BIOMG 1350 Notes Week 5
BIOMG 1350 Notes Week 5 BIOMG 1350
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This 8 page Class Notes was uploaded by genehan on Saturday September 3, 2016. The Class Notes belongs to BIOMG 1350 at Cornell University taught by Garcia-Garcia, M; Huffaker, T in Fall 2015. Since its upload, it has received 6 views. For similar materials see Introductory Biology: Cell and Developmental Biology in Molecular Biology and Genetics at Cornell University.
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Date Created: 09/03/16
BIOMG 1350 Professor Bretscher & GarciaGarcia Spring 2016 Week 5: Lecture 1 of 2 Monday, Feb 22, 2016 Lecture Title: Membrane Structure ß Lecture Keywords: “cocked state,” “power stroke,” rigor, plasma membrane, Fluid Mosaic Model, amphipathic, phospholipids, cholesterol, flippases, glycolipids, pore, FRAPß I. Unlike kinesin, myosin moves nonprocessively and independently because it must be able to release itself from the actin filament to move. Only one of the myosin heads needs to be considered to understand its movement, so coordination between the heads is not necessary. a. Myosin couples ATP hydrolysis to conformational change. b. It hydrolyzes ATP to ADP and P and the myosin head becomes “cocked” and then the myosin head binds to the actin filament. c. This induces the release of the phosphate, making the head swing backwards and moves the filament to the left. This is the “power stroke.” d. ADP remains bound until ADP is replaced by ATP and then the head is released from actin. e. When you die, you go into rigor because your ATP is hydrolyzed so it is in the ADP state, tightly associated with actin, so your muscles are stiff and tense. II. Iclicker question – Which of the following is false? a. Kinesin and myosin both move towards the plus end of their respective filaments. (True – kinesin moves towards plus ends of microtubules and myosin moves towards plus end of actin.) b. Kinesin moves in a hand over hand fashion but muscle myosin does not. (True – kinesin moves processively but coordination between heads for myosin is not observed.) c. Kinesin binds microtubules more tightly in the ATPbound state whereas myosin binds actin filaments more tightly in the ADP bound state. (True – remember rigor mortis) d. The ER is distributed by dynein, whereas the Golgi complex is concentrated by kinesin. (False – it is the opposite) e. Muscle myosin heads in thick filaments pull in opposite directions in a sarcomere. (True) III. Membranes act as selective barriers. a. Prokaryotes have only a plasma membrane while eukaryotes have internal membranes as well to enclose intracellular molecules. b. The plasma membrane is a barrier with proteins that allow it to receive information, import and export molecules, and have the capacity to shape the membrane to move and expand. It is the interface with the cell’s environment. c. Membranes are made of 50% lipids and 50% proteins, known as the Fluid Mosaic Model. The lipid bilayer measures 5 nanometers (If the cell were the size of a watermelon, the bilayer would be as thick as a sheet of paper.) IV. Membrane lipids are amphipathic and the most abundant lipids in membranes are phospholipids. a. If the tails are saturated, the tails are relatively straight. b. Some major phospholipids in membranes are phosphatyidylethanolamine PE (net zero charge), phosphatyidylserine PS (net negative charge, phosphatyidyl choline PC (net zero charge), and sphingomyelin. c. In addition to phospholipids, membranes have cholesterol and glycolipids that are both amphipathic as well. V. A phospholipid has a hydrophilic head and two hydrophobic fatty acid tails, so phospholipids form a bilayer and the heads interact with water while the tails do not. a. They spontaneously close to form sealed compartments because this is energetically favorable. VI. Iclicker question – Which of the following statements about a pure lipid bilayer is false? a. Lipids readily exchange places with one another by lateral movement. (True – lipids can diffuse rapidly within one plane of bilayer.) b. Lipids readily rotate around their long axis. (True) c. Lipids rarely flip from one side of the bilayer to the other. (True, this would require the hydrophilic polar head to pass through the hydrophobic bilayer.) d. Lipid lateral motion/fluidity increases when hydrocarbon tails are shortened. (True – there is less interaction between tails) e. Lipid fluidity decreases with increasing proportion of unsaturated hydrocarbon tails. (False – kinked tails increase fluidity.) VII. Phospholipids rapidly rotate and readily laterally diffuse in the membrane, but flipping across the layer is rarely observed. a. If double bonds are