow, highly selective pores. Ex: aquaporins allow passive diffusion of uncharged H2O--prohibiting movement of ions. Ion channels show ion selectivity, permitting some inorganic molecules through but not others--depends on the diameter and shape of ion channel and the distribution of charged amino acids that line it. Each ion is surrounded by a small shell of H2O that shed to allow ions to pass. Ion channels are not continuously open. Most are gated--specific stimulus triggers them to switch between closed and open states by changing conformation. Does not need to undergo conformational changes with each ion it passes, and so has a large advantage over a transporter with respect to the maximum rate of transport 1000x greater. But cannot couple ion to energy source for active transport. Changes of membrane potential are the basis for electrical signaling in many cells--mediated by alterations in permeability of membranes to ions. In an animal cell, unstimulated or _resting+, negative charges on organic molecules are largely balanced by K+. K+ is actively imported. Plasma membrane also has K+ leak channels that randomly flicker between open/closed states no matter the conditions--when open, K+ has a tendency to flow out of the cell down a steep concentration gradient. Leaves unbalanced negative charges, creating membrane potential. Forces all other K+ inside to counteract K+ flow out--here, E.G. for K+=0, even though higher K+ exists inside the cell than out. Resting membrane potential with Nernst equation expresses equilibrium--flow of negative and positive ions across the membrane is precisely balanced. (-20--200 mV (animal cells))-- reflection of E.G. of K+. When the cell is stimulated other ion channels in the plasma membrane open, changing permeability--whether ions enter/leave depends on the direction of the E.G. Membrane potential at any time depends on both state of ion channels and the concentration on either side of the plasma membrane. Rapid opening and closing of ion channels matters most in cell signaling. Measuring changes in electrical current is the main method to study ion movements and channels--proton-clamp recording provides direct picture of how ion channels behave. Fine glass tube is used as microelectrode to isolate and make electrical contact with a small area of the membrane at the surface of the cell. Technique makes it possible to record the activity of ion channels in all sorts of cell types--particularly large nerve/muscle cells. By varying concentrations in ions on either side of the patch, can test which ions will go through channels in the patch--voltage/membrane potential can be set and clamped at a value. Random thermal environments trigger on/off channels even though conditions are constant. “All or none” reaction for channels--fully open or closed. Ion channels differ with one another primarily with respect to ion selectivity and gating. For voltage-gated channels, the probability of having open controlled by membrane potential--for ligand- gated channel, controlled by binding of some molecule (ligand) to the channel. For mechanically-gated channels, opening is controlled by mechanical force applied to the channel. Auditory hair cells are a good example of cells that depend on mechanically gated channels. Vibrations pull channels open, flowing ions into hair cells--sets up electrical signal to auditory nerve to the brain. Voltage gated ion channels have domains of voltage sensors that are extremely sensitive to changes in membrane potential and changes above certain threshold value, exert sufficient electrical force on these domains to encourage the channel to switch from closed to open conformation (membrane potential change) alters probability that gate will open. When one type of voltage-gated ion channel opens, membrane potential can change, activate/inactivate other voltage channels--fundamental to electrical signaling. Neuron’s task is to receive, integrate, transmit signals and carry signals inward from sense organs such as eyes and ears to the central nervous system--brain and spinal cord. In CNS, neurons signal through networks. Every neuron has a cell body with a nucleus and long extensions radiating. Usually has one long extension--axon--which conducts electrical signals away from the cell body to distant target cells; shorter dendrites which radiate like antennae and provide larger surface area receive signals. Axon commonly divides at the far end into many branches, each which end in a nerve terminal so a message can be passed simultaneously to many target cells. Branches of dendrites can be extensive too. Neuron is stimulated by a signal delivered to localized site on the surface, initiated change in membrane potential--to spread, goes from dendrite to axon terminals to next dendrite, forming a neural circuit. Local change in membrane potential generated by a signal can be spread passively but rapidly becomes weaker with distance. Over short distances, unimportant, but for long-distance, pressure spread inadequate. Employ active signaling mechanism here, local electrical stimulus of sufficient strength triggers explosion of electrical activity in the plasma membrane that propagates constantly renewing--action potential or nerve impulse can carry long distance message. When neuron stimulated, membrane potential of the plasma membrane shifts to less negative value (towards zero). If depolarization sufficiently large, causes voltage-gated