Chapt. 12, Movement Across Membranes. Chapt. 12, Movement through lipid bilayer. Chapt. 12, Movement through lipid bilayer

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1 Chapt. 12, Movement Across Membranes Two ways substances can cross membranes Passing through the lipid bilayer Passing through the membrane as a result of specialized proteins 1 Chapt. 12, Movement through lipid bilayer Hydrophobic molecules and small polar molecules can diffuse through a synthetic lipid bilayer or the lipid bilayer of a real biological membrane. (Fig. 12-2) 2 Chapt. 12, Movement through lipid bilayer Larger polar molecules, cannot rapidly diffuse through the bilayer. 3 1

2 Chapt. 12, Movement through lipid bilayer Larger polar molecules, cannot rapidly diffuse through the bilayer. 4 Chapt. 12, Movement through lipid bilayer Ions or charged molecules cannot rapidly diffuse through the bilayer. (Fig ) Ions are small. Why can t they diffuse through? 5 Chapt. 12, Protein Based Transport Many charged or large polar molecules do enter and exit cells. This requires membrane proteins. A simple proof: 6 Fig

3 Chapt. 12, Protein Based Transport The two classes of membrane transport proteins. Similarities and differences. (Fig. 12-2) 7 Chapt. 12, Protein Based Transport The cellular concentrations of ions and metabolites are very different on the inside and the outside of the cell. 8 Chapt. 12, Protein Based Transport Ions The inside has much less Na+ and much more K+ than outside. Other ions more common outside include Ca++, Mg++, and Cl-. Fixed anions are much more common inside (but never diffuse out) Summary table

4 Table Chapt. 12, Protein Based Transport Metabolites or other organic molecules One of the major functions of the plasma membrane is to contain metabolites or other molecules necessary for cellular functioning. Some organic substances are rapidly imported into certain cells. 11 Chapt. 12, Carrier Proteins Carrier Proteins are largely responsible for the differences in concentration of substances inside and outside of cells. 12 4

5 Chapt. 12, Carrier Proteins Some examples of carrier proteins in cells. (Fig. 12-5) 13 Chapt. 12, Carrier Proteins Nomenclature -- types of transport mediated by carrier proteins. (Fig ) 14 Chapt. 12, Carrier Proteins Mechanism of action. (Fig. 12-7) Molecular recognition/binding Allosteric conformational change Solute release Return to original conformation 15 5

6 16 Chapt. 12, Carrier Proteins Sound familiar? (I hope) 17 Chapt. 12, Carrier Proteins Similarities between enzymes and carrier proteins: 18 Specificity in binding Release of products Can only carry out events with a negative G Can be coupled to an energy source to carry out half reactions that otherwise would have a positive G. 6

7 Chapt. 12, Carrier Proteins Further similarities between enzymes and carrier proteins: Speeds up a permissible (=spontaneous) reaction. It does so by lowering the energy of the transition state. 19 Chapt. 12, Carrier Proteins Compare typical reaction: A ----> B with carrier based transport: X in ----> X out 20 Chapt. 12, Carrier Proteins Similarities in kinetics: Vmax Km Design an experiment to determine Vmax and Km. Be specific. 21 7

8 Chapt. 12, Carrier Proteins Active and Passive Transport (=facilitated diffusion) Fig Chapt. 12, Carrier Proteins There is something wrong with this figure. What is it? 23 Chapt. 12, Carrier Proteins For uncharged molecules the free energy gradient is really the same as the concentration gradient and the diagram is O.K. 24 8

9 Chapt. 12, Carrier Proteins However, for any charged particle, the free energy differences is a composite of the concentration gradient and the charge gradient. This combined gradient is called the electrochemical gradient, and the energy difference for the particle is called the electrochemical potential. (Fig 12-7) 25 Chapt. 12, Carrier Proteins (Fig 12-8; alternative version) Extra panel 26 Chapt. 12, Carrier Proteins Passive transport thus can be defined as transport in which the transported molecule drops down the electrochemical gradient (and thus the free energy gradient) Active transport can be defined as transport in which the transported molecule is moved up the electrochemical gradient. 27 9

