In cellular respiration, one complete turn of the citric acid cycle (Krebs cycle) per acetyl CoA molecule results in which of the following products?

Introduction / Context:
The citric acid cycle, also known as the Krebs cycle or tricarboxylic acid (TCA) cycle, is a central pathway in aerobic cellular respiration. It oxidises acetyl CoA and captures high energy electrons in the form of NADH and FADH2, which then feed into the electron transport chain. Understanding the products of one complete turn of the cycle is a standard exam topic in biochemistry and general biology. This question asks which of several statements about the outputs of one turn of the cycle is correct.

Given Data / Assumptions:

• We consider one turn of the citric acid cycle per acetyl CoA molecule entering the cycle.
• The liver and most textbooks describe the stoichiometry for one cycle turn in a consistent way.
• The statements mention carbon dioxide release, generation of NADH and FADH2, and production of ATP or GTP.
• We ignore the preparatory steps that convert pyruvate to acetyl CoA and focus only on the cycle itself.

Concept / Approach:
For each acetyl CoA molecule entering the citric acid cycle, the cycle performs a series of oxidation and decarboxylation reactions. The accepted stoichiometry for one turn of the cycle is: two molecules of CO2 are released, three molecules of NADH and one molecule of FADH2 are produced, and one high energy phosphate molecule (ATP or GTP, depending on the cell type) is generated by substrate level phosphorylation. Additionally, one molecule of oxaloacetate is regenerated to allow the cycle to continue. The question lists each of these product types separately and then offers the combined option, so the correct answer is that all of the listed statements are true.

Step-by-Step Solution:
Step 1: Recall that acetyl CoA (2 carbon units) enters the cycle by combining with oxaloacetate (4 carbons) to form citrate (6 carbons). Step 2: Follow the decarboxylation steps in the cycle where two carbons are removed as CO2 in a series of reactions, leading to the release of two molecules of carbon dioxide per turn. Step 3: Remember that oxidation steps generate three molecules of NADH and one molecule of FADH2 per cycle turn, capturing high energy electrons. Step 4: Note that one step in the cycle performs substrate level phosphorylation, producing one molecule of GTP (in some tissues) or ATP, which can readily be converted to ATP. Step 5: Recognise that options A, B, and C each describe accurate aspects of these products, so the combined statement in option D (All of the above) must be correct.

Verification / Alternative check:
Biochemistry references on cellular respiration provide a standard summary: per acetyl CoA, one turn of the citric acid cycle yields 2 CO2, 3 NADH, 1 FADH2, and 1 GTP or ATP, along with regeneration of oxaloacetate. When glucose is fully oxidised, two acetyl CoA molecules enter the cycle, so the total cycle yields per glucose double these numbers. This consistent stoichiometric information supports each of the statements listed in the question and confirms that the correct answer is the combination of all of them together.

Why Other Options Are Wrong:
Selecting only one of the individual statements A, B, or C would ignore the fact that all of these events occur in a single turn of the citric acid cycle. Option A alone would account only for carbon dioxide release, not for electron carriers or ATP. Option B alone would ignore carbon dioxide and ATP production. Option C alone would address only the ATP or GTP generation. Since all of these are known products of the same cycle turn, choosing any single statement would be incomplete and therefore incorrect.

Common Pitfalls:
A frequent source of confusion is mixing up the yields per acetyl CoA with yields per glucose. Because each glucose molecule produces two acetyl CoA molecules, some students mistakenly double the citric acid cycle yields and then misapply the numbers to questions about a single turn. Another pitfall is forgetting that GTP is equivalent to ATP in terms of energy and is often converted to ATP in the cell. To avoid mistakes, carefully read whether the question refers to one turn of the cycle or to the complete oxidation of glucose and remember the standard per cycle stoichiometry.

Final Answer:
One complete turn of the citric acid cycle per acetyl CoA produces two molecules of carbon dioxide, three molecules of NADH, one molecule of FADH2, and one molecule of ATP or GTP, so all of the listed statements are correct.