Which of the following is a final product of aerobic respiration?
Pyruvate
Lactate
Glucose
Oxygen
Carbon dioxide
The Correct Answer is E
A. Pyruvate: This 3-carbon carboxylate is the end-product of cytosolic glycolysis. In aerobic conditions, it undergoes oxidative decarboxylation to form acetyl-CoA for entry into the Krebs cycle. It represents a metabolic intermediate rather than the terminal molecular output of respiration.
B. Lactate: Cells produce this conjugate base during anaerobic fermentation when oxygen availability is insufficient. Pyruvate is reduced to lactate to regenerate NAD+ for continued glycolytic flux. It is a marker of anaerobic metabolism and not produced during complete aerobic oxidation.
C. Glucose: This monosaccharide serves as the primary hexose substrate or reactant for the respiratory pathway. It is consumed during the initial stages of phosphorylation to initiate energy extraction. Metabolism focuses on the catabolism of glucose rather than its synthesis as a product.
D. Oxygen: Molecular oxygen acts as the final electron acceptor at complex 4 of the electron transport chain. It is consumed to form water during the reduction process in the inner mitochondrial membrane. As a reactant, its concentration decreases as aerobic respiration proceeds.
E. Carbon dioxide: Aerobic respiration is the process by which cells break down glucose in the presence of oxygen to produce usable energy. The complete chemical reaction is summarized as: 
C6H12O6+6O2→6CO2+6H2O+Energy (ATP). The final products of this reaction are carbon dioxide, water, and ATP. Carbon dioxide is released as a waste product primarily during the Krebs cycle and pyruvate oxidation.
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Naxlex Comprehensive Predictor Exams
Related Questions
Correct Answer is B
Explanation
A. glucagon, lowers: Glucagon is the correct hormone secreted by alpha cells during a fasting state, but its physiological action is the opposite of lowering glucose. Glucagon is a hyperglycemic agent that works to restore blood sugar levels. Lowering blood glucose is the primary function of the hormone insulin.
B. glucagon, raises: During the post-absorptive state, decreasing blood glucose levels trigger the alpha cells of the pancreatic islets to release glucagon. This hormone stimulates glycogenolysis and gluconeogenesis in the liver to release glucose into the bloodstream. This homeostatic mechanism ensures a steady energy supply for the brain.
C. Insulin, lowers: Insulin is secreted by the beta cells of the pancreas, not the alpha cells, typically following a meal when glucose levels are high. While it effectively lowers blood glucose, it would not be secreted "many hours after a meal" during a fasting state. Its secretion is inhibited during hypoglycemia.
D. insulin, raises: This choice incorrectly identifies the cell type, the timing of secretion, and the physiological effect of the hormone. Insulin lowers blood sugar by promoting cellular uptake and is not a product of alpha cells. It never functions to raise systemic blood glucose concentrations.
E. glucocorticoids; raises: Glucocorticoids like cortisol are secreted by the adrenal cortex, not the pancreatic islets, and they do raise blood glucose. However, they are regulated by the pituitary-adrenal axis rather than directly by local pancreatic islet cells. Alpha cells are specifically dedicated to the secretion of glucagon.
Correct Answer is B
Explanation
A. C6H12O6 + 6 H2O -> 6 CO2 + 6 O2: This equation incorrectly lists water as a reactant instead of oxygen. Aerobic catabolism requires molecular oxygen as the terminal electron acceptor in the electron transport chain. Metabolism of glucose without oxygen input prevents oxidative phosphorylation.
B. C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O: Hexose oxidation involves the complete breakdown of glucose in the presence of oxygen. This metabolic pathway yields carbon dioxide and water as primary byproducts while capturing chemical energy. It accurately reflects the stoichiometric balance of aerobic cellular respiration.
C. C6H12O6 + 6 CO2 -> 6 O2 + 6 H2O: Carbon dioxide functions as a metabolic waste product rather than a reactant in human cellular respiration. Glucose does not react with carbon dioxide to produce oxygen during heterotrophic metabolism. This chemical arrangement reverses the standard physiological gas exchange.
D. CO2 + 6 H2O -> C6H12O6 + 6 O2: This formula represents the endergonic process of photosynthesis occurring in photoautotrophic organisms. It describes the fixation of inorganic carbon into organic compounds using light energy. Eukaryotic animal cells lack the chloroplasts necessary to drive this specific anabolic reaction.
E. 6 O2 + 6 H2O -> C6H12O6 + 6 CO2: The combination of oxygen and water does not spontaneously synthesize glucose molecules in biological systems. This equation fails to account for the carbon source required for carbohydrate formation. It violates the fundamental thermodynamic principles governing respiratory substrate breakdown and energy release.
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