Energy Considerations – Oxidation and Reduction in Metabolism

by Kevin Ahern, PhD

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    00:01 We live in an era where energy needs are very visible to everybody. And not only are energy needs important in the real outside world that we live in, but they're also very big considerations for the cellular environment. In this lecture, I will be talking about metabolism particularly with views with respect to energy and oxidation and reduction. First in this lecture I'll talk about energy considerations and how they relate to metabolic pathways.

    00:26 And then I'll follow this up with a discussion of how oxidation and reduction plays into this.

    00:33 Now when we can think about energy, it's very important to remember that we need to measure some way understand the needs of energy for cell. The needs of energy for cell are measured in terms of useful energy, and the useful energy for a cell is the Gibbs free energy. Now the Gibbs free energy is a measure of what's available to do something as needed. It's called G, and G is a very important concept because the change in the Gibbs free energy, which is called the delta G, is something that tells us immediately whether or not a chemical reaction will occur. So chemists worry about delta G, but so too do biochemists because biochemists have biochemical reactions going on and biochemical reactions obey the same universal laws the chemical reactions outside of cells do. The principle of Gibbs free energy as I said helps us to understand whether a reaction is favorable in one direction or another and I'll talk in a little bit about what it is that makes a reaction favorable or unfavorable. But if the delta G for a given reaction has a value that's less than zero, the reaction will move forward. If a reaction has a delta G that's equal to zero that reaction will be at equilibrium and no change will occur. Last, if a delta G for reaction is greater than zero, then that means that the reaction will move backwards. Now for metabolic processes, this is very, very important because metabolic process moving forward may involve breakdown or synthesis of a compound, but moving that same reaction backwards will accomplish the reverse of that process. Cells have to work within the energy environments in which they exist and as we will see the only control cells have over their energy environments is by manipulating the concentrations of reactants and products for a given reaction.

    02:25 So for reaction A going to B, we can write the delta G for that reaction with the following equation. The change in the Gibbs free energy, delta G, is equal to the change in the Gibbs free energy, a standard state that's given by the delta G 0 prime (ΔG0'), plus a term that includes the gas constant, times the temperature in Kelvin, times the natural log of the concentration of the product, in this case B, divided by the concentration of the reactant A. Now R as I said is the gas constant and is a constant for a given circumstance and T is the temperature measured in Kelvin. Now the ΔG0' is a constant for a given reaction, so for any particular reaction that we pick, it will have a specific ΔG0', but that ΔG0' will be invariant for that reaction, because it will always have the same value. We note that the gas constant R, is a constant value as well, it doesn't change. The temperature for a biochemical reaction typically doesn't change either, especially if we're talking about a reaction that's occurring inside of a thermal regulated entity like a human being. That means that the only things that affect the delta G then are the concentration of the products and the reactants, B and A. Okay. Increasing the quantities of products will increase the value of delta G. And increasing the amount of reactants, will decrease the value of delta G, and that's because the value of the log term is changing according to the ratio of B over A. As B gets larger, the log term gets larger, which means the delta G gets larger. As A gets larger the log term gets smaller, which means the delta G gets smaller.

    04:08 The ΔG0' within this equation as I said, is a constant and what it corresponds to is the Gibbs free energy that's determined under standard biological conditions. Now standard biological conditions include a temperature of 298 K, which is about 25°C and one more concentration of all the reactants and products except for protons. Now protons are very important because they control the pH of a solution and a high concentration of protons corresponds to a low pH and that low pH would be in conducive to a biological system. So we keep protons at a very low concentration for this consideration.

    04:49 Now under standard conditions, I notice that I said that the concentration of all solutes was one molar. That means therefore that the concentration of B over A is equal to one, because B and A are both equal one to start with. That means therefore that the log term, RT log times the concentration of B divided by the concentration of A, that log term has a value of zero, because the natural log of one is equal to zero. When that happens of course, delta G equals ΔG0' and that's how I started the slide, basing it corresponds to the Gibbs free energy under standard biological conditions. Now delta G is a value that can change depending upon how the conditions change, so delta G will not always be equal to ΔG0', because the reaction not always be at standard conditions.

    About the Lecture

    The lecture Energy Considerations – Oxidation and Reduction in Metabolism by Kevin Ahern, PhD is from the course Biochemistry: Basics.

    Included Quiz Questions

    1. If it is less than zero, the reaction moves backward.
    2. It determines the direction of a reaction.
    3. It is zero when a reaction is at equilibrium.
    4. It is related to the change in standard Gibbs free energy.
    5. If it is greater than zero, the reaction moves backward.
    1. Increasing quantities of products will increase the value of delta G.
    2. The value of R varies from one reaction to another.
    3. Increasing quantities of products will not affect the value of delta G.
    4. It is negative only if the standard Gibbs free energy change is negative.
    5. In the human body, the value of T varies considerably.
    1. Free energy
    2. Entropy
    3. Activation energy
    4. Temperature
    5. Concentration of protons

    Author of lecture Energy Considerations – Oxidation and Reduction in Metabolism

     Kevin Ahern, PhD

    Kevin Ahern, PhD

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