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Thermodynamics & Acidity 3 Thermodynamics & Acidity To understand how a cell Metabolism is a defining characteristic of living systems. All living things Goal drives unfavorable reactions. carry out metabolic processes, many of which are unfavorable and require energy to proceed. Indeed, energy is required for the maintenance of life. Why does life require energy, how is it obtained, and where does it go? These Objectives questions can be addressed using thermodynamics. Thermodynamics is After this chapter, you should be able to the study of energy flow within a system. Thermodynamics can be used to predict how systems will behave under different circumstances and whether • explain the concept of Gibbs free energy. reactions are energetically favorable. • explain the relationship of ΔG° to the rxn Energy cannot be created or destroyed favorability of a reaction. The first law of thermodynamics states that energy cannot be created or • relate ΔG°rxn to Keq. destroyed; it can only be converted from one form to another or used to do • apply Le Châtelier’s Principle to chemical equilibria. work. For example, when energy is added to solid water (ice) by heating, the energy of the molecules within the solid ice increases, causing them • predict protonation state from the pH to vibrate more rapidly. As the heating continues, some water molecules of a solution and pK . a gain enough energy to break away from the other water molecules. Because water molecules interact through hydrogen bonding and because energy is needed to break those interactions, the water molecules are converting thermal energy into chemical energy when water is heated. Chemical reactions either release or absorb energy Cells require energy to carry out metabolic reactions, including the synthesis and breakdown of molecules. To obtain energy, living systems metabolize Chapter 3 Thermodynamics & Acidity 2 Glycolysis O O O O OH ATP ADP P O ATP ADP P HO P O OH O O O O HO O O O HO O O O O HO O O HO OH HO OH P OH OH OH O OH HO HO Glucose OH OH O O O OH O O O O O O O O O O P P P O O O HO O O + P P O O ATP ADP O O O NADH NAD , Pi O O + CoA-SH, NAD OH O O O O O O O O CO , NADH P P 2 ADP ATP O H2O O O O O O O Krebs Cycle O NADH, CoA O + + O OH NAD H O O FADH2 O O O O O O CoA-SH, H+ FAD O O O H2O OH O O O O O O O O O O O O GTP, CoA-SH OH O O O CoA O O GDP, Pi O CO , CoA-SH,O O NADH, NAD+ 2 + O O NADH NAD CO2 O O Figure 1 Glycolysis and the Krebs cycle are central reactions in metabolism Glycolysis and the Krebs cycle are metabolic reactions that release energy that is then used by the cell to synthesize ATP. The energy stored in ATP can be used later to do work. The cell uses this energy to grow, divide, move, and carry out metabolism. We will return to these energy-generating reactions and to how the molecules in these pathways are depicted in later chapters. nutrients, such as amino acids, fats, and sugars. One of the primary reactions that occurs during metabolism is the reaction of glucose (C6H12O6) with oxygen, which releases energy and produces carbon dioxide and water. The released energy is used in a multi-step process (the glycolytic pathway, Figure 1) to generate an energy store known as adenosine triphosphate (ATP), which can later be utilized by cellular machinery to help carry out important metabolic reactions. We will have much more to say about ATP and its myriad roles in the cell in later chapters. The reaction of glucose with oxygen depicted in Figure 2 shows glucose and oxygen (the reactants) at a higher energy level than carbon dioxide and Chapter 3 Thermodynamics & Acidity 3 Respiration an energetically favorable reaction Photosynthesis an energetically unfavorable reaction OH OH O O HO + 6 O 6 CO + 6 H O 6 CO + 6 H O HO 6 O HO OH 2 2 2 2 2 HO OH + 2 OH OH Glucose Oxygen Carbon dioxide Water Carbon dioxide Water Glucose Oxygen high energy high energy Glucose Glucose + Energy Energy + Oxygen Oxygen favorable unfavorable Carbon dioxide Carbon dioxide energy is released energy is required + + Water Water low energy low energy Figure 2 Respiration and photosynthesis interconvert glucose and O2 with CO2 and water as we will explain in more depth in a later chapter water (the products) to capture the idea that energy is released when the reaction occurs. Reactions that release energy are energetically favorable. Conversely, reactions in which the products are higher in energy than the reactants require an input of energy to proceed; such reactions are energetically unfavorable. It is important to note that the favorability of a reaction is largely unrelated to its rate; favorable reactions can be extremely slow. The determinants of reaction rates are the subject of the next chapter on kinetics. A full understanding