10 Chapt. 12, Carrier Proteins Active transport can be powered by: Co-transport of another substance down its energy gradient ATP hydrolysis Light energy Fig Chapt. 12, The Na + /K + Pump A reminder: K + is much more common inside cells than outside; Na + is much more common outside cells than inside. How did it get that way? Lets us consider what this fact alone can tell us. 29 Chapt. 12, The Na + /K + Pump Lets us consider what this fact alone can tell us. 30 We have seen that an ion can diffuse up its concentration gradient in response to an electrical gradient. Could this explain these results? No! Both ions are positive. You cannot attract both ions in different directions with an electrical gradient. 10

11 Chapt. 12, The Na + /K + Pump Lets us consider what this fact alone can tell us. If these ion distributions cannot be brought about by facilitated diffusion, what is the other alternative? A: at least one (and probably both) ions must be pumped against their electrochemical gradients. 31 Chapt. 12, The Na + /K + Pump Lets us consider what this fact alone can tell us. If you had to guess, how do you suppose that this pump would be powered? ATP is a logical choice. 32 Chapt. 12, The Na + /K + Pump Lets us consider what this fact alone can tell us. Where should the K + binding site be located? (On the portion of the pump facing the cytosolic or non-cytosolic side?) Where should the Na + binding site be located? 33 Where should the ATP binding site be located? 11

12 Chapt. 12, A Model for the Na + /K + Pump 34 Fig Chapt. 12, Functions of the Na + /K + Pump This pump is very expensive -- it can use 30% to 70% of the ATP used by an animal cell. What are these gradients used for? 36 12

13 Chapt. 12, Functions of the Na + /K + Pump This pump is very expensive -- it can use 30% to 70% of the ATP used by an animal cell. What are these gradients used for? Powering co-transport. (Fig , 12-15) Fig

14 Chapt. 12, Functions of the Na + /K + Pump What are these gradients used for? The ion gradients are responsible for electrically active cells (considered in more detail later). 40 Chapt. 12, Functions of the Na + /K + Pump What are these gradients used for? In many animals, the pump is necessary to prevent osmotic lysis. Typically more non-water molecules inside than outside; water flows down its own concentration gradient into the cell and the cell bursts. Made worse by Na+ and Cl- diffusing in. Na + /K + Pump pumps out Na+, also results in negative membrane charge which repels Cl-. 41 Chapt. 12, Other Important Pumps The H+ pump. Importance in some organelles. Importance in plants, fungi and bacteria. (Fig ) 42 14

15 Chapt. 12, Other Important Pumps The Ca++ pump. Well understood Importance 43 Fig 12-6 Chapt. 12, Ion Channels Ion channels are like doors They are often gated. They can be gated in different ways. 44 Fig Chapt. 12, Ion Channels Ion channels are like doors 45 They show ion selectivity. Sometimes pass only 1 particular ion. Sometimes pass multiple similar ions. Fig

16 Chapt. 12, Ion Channels Ion channels can be in either open or closed states. The evidence (Fig ) Fig Chapt. 12, Ion Channels Channels are either all they way open or all the way closed. (Fig ) 48 16

17 Chapt. 12, Ion Channels and Membrane What is membrane potential? 49 The difference in total charges on the opposite sides of a membrane. Membrane potential can easily be measured (as we just saw). Where does the membrane potential come from? Cannot find free negative or positive charges on the shelf of chemicals. Cell inside Cell outside 10,000 Na+ 140,000 K+ 1 Ca++ 10,000 Cl- 139,999 other neg charges 145,000 Na+ 5,000 K+ 1,000 Ca++ 110,000 Cl- 42,000 other neg charges total net charge = 0 total net charge =0 Difference in charges = 0-0 or none 50 Suppose a Na+ channel opened and 1000 Na+ diffused down their electrochemical gradient... Cell inside 11,000 Na+ 10,000 Na+ 140,000 K+ 1 Ca++ 10,000 Cl- 139,999 other neg charges 144,000 Na+ 145,000 Na+ 5,000 K+ 1,000 Ca++ 110,000 Cl- 42,000 other neg charges Cell outside total net charge = total net charge = Difference in charges = 1000 minus = 2,000 17