Na+ channels in the membrane to open transiently at the site. As these channels flicker open, allows a small amount of Na+ to enter the cell down a steep E.G. Influx of positive depolarizes further, opening more Na+ channels. Happens until the membrane potential is positive (+40 mV)--where electrochemical driving force for driving Na+ channels is zero--“timer” of Na+ channels causes them to inactivate even when the membrane is depolarized--remain inactivated until potential returns to initial negative value. During action potential, Na+ channels do not act alone--depolarized axonal membrane is helped to return to resting potential by opening voltage- gated K+ channels--open in response to depolarization, but not as fast a Na+ and stay open as long as the membrane is depolarized. When depolarization reaches peak, K+ ions start to flow out of the cell--brings back to resting state. Action potential spreads outwards as travelling wave from the initial side of depolarization, eventually reaching axon terminals. After action, Na+ pumps in the axon plasma to restore ion gradients of resting cell. 20% of the brain consumes the energy generated from metabolism of food to power the pump. When an action potential reaches nerve terminals at the end of an axon, the signal must be relayed to target cells that signal contact (muscle/nerve cells). Signal is transmitted to junction synapses. At most, the plasma of cells presynaptic (receiving)/postsynaptic (transmitting) the message are separated by the synaptic cleft, to which the electrical signal cannot cross. TO transmit, electrical signal is turned to chemical in the form of a neurotransmitter. Initially stored in the nerve terminals within membrane enclosed synaptic vesicles. When the action potential reaches the nerve terminal, some of the synaptic vesicles fuse with the plasma membrane, releasing neurotransmitters to the synaptic cleft--activates voltage-gated- Ca2+ channels, concentrated in the plasma membrane of the presynaptic nerve terminal. Because the concentration outside the terminal is 1000x greater than free Ca2+ in the cytosol, Ca2+ rushes in through the open channels--increase triggers membrane fusion that releases neurotransmitters--secreted into the synaptic cleft--binds to neurotransmitter receptors concentrated in the postsynaptic plasma membrane of the target cell--causes change in the membrane potential of the target cell which if big enough can fire action potential. Neurotransmitter is renewed from the synaptic cleft by enzymes/uptake into neighboring non-neural cells. Limits duration and spread of signal to quiet both pre/postsynaptic cell. Different types of neurotransmitters--rapid responses depend on receptors that are transmitter-gated ion channels--constitute a subclass of ligand-gated ion channels and function is to convert a chemical signal carried by neurotransmitters back into an electrical one. Channels open transiently in response to binding and changing ion permeability of postsynaptic membrane. Causes change in membrane potential. If the change is big enough, it will depolarize the post-synaptic cell and trigger an action potential. Good example is at the neuromuscular junction--special synapse is formed between the motor neuron and skeletal muscle cells-in vertebrae, the neurotransmitter is acetylcholine, and gate ion channel is acetyl receptor. But not all neurotransmitters excite the postsynaptic cell. Can either excite/inhibit, character of receptor that determines how the postsynaptic cell will respond. Receptors for excitatory neurotransmitters are ligand-gated cation channels-when bound, open for influx of Na+ which depolarizes the plasma membrane and activated. Inhibitor receptors for example: GABA and glycine are ligand-gated Cl- channels--increases permeability to Cl-, inhibits postsynaptic cell by making it harder to depolarize (curare (toxin)) causes muscle paralysis when bound to receptors--strychnine (rat poison) causes muscle spasms, death by blocking inhibitory glycine receptors on neurons in CNS. Many drugs for treatment for insomnia, anxiety, depression, schizophrenia act by binding to the transmitter-gated ion channels. Sedatives/tranquilizers affect by binding to GABA-gated Cl- channels--makes even more sensitive to GABA inhibition. Number of distinct types of neurotransmitter receptors that fall into a small number of families with subtle electrophysiological properties. For a signal to pass from one neuron to the next, nerve terminal of presynaptic cell converts an electrical signal to chemical, then diffusion across the synaptic cleft to the postsynaptic cell can convert to electric signal. Chemical synapses allow for combining signals, interpreting, recording, computing appropriate output from many inputs advanced by interplay of different types of ion channels in a neuron’s plasma membrane. In addition to integrating chemical inputs, can adjust the magnitude of the response--synaptic plasticity triggered by Ca2+ influx through special channel in the postsynaptic plasma membrane, which leads to functional alterations, number of neurotransmitters released, way postsynaptic cell responds or both--play important part in learning/memory. Photosynthetic algae use light gated channels to sense and go to sunlight. In response to blue light--channel rhodopsin (ex: aggressive mouse when expose to blue light in hypothalamus)--optogenetics.
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