Multiple Choice
The formation of glucose-6-phosphate from glucose is an endergonic reaction and is coupled to which of the following reactions or pathways?
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7. Energy and Metabolism
ATP
1:17 minutesProblem 7
Textbook Question
In Figure 8.10, the energetic coupling of substrate phosphorylation and an endergonic reaction are shown. If the hydrolysis of ATP releases 7.3 kcal of free energy, use the graph in this figure to estimate what you would expect the ∆G values to be for the uncoupled reaction and the two steps in the coupled reaction.
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Examine the graph to understand the energy changes involved in both the uncoupled and coupled reactions. The graph shows free energy changes relative to the reactants A and B.
Identify the uncoupled reaction, which is endergonic, meaning it requires an input of energy. The graph indicates that the free energy change (∆G) for the uncoupled reaction is positive, as energy is required to synthesize AB from A and B.
Note that the hydrolysis of ATP releases 7.3 kcal/mol of free energy, which is used to drive the coupled reaction. This energy release is depicted in the graph as the exergonic step where ATP is converted to ADP and inorganic phosphate (Pi).
In the coupled reaction, the energy released from ATP hydrolysis is used to form an activated intermediate (BP), which then reacts with A to form AB. The graph shows that the ∆G for the coupled reaction is negative, indicating that the reaction is exergonic overall.
Estimate the ∆G values for the uncoupled reaction and the two steps in the coupled reaction by analyzing the graph. The uncoupled reaction has a positive ∆G, while the coupled reaction has two steps: the first step (ATP hydrolysis) has a ∆G of -7.3 kcal/mol, and the second step (formation of AB) results in a net negative ∆G due to the energy released from ATP hydrolysis.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Energetic Coupling
Energetic coupling refers to the process where an exergonic reaction (which releases energy) drives an endergonic reaction (which requires energy). In biological systems, ATP hydrolysis is a common exergonic reaction that provides the necessary energy to power various cellular processes, including the synthesis of biomolecules. This coupling allows cells to efficiently manage energy resources and perform work.
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Gibbs Free Energy (∆G)
Gibbs Free Energy (∆G) is a thermodynamic quantity that indicates the spontaneity of a reaction. A negative ∆G value signifies that a reaction is exergonic and can occur spontaneously, while a positive ∆G indicates an endergonic reaction that requires energy input. Understanding ∆G is crucial for predicting the direction and feasibility of biochemical reactions, especially in the context of metabolic pathways.
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Substrate Phosphorylation
Substrate phosphorylation is a biochemical process where a phosphate group is transferred from a donor molecule (often ATP) to a substrate, resulting in the formation of a phosphorylated product. This process is essential in energy transfer within cells, as it often activates or deactivates enzymes and other proteins, thereby regulating metabolic pathways. It plays a key role in the coupling of reactions, as seen in the graph.
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