present, it becomes more fluid (unsaturated). b. Cholesterol stiffens membranes, reducing fluidity in the membrane bilayer. c. The plasma membrane lipid bilayer is asymmetric, due to the activity of enzymes called flippases. i. Glycolipids are always on the outer extracellular space. ii. PS and PE are in the inner leaflet in the cytosol, which is net negatively charged. d. Synthetic lipid bilayers block the passing of most watersoluble molecules. i. Small hydrophobic molecules such as oxygen, carbon dioxide, steroid hormones can get through. ii. Small uncharged polar molecules such as water, glycerol and ethanol can get across but not as efficiently. iii. Larger uncharged polar molecules such as amino acids cannot get across such a bilayer. VIII. Plasma membrane proteins have different functions, such as transporters, anchors, receptors, and enzymes. IX. Membrane proteins associate with the lipid bilayer in different ways such as transmembrane (alpha helices across the membrane), monolayerassociated, some are modified to attach a fatty acid (lipidlinked) and some are indirectly associated with proteins (proteinattached). a. A peptide chain crosses the bilayer as an alphahelix so that the polar groups are hydrogen bonded in the middle of the helix so the hydrophobic R groups point outwards. b. The primary sequence of an amino acid can predict the number of membrane spanning segments by determining the number of hydrophobic and hydrophilic segments. c. A hydrophilic highly selective pore can be formed by multiple transmembrane alpha helices through the lipid bilayer. X. Fluorescence Recovery After Photobleaching FRAP is an experiment to test protein mobility in membranes. (more in active learning section) a. Membrane proteins that are not anchored are fused with GFP to make certain proteins visible as they diffuse. b. An area in the membrane is bleached with a laser beam, and the fluorescence comes back at a certain rate, which can tell us the diffusion coefficient of that protein in the bilayer. XI. Lateral mobility of plasma membrane proteins is restricted by attachment to molecules inside or outside the cell, adhesions between cells, or there can be a barrier between one region of the cell so that proteins from one region cannot diffuse into another region. XII. A spectrin meshwork forms the cell cortex in red blood cells and lots of diseases are associated with defects in red blood cells that rupture because they cannot withstand the torrent of the heart. BIOMG 1350 Professor Bretscher & GarciaGarcia Spring 2016 Week 5: Lecture 2 of 2 Wednesday, Feb 24, 2016 Lecture Title: Membrane Transport Lecture Keywords: Channels, transporters, passive transporter, glucose transporter, membrane potential, electrochemical gradient, active transport, uniport, symport, antiport, glucoseNa+ coupled active transport, sodiumpotassium pump, ion channels, gated channels Prelim #1 on Wednesday March 2 during class time ((9:059:55) – covers all materials from lectures 18, required readings, and active learning sections. I. Iclicker question – Using FRAP to measure diffusion coefficients in the plasma membrane, which of the following statements is correct? a. Lipids diffuse slower than proteins. (No, lipids diffuse faster than proteins.) b. Lipids with saturated fatty acids will diffuse faster than lipids with unsaturated fatty acids. (No, lipids with saturated fatty acids are less fluid.) c. The diffusion rate of proteins is influenced by the lipid composition of the membrane. (True – a membrane in which the lipids diffuse slowly will slow the diffusion of membrane proteins. Imagine wading through bacon fat.) d. Large proteins always diffuse more slowly than small ones. (No, it depends on what the proteins are attached to.) e. Membrane proteins interacting with the cytoskeleton do not diffuse in the plane of the membrane. (No, they diffuse more slowly.) f. The first prelim is in a week from today and includes all material in active learning sections and lectures up to, and including, today’s. II. Small nonpolar molecules and uncharged polar molecules can get across a synthetic lipid bilayer, but large uncharged polar molecules and ions such as amino acids, glucose, Na+, H+, K+, etc. cannot without assistance. a. Pure lipid bilayers are electrical insulators. b. Transport proteins are used to move watersoluble molecules. The selective pore transports specific molecules across. III. Membrane transport proteins include channels and transporters. a. Channels act like a door opening but only allowing a large amount of a certain molecule through. All channels only do passive transport. b. Transporters change between two conformation states and act like a turnstile, letting a smaller