1. Sample Problem 23.1 The Citric Acid Cycle
When one acetyl CoA moves through one turn of the citric acid cycle, how many of each of the following is produced?
a. NADH b. CO2
Solution
a. One turn of the citric acid cycle produces three molecules of NADH.
b. Two molecules of CO2 are produced by the decarboxylation of isocitrate and α-ketoglutarate.
Study Check 23.1
When one acetyl CoA moves through one turn of the citric acid cycle, how many FADH2 would be produced?
Answer
one FADH2

2. Sample Problem 23.2 Oxidation and Reduction
Identify each of the following steps in electron transport as oxidation or reduction:
a. CoQH2 → CoQ + 2H+ + 2 e−
b. cyt c (Fe3+) + e− → cyt c (Fe2+)
Solution
a. The loss of electrons is oxidation.
b. The gain of an electron is reduction.
Study Check 23.2
What is the final substance that accepts electrons in electron transport?
Answer
oxygen (O2)

3. Sample Problem 23.3 ATP Synthesis
Why does the oxidation of NADH provide energy for the formation of 2.5 ATP molecules, whereas FADH2 produces 1.5 ATP molecules?
Solution
The oxidation of NADH at complex I provides electrons and hydrogen ions (H+), which are pumped from the matrix into the intermembrane space eventually producing enough energy for the synthesis of 2.5 ATP molecules. However, FADH2 is oxidized at complex II, which provides energy for the synthesis of only 1.5 ATP molecules.
Study Check 23.3
How do hydrogen ions return to the matrix?
Answer
Hydrogen ions return to the matrix by passing through ATP synthase, complex V.

4. Sample Problem 23.4 ATP Production
As Natalie is walking on the treadmill during her fitness test, how many ATP are produced during each of the following oxidations?
a. two pyruvate to two acetyl CoA
b. one glucose to two acetyl CoA
Solution
a. The oxidation of two pyruvate to two acetyl CoA produces two NADH, which yields five ATP. We calculate
this as:
2 NADH × 2.5 ATP/NADH = 5 ATP
b. Seven ATP are produced, two by direct phosphorylation and five from the two NADH, when glucose is
oxidized to two pyruvate. Five ATP (from two NADH) are synthesized from the oxidation of two pyruvate to
two acetyl CoA. Thus, a total of 12 ATP are produced when glucose is oxidized to yield two acetyl CoA.
Study Check 23.4
What are the sources of ATP for two turns of the citric acid cycle?

5. Sample Problem 23.4 ATP Production
Continued
Answer
Six NADH provide 15 ATP, two FADH2 provide three ATP, and two direct phosphate transfers provide two ATP.