Lesson 3 – Video 1
1085 [00:00:00.77] SPEAKER: Hi. In this set of lectures, we'll talk about Cell Energetics and
1086 Catalysis. We'll show that basic thermodynamic considerations apply equally well to life
1087 phenomena. We'll talk about reactions and how free energy controls their direction,
1088 relative amounts of equilibrium. We'll talk about activation energies controlling reaction
1089 [00:00:26.22] We'll talk about how the cell does energetically unfavorable reactions by
1090 them to energetically favorable ones, which are always more favorable in magnitude
1091 desired unfavorable reactions are. We'll give an example of how enzymes reduce
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
1094 solve that issue.
1095 [00:01:05.62] The simplest view of energetics of cells is that we consume food. And we
1096 two things; we break the food down, we get useful forms of energy, and we get building
1097 of molecules that we want to make. Then we can combine these building blocks using
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
1100 photosynthesis sugars from sunlight, and carbon dioxide. And then use that energy
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
1106 system. And that living cells to create their order, create more than sufficient disorder
1107 up heat to the environment that overall the systems disorder increases our entropy
1108 disorder. And although inside the cell, we have areas of highly regulated order.
[00:02:37.44] It is the Gibbs free energy that determines whether 1109 reactions occur
1110 And on this slide, we see that the free energy of this reaction would be the difference
1111 and C. Because of the law of microscopic reversibility, sum Y will convert to X, a
1112 amount of X will convert to Y, there's an equilibrium. The amounts of reactant and
1113 be in equilibrium. They are determined by the ratio of the forward and reverse reaction
1114 [00:03:13.43] The forward reaction rate is much faster because the activation energy
1115 forward reaction is the difference between A and B. Where as the activation for the
1116 reaction is the difference between A and C. Since this has such a large activation
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
1119 equilibrium amounts. When enzymes or other catalysts speed reactions, they do so by
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
1122 speeds up. But again, there relative ratios have a relationship governed by their free
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
1126 changed the direction of the reaction, you have not changed the relative amounts of
1127 and reactants when you're at equilibrium. All that you've changed is the rate at which
1128 approach equilibrium.
1129 [00:04:41.90] And here I've put in some numbers just to discuss that the free energy of
1130 reaction is minus 15 Kcals, starting with the react B down to C. And that's the direction
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
1133 amounts of X, faster rates of conversion from Y to X, and slower rates converting X to
1134 two reactions, you can add the delta G val