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Lecture 3

Cell Biology - Lecture 3 - Video 1 - Notes

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Boston University
CAS BI 203
Martin Steffen

Lesson 3 – Video 1 [00:00:00.00] 1085 [00:00:00.77] SPEAKER: Hi. In this set of lectures, we'll talk about Cell Energetics and Enzyme 1086 Catalysis. We'll show that basic thermodynamic considerations apply equally well to life as all 1087 phenomena. We'll talk about reactions and how free energy controls their direction, and the 1088 relative amounts of equilibrium. We'll talk about activation energies controlling reaction kinetics. 1089 [00:00:26.22] We'll talk about how the cell does energetically unfavorable reactions by coupling 1090 them to energetically favorable ones, which are always more favorable in magnitude then the 1091 desired unfavorable reactions are. We'll give an example of how enzymes reduce activation 1092 energies and then we'll talk a little bit about protein folding, which has some conceptual 1093 difficulties, which the proteins solve automatically. But it's hard for us to fully grasp how they 1094 solve that issue. 1095 [00:01:05.62] The simplest view of energetics of cells is that we consume food. And we produce 1096 two things; we break the food down, we get useful forms of energy, and we get building blocks 1097 of molecules that we want to make. Then we can combine these building blocks using some of 1098 the energy we capture in to the desired forms that we wish. 1099 [00:01:30.30] The exception to this would be plants or bacteria that instead of ingesting food, 1100 photosynthesis sugars from sunlight, and carbon dioxide. And then use that energy and those 1101 building blocks to make molecules they need. Cells do not violate the second law of 1102 thermodynamics, which states that the entropy of the system always increases. At some points 1103 there was some confusion because cells obviously become more ordered, highly ordered in fact. 1104 And this was originally seen as a contradiction. 1105 [00:02:09.71] However, the answer to this situation is that you have to consider the entire 1106 system. And that living cells to create their order, create more than sufficient disorder by giving 1107 up heat to the environment that overall the systems disorder increases our entropy rather than 1108 disorder. And although inside the cell, we have areas of highly regulated order. 31 [00:02:37.44] It is the Gibbs free energy that determines whether 1109 reactions occur spontaneously. 1110 And on this slide, we see that the free energy of this reaction would be the difference between B 1111 and C. Because of the law of microscopic reversibility, sum Y will convert to X, a smaller 1112 amount of X will convert to Y, there's an equilibrium. The amounts of reactant and product will 1113 be in equilibrium. They are determined by the ratio of the forward and reverse reaction rates. 1114 [00:03:13.43] The forward reaction rate is much faster because the activation energy for the 1115 forward reaction is the difference between A and B. Where as the activation for the reverse 1116 reaction is the difference between A and C. Since this has such a large activation energy, not 1117 many molecules make it over that barrier. 1118 [00:03:34.79] This difference between B and C, the reactant in product, will also determine their 1119 equilibrium amounts. When enzymes or other catalysts speed reactions, they do so by reducing 1120 the activation energy from A to B to a smaller value, which is shown here as D to B. Now this 1121 activation energy is smaller and Y can go even faster to the product X. The reverse reaction also 1122 speeds up. But again, there relative ratios have a relationship governed by their free energy 1123 differences. 1124 [00:04:14.72] The notice that in the UN catalyzed case and the catalyzed case, the energy of the 1125 reactants-- sorry it's so wiggly-- and the energy of the products does not change. You have not 1126 changed the direction of the reaction, you have not changed the relative amounts of the products 1127 and reactants when you're at equilibrium. All that you've changed is the rate at which you 1128 approach equilibrium. 1129 [00:04:41.90] And here I've put in some numbers just to discuss that the free energy of the 1130 reaction is minus 15 Kcals, starting with the react B down to C. And that's the direction of a 1131 spontaneous reaction. It will always be specified by a negative Gibbs free energy. 1132 [00:05:01.23] And here we schematize the amounts at equilibrium smaller amounts of Y, larger 1133 amounts of X, faster rates of conversion from Y to X, and slower rates converting X to Y. For 1134 two reactions, you can add the delta G val
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