of the reactions that drive living systems requires an appreciation of both thermodynamics and kinetics. Reactions can proceed in both directions. For example, the conversion of glucose and oxygen to carbon dioxide and water can take place in the reverse direction to synthesize glucose, as occurs during photosynthesis. Because glucose and oxygen (the products of photosynthesis) are higher in energy than carbon dioxide and water (the reactants in photosynthesis), photosynthesis requires an input of energy (Figure 2). In the case of photosynthesis, this energy comes directly from the sun, but other energy sources, such as ATP, are commonly used by cells to drive unfavorable processes. Reactions that occur in both directions are indicated with stacked arrows in which one arrow points to the left and the other points to the right. The concentrations of reactants and products do not change at equilibrium How do we determine whether the reactants or the products of a reaction are higher in energy? One way to do this is to measure the concentrations of the reactants and products when the reaction is at equilibrium, as we will explain. A reaction is at equilibrium when the concentrations of its reactants and products no longer change with time. The reaction does not stop at equilibrium; instead, the forward and reverse reactions occur at the same rate. As a consequence, reactants are consumed by the forward Chapter 3 Thermodynamics & Acidity 4 reaction as fast as they are generated by the reverse reaction, and products are consumed by the reverse reaction as fast as they are generated by the forward reaction. Consider the example of two hypothetical molecules, A and B, which interconvert as described in the following reaction: A B If a reaction begins with only molecule A present, the concentration of molecule A will decrease with time as the reaction proceeds, and the concentration of molecule B will increase. Because no molecules of B are present initially, the reverse reaction cannot occur at first. As time passes and molecule B accumulates, however, the rate of the reverse reaction increases, which leads to a slower net rate of molecule B accumulation because some molecules of B are being converted back into molecule A. Ultimately, the reaction reaches an equilibrium, in which the rates of the forward and reverse reactions are equal; at that point, there is no further net change in the concentration of either molecule A or molecule B. The equilibrium constant is the ratio of products to reactants at equilibrium All reactions have a characteristic equilibrium constant (Keq) under a given set of conditions. Keq is defined as the ratio of the mathematical product of the equilibrium concentrations of the species on the right (i.e., the concentrations of the chemical products multiplied together) divided by the mathematical product of the equilibrium concentrations of the species on the left (i.e., the reactants). A hypothetical reaction in which one molecule of A reacts with one molecule of B to reversibly yield one molecule of C and one molecule of D would have an equilibrium constant equal to ([C][D])/ ([A][B]), as shown below. The brackets (“[]”) surrounding each molecule signify the concentration for that reactant or product in units of molarity (i.e., moles per liter). [C] [D] A + B C + D K = eq [A] [B] If Keq is less than one, then the concentrations of the products are lower than the concentrations of the reactants at equilibrium. Conversely, if Keq is greater than one, then the concentrations of products are greater than the concentrations of the reactants at equilibrium. If Keq is exactly one, then there is an equal mixture of reactants and products at equilibrium. At one extreme, a Keq that approaches zero means that there will be almost no product at equilibrium. At the other extreme, a Keq that approaches infinity means that there will be nearly no reactant at equilibrium. Each chemical reaction has its own value of Keq that is constant and only changes with temperature. The value of the equilibrium constant indicates whether or not a reaction is favorable. Reactions for which Keq is greater than one are favorable, meaning that the reactants are higher in energy than the products and signifying that energy is released as the reaction proceeds. Similarly, reactions for which Keq is less than one are unfavorable, meaning that the reactants are lower in energy than the products and signifying that the reaction absorbs energy Chapter 3 Thermodynamics & Acidity 5 Example 1 Writing an equilibrium constant Write an equation that expresses the equilibrium constant for the reaction shown below in terms of reactant (water and pyrophosphate) and product (hydrogen phosphate) concentrations.
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