The following reactions occur during the catabolism of glucose. Which are exergonic? Which is endergonic? Which proceeds farthest toward products at equilibrium?
b. Phosphoenol pyruvate + H₂O → Pyruvate + Phosphate(P_i)
∆G = –14.8 kcal/mol (–61.9 kJ/mol)

Verified step by step guidance

1. Step 1: Understand the terms 'exergonic' and 'endergonic'. Exergonic reactions release energy and have a negative ΔG value, while endergonic reactions require energy input and have a positive ΔG value. In this case, the ΔG value is given as -14.8 kcal/mol (-61.9 kJ/mol), which is negative, indicating that the reaction is exergonic.
2. Step 2: Recall the relationship between ΔG and the direction of a reaction at equilibrium. A highly negative ΔG value suggests that the reaction proceeds far toward the products at equilibrium because it is thermodynamically favorable.
3. Step 3: Analyze the reaction provided: Phosphoenol pyruvate + H2O → Pyruvate + Phosphate (Pi). The breakdown of phosphoenol pyruvate (PEP) into pyruvate and phosphate is a key step in glycolysis and is known to be highly exergonic, as indicated by the large negative ΔG value.
4. Step 4: Conclude that this reaction is exergonic due to the negative ΔG value. Additionally, because the ΔG value is significantly negative, the reaction proceeds far toward the products at equilibrium, favoring the formation of pyruvate and phosphate.
5. Step 5: Summarize the findings: The reaction is exergonic, and it proceeds far toward the products at equilibrium due to the highly negative ΔG value. No energy input is required for this reaction to occur.

Key Concepts

Exergonic Reactions

Exergonic reactions are chemical processes that release energy, typically characterized by a negative change in Gibbs free energy (∆G). In the context of glucose catabolism, these reactions are spontaneous and proceed in the direction of product formation, contributing to the overall energy yield of cellular respiration.

Endergonic Reactions

Endergonic reactions require an input of energy to proceed, resulting in a positive change in Gibbs free energy (∆G). These reactions are non-spontaneous and often involve the synthesis of complex molecules from simpler ones, which is essential for cellular functions but must be coupled with exergonic processes to drive them forward.

Equilibrium in Chemical Reactions

Equilibrium in chemical reactions refers to the state where the rates of the forward and reverse reactions are equal, resulting in constant concentrations of reactants and products. The position of equilibrium can be influenced by factors such as temperature and concentration, and reactions that proceed farthest toward products at equilibrium typically have a larger negative ∆G, indicating a strong tendency to form products.