Respiratory Substrates and Carbohydrates — NEET Biology

Correct answer: C — S1, S3 and S4 only

The core concept tested is the nature of respiratory substrates and the amphibolic nature of the respiratory pathway. S1 is NOT correct. While carbohydrates are usually favored, proteins, fats, and organic acids can also be used as respiratory substrates under certain conditions, meaning carbohydrates are not exclusive under all conditions. S2 is correct. According to NCERT, fats are broken down into glycerol and fatty acids. Glycerol is converted to PGAL, and fatty acids are degraded to acetyl CoA before entering the respiratory pathway. S3 is NOT correct. Amino acids, after deamination, enter the pathway at various points depending on their structure, including as pyruvate, acetyl CoA, or at different stages within the Krebs’ cycle, not primarily or specifically as 3-phosphoglyceraldehyde. S4 is NOT correct. The respiratory pathway is considered an amphibolic pathway, not purely catabolic, because its intermediates are involved in both breakdown (catabolism) and synthesis (anabolism) of other compounds. S5 is correct. Organic acids can indeed be used as respiratory substrates by plants under certain conditions, demonstrating the adaptability of cells in utilizing different energy sources. Therefore, statements S1, S3, and S4 are NOT correct. This question evaluates the understanding of alternative respiratory substrates and the amphibolic nature of the respiratory pathway.