Transport Across Cell Membranes PDF

Transport Across Cell Membranes Learning Objectives: Understand how transport proteins move specific molecules across a membrane Understand the principles that drive passive, coupled and active transport Understand how several tra...

Transport Across Cell Membranes Learning Objectives: Understand how transport proteins move specific molecules across a membrane Understand the principles that drive passive, coupled and active transport Understand how several transport proteins can be used together in the context of a cell Understand the This is movie 12.4 (also on Canvas in Lecture 8 mod concept of channels Understand how an electrochemical gradient is established Today’s topics: 1. Transporters vs channels 2. Electrochemical gradients 3. Passive transport 4. Active transport 5. How to make ATP  Eukaryotic cells have membrane-bound organelles plasma membrane Figure 1-24A Essential Cell Biology  Membrane transport proteins provide cells with control over their composition These molecules cross membranes through membrane Figure 12-1 and 12-2 Essential Cell Biology transport proteins  Membrane transport proteins fall into two classes: channels and transporters channels K + on ly ! Discriminate based on: size and charge many ions can pass quickly transporters y! cos e onl G lu Discriminate based on: direct binding transports one (or few) molecule(s) at a time Figure 12-3 Essential Cell Biology Passive Transport does not require additional energy (uncharged) molecules transporter Passive transport No additional energy needed: molecules move “down” (in direction of) their concentration gradient. Active Transport requires additional energy (uncharged) molecules Active transport Energy is needed: molecules move “up” (against) their concentration gradient. What about charged molecules? Today’s topics: 1. Transporters vs channels 2. Electrochemical gradients 3. Passive transport 4. Active transport 5. How to make ATP  For charged molecules, the ‘electrochemical gradient’ determines the direction of passive transport the ‘electrochemical gradient’ is a combination of two gradients: (1) the voltage (electrical) gradient and (2) the concentration (chemical) gradient membrane electrochemical potential gradient charge differential) For an uncharged molecule, membrane potential is irrelevant: The ‘electrochemical gradient’ is equal to the ‘concentration’ gradient Figure 12-5 Essential Cell Biology Today’s topics: 1. Transporters vs channels 2. Electrochemical gradients 3. Passive transport 4. Active transport 5. Putting it all together Passive transport by channels: molecules move “down” their electrochemical gradient Cell electrochemical membrane gradient Channels can only mediate passive transport  Channels allow specific molecules to diffuse across cell membranes “side” view “top-down” view K+ ion Figure 12-1 and 11-24 Essential Cell Biology Figure 11-48 Lehninger: Principles of Biochemistry  Many channels can open and close top view Figure 12-21 Essential Cell Biology and Figure 11-25 Molecular Biology of the Cell ted” channels respond to different types of sti Figure 12-27 Essential Cell Biology Passive transport by Transporters moves molecules “down” (with) their electrochemical gradient Cell electrochemical membrane gradient Some transporters mediate passive transport (other transporters mediate active transport) Passive transporters permit flow of a solute, but play no role in determining the direction of flow!  The glucose transporter mediates passive transport Transport requires conformational changes of the transporter ells evolve transporters, rather than channels, for molecules like Figure 12-9 Essential Cell Biology Today’s topics: 1. Transporters vs channels 2. Electrochemical gradients 3. Passive transport 4. Active transport 5. Putting it all together  Active transport uses energy to move molecules “up” (against) their electrochemical gradients Active transport proteins use the same energy “coupling” principle that we discussed for enzyme-catalyzed coupled reactions. Gradient-driven ATP-driven Light-driven pump pump pump Figure 12-10 Essential Cell Biology  The Na+-K+-ATPase is an ATP-driven pump (aka “Na+-K+ pump” or “Na+ pump”) It utilizes about 30% of the total ATP