Which of the following molecules will produce the most ATP per mole?
d. lauric acid (C₁₂) or palmitic acid (C₁₆)

Verified step by step guidance

1. Step 1: Understand the problem. The question asks which molecule, lauric acid (C₁₂) or palmitic acid (C₁₆), will produce the most ATP per mole. This involves comparing the energy yield of these two fatty acids during their metabolism, specifically through β-oxidation and the citric acid cycle.
2. Step 2: Recall the process of β-oxidation. Each cycle of β-oxidation shortens the fatty acid chain by two carbons, producing 1 FADH₂, 1 NADH, and 1 acetyl-CoA. The number of β-oxidation cycles is determined by the number of carbons in the fatty acid minus 2, divided by 2. For lauric acid (C₁₂), there are 5 cycles, and for palmitic acid (C₁₆), there are 7 cycles.
3. Step 3: Calculate the total number of acetyl-CoA molecules produced. Each fatty acid produces acetyl-CoA molecules equal to the number of carbons divided by 2. Lauric acid (C₁₂) produces 6 acetyl-CoA molecules, while palmitic acid (C₁₆) produces 8 acetyl-CoA molecules.
4. Step 4: Determine the ATP yield from each product. Each FADH₂ yields approximately 1.5 ATP, each NADH yields approximately 2.5 ATP, and each acetyl-CoA entering the citric acid cycle yields approximately 10 ATP. Multiply these values by the number of FADH₂, NADH, and acetyl-CoA molecules produced during β-oxidation and the citric acid cycle for each fatty acid.
5. Step 5: Compare the total ATP yield. Since palmitic acid (C₁₆) has more carbons than lauric acid (C₁₂), it undergoes more β-oxidation cycles and produces more acetyl-CoA, resulting in a higher total ATP yield per mole. Therefore, palmitic acid will produce the most ATP per mole.

Key Concepts

Beta-Oxidation

Beta-oxidation is the metabolic process by which fatty acids are broken down in the mitochondria to generate acetyl-CoA, which then enters the citric acid cycle. Each cycle of beta-oxidation shortens the fatty acid chain by two carbon atoms and produces NADH and FADH2, which are crucial for ATP production in the electron transport chain.

ATP Yield from Fatty Acids

The ATP yield from fatty acids depends on their carbon chain length. Longer fatty acids, such as palmitic acid (C₁₆), produce more acetyl-CoA and thus more ATP through subsequent metabolic pathways compared to shorter fatty acids like lauric acid (C₁₂). This is because each acetyl-CoA can generate multiple ATP molecules during cellular respiration.

Energy Density of Fatty Acids

Fatty acids are energy-dense molecules, providing more energy per gram than carbohydrates or proteins. This is due to their long hydrocarbon chains, which contain many carbon-hydrogen bonds that release energy when oxidized. Understanding the energy density helps in comparing the ATP yield of different fatty acids based on their structure.