The table below shows the characteristics of four unknown respiratory substrates, PPP, QQQ, RRR, and SSS, isolated from mammalian tissues.
| Substrate | Mean energy density (kJ g−1\text{kJ g}^{-1}kJ g−1) | H:O H:O\,H:O ratio in molecular formula | Respiratory Quotient (RQ) |
|---|---|---|---|
| PPP | 39.4 | 18.0:118.0 : 118.0:1 | 0.70 |
| QQQ | 17.3 | 2.0:12.0 : 12.0:1 | 1.00 |
| RRR | 17.0 | 4.1:14.1 : 14.1:1 | 0.85 |
| SSS | 10.2 | 1.2:11.2 : 11.2:1 | 1.33 |
Which of the following statements is a correct deduction based on these data and the biochemistry of respiration?
Substrate PPP represents a lipid, which releases more energy per gram than carbohydrate QQQ because its higher H:OH:OH:O ratio enables a greater yield of ATP directly via substrate-level phosphorylation.
Substrate SSS represents an organic acid that is more oxidized than carbohydrate QQQ; because it is already more oxidized, it releases a greater amount of energy per gram during glycolysis.
Substrate RRR represents a protein; after deamination, its remaining carbon skeletons are respired, yielding an intermediate energy value (≈17.0 kJ g−1\approx 17.0\text{ kJ g}^{-1}≈17.0 kJ g−1) and an intermediate RQ (≈0.85\approx 0.85≈0.85) due to their moderate hydrogen-to-oxygen ratio.
During the complete aerobic respiration of 1 g1\text{ g}1 g of each substrate, QQQ will produce a larger mass of metabolic water than PPP because QQQ contains more internal oxygen atoms to act as the terminal electron acceptors.