hydrolysis in an animal. How does it work? Figure 12-11 Essential Cell Biology e Na+-K+ pump transports ions in cyclical mann ATP ADP Hydrolysis of one ATP transfers 3Na+ to the outside and 2K+ to the inside per cycl Figure 12-12 (modified) (see movie 12.2) Essential Cell Biology You have prepared lipid vesicles that contain Na +-K+ pumps as the sole membrane protein. All pumps are oriented so that the portion of the molecule that is normally in the cytosol faces the outside of the vesicle (except in D where they have a random orientation). You set up the system to have significant concentrations of the ions indicated. Which of the following conditions will generate a Na+ concentration gradient? Inside Outside vesicle vesicle A. The solution inside and outside the vesicles K+ Na+ contains both Na+ and K+, but no ATP. Na+ K+ B. The solution inside the vesicles contains both Na+ K + ATP Na+ and K+; the solution outside contains Na+ K + both ions and ATP. C. The solution inside contains Na+; the Na+ ATP solution outside contains Na+ and ATP Na + D. The solution is as in B, but the Na+-K+ Na+ pump molecules are now randomly K + ATP Na+ K+ oriented, some facing one direction and some the other. E. More than one of the conditions listed above will generate a Na+ concentration gradient.  You have prepared lipid vesicles that contain Na +-K+ pumps as the sole membrane protein. All pumps are oriented so that the portion of the molecule that is normally in the cytosol faces the outside of the vesicle (except in D where they have a random orientation). You set up the system to have significant concentrations of the ions indicated. Which of the following conditions will generate a Na+ concentration gradient? Inside Outside vesicle vesicle A. The solution inside and outside the vesicles K+ Na+ No, need ATP contains both Na+ and K+, but no ATP. Na+ K + B. The solution inside the vesicles contains both Na+ and K+; the solution outside contains Na+ Yes! K + ATP both ions and ATP. Na+ K + C. The solution inside contains Na+; the No, need K+ solution outside contains Na+ and ATP Na+ ATPto Na transport + Na+ Yes! D. The solution is as in B, but the Na -K + + The inside- pump molecules are now randomly Na+ out pumps K+ ATPdon’t have oriented, some facing one direction and Na+ K+ access to some the other. ATP on their cytosolic E. More than one of the conditions listed above will domain, so generate a Na+ concentration gradient. they can’t pump  Gradient-driven transport is driven by the flow of a molecule down its electrochemical gradient Figure 12-13 Essential Cell Biology  Gradient-driven transport is driven by the flow of a molecule down its electrochemical gradient (passive) or symport and antiport: Most of the examples we know of are considered “active” transport, but some examples are passive. (your textbook implies that symport and antiport always involve active transp Active’ if at least one molecule is moving up its electrochemical gradient Passive’ if both molecules are moving down their electrochemical gradients Figure 12-15 Essential Cell Biology echanism for glucose-Na+ coupled active transp Transport of glucose up its concentration gradient is driven by Na+ moving down its electrochemical gradient Both molecules are moved into the cell (example of symport) Na+ gradient is established Pump changes conformations by the Na+-K+ randomly ATPase! But glucose and Na+ only bind strongly when Figure 12-16 Essential Cell Biology both are bound at the same time ifferent transporters are involved in glucose uptake fr 1. glucose-Na+ symport (active) 2. glucose uniport (passive) 3. Na+-K+ pump (active) Figure 12-17 (see movie 12.5) Essential Cell Biology Today’s topics: 1. Transporters vs channels 2. Electrochemical gradients 3. Passive transport 4. Active transport 5. How to make ATP  The inner structure of mitochondria (RFP) mitochondria nucleus Outer membrane Inner membrane Folded inner membrane (“cristae”) A section through a mitochondrion imaged by electron microscope  Three H+ pumps generate an electrochemical gradient (H+) across the inner membrane of mitochondria glucose ATP synthase uses the H+ gradient to synthesize ATP glucose ATP synthase  If PollEV did not work for you, Please sign the attendance sheet in the front