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TAMU BIOL 213 - The Basics of Membrane Transport
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BIOL 213 1st Edition Lecture 8 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 accomplished i. Flipase V. Membrane proteinsa. Different functionsb. Different associationsc. Mobility VI. Detergentsa. Useful for studying integrated proteinsThese 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.VII. The cell’s surface is coated with carbohydratesa. Lubricantb. Cell-cell signalingOutline of Current Lecture I. Remember that membranes are selectively permeablea. What can and can’t diffuse simplyII. Membrane transport proteinsa. Carrier and channel proteinsb. Active or passive transportc. Na+ - K+ gradientIII. Passive transporta. Channel proteinsb. Carrier proteinsi. Uncharged moleculesii. Charged moleculesIV. Active transporta. ATPb. Coupled transportersV. Differentiating between carrier and channel proteinsCurrent LectureI. Remember that membranes are selectively permeablea. Some molecules can cross via simple diffusioni. Small hydrophobic moleculesii. Small uncharged polar molecules1. These can diffuse because they’re small enough so that the polarity doesn’t interact with the hydrophobic interior of the membrane2. Water is NOT included in this list – it is aided in diffusion by transport proteins called aquaporin3. The amount of water that does simply diffuse is insignificant to biologyb. Other molecules can’t cross via simple diffusioni. Large uncharged polar molecules – too bigii. Ions 1. Even though they’re small, they’re extremely charged and they don’t want to interact with the hydrophobic interiorII. Membrane transport proteinsa. These aid in the transport of molecules that can’t cross the membrane via simplediffusion i. Ex: nucleotides, sugars and amino acids – these are needed to growb. Two main kinds of proteinsi. Carrier proteins1. They have an outward and inward conformation2. The conformation changes depending on whether or not a specificmolecule is bound to it3. Can be passive or active4. Three main kinds: uniporter, symporter, and antiporter (discussed later)ii. Channel proteins 1. These are either open or closed at both ends2. They are like a channel, as the name suggests3. These ALWAYS transport molecules passively c. Either passive or active transporti. Passive transport1. Also known as facilitated diffusion2. No energy input is required to transport molecules3. The molecules move down their gradientsa. This movement has a negative ΔG4. The proteins involved in passive transport increase the rate of diffusiona. The rate of facilitated diffusion eventually plateaus as the diffusing molecule concentration increases because eventually all of the transport proteins will be in useb. The rate of simple diffusion never plateaus in biology because there is essentially and infinite number of places in the cell membrane that a molecule can diffuse through5. Both carrier and channel proteins are involved in passive transportii. Active transport1. Energy must be spent to transport a molecule against its gradient2. Only carrier proteins transport molecules activelyd. They create the Na+ and K+ gradients that are important to a celli. Concentration gradient: a molecule will naturally diffuse from an area of high concentration to an area of low concentration1. This diffusion is energetically favorable2. Therefore some concentration gradients can be coupled with energetically unfavorable reactions to drive themii. Na+ concentration is higher inside the cell than outside1. Cells use transport proteins to shuttle Na+ out of the cell, against its concentration gradient2. The Na+ ions naturally want to diffuse back into the cell3. This diffusion back into the cell can be used to drive energetically unfavorable reactions or diffusionsiii. K+ concentration is higher inside the cell than outside1. Cells use transport proteins to shuttle K+ in the cell, against its concentration gradient2. K+ ions naturally want to diffuse out of the cell3. This diffusion out of the cell can be used to drive energetically unfavorable reactions/diffusionsiv. The concentration gradients of Na+ and K+ are commonly used in a cell to drive unfavorable reactions1. For every 2 K+ pumped out of the cell, 3 Na+ are pumped in 2. It is used to maintain osmotic pressure by controlling the Na+ and (indirectly) the Cl- concentrations, which influences the diffusion of watera. The water will go towards wherever there is a higher concentration of solutes3. It can cause “movement” in plantsa. Venus fly trapb. Mimosa plantc. This causes a change in osmotic pressure in the cells, which allows the plant to “move”III. Passive transporta. Channel proteinsi. They are always passive because they allow molecules that can’t diffuse simply to diffuse down their concentration gradientsb. Carrier proteinsi. Uncharged molecules1. A protein is in its outward facing conformation, meaning it’s open to the outside of the cell2. A solute from the outside binds to the binding site3. This causes a conformational change in the protein4. The protein changes to its inward facing conformation (it’s open tothe inside of the cell)5. The solute detaches from the protein6. The solute is now inside the cell7. No energy was require to drive this; the solute moved down its concentration gradient8. The direction of transport depends entirely upon the concentrationgradientii. Charged molecules1. The direction of transport also depends on the electrochemical gradienta. The electrochemical gradient is caused by the net charge on a cell membranei. This is known as the membrane potential (E) ii. The net charge on the inside of a cell is negative relative to the outside of the celliii. It exerts a force on charged moleculesiv. ΔG = RT ln [solute]inside/[solute]outside + FEM b. The electrochemical gradient influences the amount of diffusioni. Positive molecules will diffuse more across the membrane if the other side of the membrane is negatively chargeii. They will not diffuse as easily if the membrane is positively chargediii. Negative molecules will diffuse more easily across the membrane if the other side of the membrane


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TAMU BIOL 213 - The Basics of Membrane Transport

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