18 Suppose a K+ channel opened and 1000 K+ diffused down their electrochemical gradient... Cell inside 10,000 Na+ 139,000 K+ 140,000 K+ 1 Ca++ 10,000 Cl- 139,999 other neg charges 145,000 Na+ 6,000 K+ 5,000 K+ 1,000 Ca++ 110,000 Cl- 42,000 other neg charges Cell outside total net charge = total net charge = Difference in charges = minus = - 2,000 Chapt. 12, Ion Channels and Membrane What have we learned? The membrane potential is due to differing net charges on each side of the membrane. Changes in membrane potential are due to ions moving across the membrane. Because ions do not penetrate the hydrophobic interior of the lipid bilayer, they must pass through carrier proteins or channel proteins. 53 Chapt. 12, Ion Channels and Membrane The equilibrium potential: Cell inside Let us consider again this figure. The inside of the cell is to the left. There is a large difference in Na + concentrations. What happens if we open up if we open up Na + channels only? 10,000 Na+ 140,000 K+ 1 Ca++ 10,000 Cl- 139,999 other neg charges 145,000 Na+ 5,000 K+ 1,000 Ca++ 110,000 Cl- 42,000 other neg charges total net charge = 0 total net charge =0 Difference in charges = 0-0 or none Cell outside 54 18

19 Chapt. 12, Ion Channels and Membrane What happens if we open down their electrochemical gradient... up Na+ channels only? Na + flows in. Changes membrane potential. Changes Na + concentration. Will Na + continue to diffuse in until [Na + ] in = [Na + ] out? 55 Suppose a Na+ channel opened and 1000 Na+ diffused Cell inside 11,000 Na+ 10,000 Na+ 140,000 K+ 1 Ca++ 10,000 Cl- 139,999 other neg charges 144,000 Na+ 145,000 Na+ 5,000 K+ 1,000 Ca++ 110,000 Cl- 42,000 other neg charges Cell outside total net charge = total net charge = Difference in charges = 1000 minus = 2,000 Chapt. 12, Ion Channels and Membrane Will Na + continue to diffuse in until [Na + down their electrochemical gradient... ] in = Cell inside [Na + 11,000 Na+ 144,000 Na+ ] out? No, before long the positive interior of the cell will balance out the greater concentration of Na + on the outside. Suppose a Na+ channel opened and 1000 Na+ diffused 10,000 Na+ 140,000 K+ 1 Ca++ 10,000 Cl- 139,999 other neg charges 145,000 Na+ 5,000 K+ 1,000 Ca++ 110,000 Cl- 42,000 other neg charges Cell outside total net charge = total net charge = Difference in charges = 1000 minus = 2, Chapt. 12, Ion Channels and Membrane There are so many ions on the inside and outsides of cells that this usually does not changes the ion s concentration very much. Fig

20 Chapt. 12, Ion Channels and Membrane So, now we can define the equilibrium potential: The membrane charge where the component of the electric portion of the electrochemical gradient exactly balances the concentration portion of the electrochemical gradient. Different for every ion. Depends on: 58 The relative concentrations of the ion on the inside v.s. the outside of the cell. The charge on that ion. Chapt. 12, Ion Channels and Membrane The resting potential of most cells is negative. The Na + /K + pump (a minor contributor) K + leak channels 59 Chapt. 12, Ion Channels and Membrane The voltage gated Na+ channel is responsible for the action potential of electrically active cells including nerve and muscle. What is an action potential? Fig

21 Fig Chapt. 12, Ion Channels and Membrane The three states of the voltage gated Na+ channel. 62 Fig Movement of the Na+ ion and the action potential. 63 Fig

22 The action potential propagates (=regenerates) along the membrane in one direction. 64 Fig The explanation for unidirectional propagation. 65 Chapt. 12, Ion Channels and Membrane Other channels participate in nerve transmission. The voltage gated K+ channel. The voltage gated Ca++ channel at the axon terminus. (Fig ) 66 22

23 Fig Chapt. 12, Ion Channels and Membrane Other channels participate in nerve transmission (cont.) The acetlycholine gated cation channel. Fig Chapt. 12, Ion Channels and Membrane How does the acetylcholine gated cation channel initiate a response? 69 Fig

24 Chapt. 12, Ion Channels There are synapses that make an action potential more likely (excitatory) or less likely (inhibitory) 70 Fig

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