Which molecule is the primary energy carrier produced during respiration?

• A. Adenosine triphosphate
• B. Nicotinamide adenine dinucleotide
• C. Glucose
• D. Lactic acid

Answer: (A)

Energy in respiration is captured and stored mainly in adenosine triphosphate (ATP). While respiration also generates electron carriers like NADH and FADH2 that shuttle high-energy electrons to the inner mitochondrial membrane, the molecule used directly to power cellular work is ATP. When ATP is hydrolyzed to ADP and inorganic phosphate, energy is released to drive processes such as muscle contraction, active transport, and biosynthesis. ATP is produced at various steps, with some ATP generated directly in glycolysis and the Krebs cycle, and most of it formed during oxidative phosphorylation in the mitochondria. Glucose is the fuel that starts the process, not the energy carrier itself, and lactic acid is a byproduct of anaerobic metabolism. Therefore ATP is the primary energy carrier produced during respiration.