number of molecules across. IV. Passive transport is moving uncharged molecules down their concentration gradient. This permits a flux of a solute, but plays no role in determining its direction. a. When a channel opens, the molecule will go down its concentration gradient. Similarly, if the transport is transportermediated, a molecule can be transported down its gradient. b. Example – glucose transporter i. A glucose molecule is transported down its gradient until equilibrium is reached. c. For charged molecules, the membrane potential is important. i. The outside of the cell is positively charged and negatively charged on the inside. Therefore, an electrochemical gradient is established (a combination of concentration gradient, and voltage gradient). ii. In comparison, for an uncharged molecule, the membrane potential is irrelevant so the electrochemical gradient is just the concentration gradient. V. Active transport is when energy is used to move molecules against their concentration or electrochemical gradient. a. There are many ways active transport can take place – through a coupled transporter, ATPdriven pump, or a lightdriven pump. b. Both passive and active transport can be coupled. i. In a uniport, one molecule is transported ii. In a symport, two molecules must be cotransported together. iii. In an antiport, two molecules are transported but in different directions. c . GlucoseNa+ coupled active transport i. Glucose is at a lower concentration on the outside of the cell and sodium is at a higher concentration on the outside of the cell. ii. The transporter doesn’t bind glucose unless it has sodium, and vice versa. iii. The net result is that glucose is transported up its concentration gradient, driven by sodium moving down its concentration gradient. iv. The binding of NA induces a conformational change in the transporter and this increases affinity for glucose and vice versa. Thus, both molecules bind effectively only if both are present so you get net transport in one direction. v. Because of this coupling, you do not run down the sodium electrochemical gradient. d. Sodiumpotassium pump is an antiport system. i. From each ATP expended, it will pump 2 K+ ions in and 3 Na+ out. ii. This is active transport because it requires ATP hydrolysis to function. iii. 3 Na+ binds to the pump on the cytosolic side and this induces the pump to phosphorylate itself through ATP and change shape. Then, this releases Na+ outside the cell. iv. K+ comes in and binds, which induces the dephosphorylation of the pump and so it returns to the original conformation, releasing K+. v. Both ions are transported against their concentration gradient. The gradients harbor potential energy that the cell can use to do work. e. The lumen of the gut has high glucose concentration and low concentration outside through 3 mechanisms. i. On the bottom membrane, a NaK pump is used to generate the electrochemical gradient of sodium. ii. On the top membrane, glucose is transported up its gradient through a Na+ driven glucose symport. iii. On the bottom membrane, glucose is released for use by other tissues. VI. Ion channels are ionselective with a pore that only transfers specific molecules. a. K+ channels generate the membrane potential. i. The Na+K+ pump creates a high concentration of K+ inside the cell. The K+ channel allows K+ to flow out until a positive charge is established outside and a negative charge on the side. This continues until the two gradients equate. b. Gated channels respond to different types of stimuli – voltage gated, extracellular and intracellular ligandgated, and stressgated channels. i. Example of stressgated channels – in our ears, sound vibrations cause the basilar membrane to vibrate and so an electrical stimulus is activated in the hair cell, which has stressactivated channels. VII. Acid secretion in the stomach a. In its vesicle membrane, a pump transports H+ in and K+ out, but there is not enough K+ in the environment. b. Eating induces the release of a histamine hormone that binds to the receptor. This sends a signal that causes the vesicle membrane to fuse with the plasma membrane. Now there is enough K+ for the pump to function. c. K+ is cycled through the K+ channel and the pump and as H+ is pumped outside of the cell, Cl is pumped outside as well. d. Drugs are commonly used when people have too much acid secretion in their stomachs. Tagamet is a histamine receptor antagonist that inhibits binding of the histamine, which stops acid pumping into the stomach. Prilosec is a H+K+ ATPase inhibitor. VIII. Each cell membrane has its own characteristic set of transporters, not just the plasma membrane.
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