Two reactions that occur consecutively during aerobic metabolism are shown below
CHEMISTRY · Multi-part · AP · Chemistry · Thermodynamics and Electrochemistry · Thermodynamics and Electrochemistry
Problem
Two reactions that occur consecutively during aerobic metabolism are shown below. Standard thermodynamic data were measured at 298K.
(1) L-malate + NAD+→ oxaloacetate + NADH ΔG1∘=+29.7kJ mol−1
(2) NADH + 21 O2 + H+→ NAD+ + H2O ΔG2∘=−220kJ mol−1
Both reactions take place in the mitochondrial matrix.
Answer the following questions about these reactions.
(a) Identify the chemical species that appears on both sides of the combined reaction sequence and therefore acts as a common intermediate.
(b) Write the overall chemical equation obtained when reactions (1) and (2) are added together, canceling any species that appear unchanged on both sides.
(c) Without performing a numerical calculation, predict whether the overall process you wrote in part (b) is spontaneous under standard conditions. Explain your reasoning with reference to the sign and relative magnitudes of the given ΔG∘ values.
(d) Describe, in the context of cellular respiration, how coupling an energetically unfavorable oxidation–reduction step to a highly favorable one can drive the net conversion of substrates to products. In your explanation, discuss the role of the common intermediate you identified in part (a).