The table below shows the mean energy values and respiratory quotients (RQ) of three different categories of respiratory substrate: X, Y, and Z.
| Respiratory substrate | Mean energy value / kJ g−1\text{kJ g}^{-1}kJ g−1 | Respiratory Quotient (RQ) |
|---|---|---|
| X | 15.8 | 1.00 |
| Y | 39.4 | 0.70 |
| Z | 17.0 | 0.85 |
Using this information and your knowledge of respiratory pathways, which of the following statements is correct?
Substrate Y releases more energy per gram than substrate X because its molecules contain a higher proportion of oxygen atoms, allowing more protons to be transferred to NAD and FAD.
Under anaerobic conditions, only substrate X can be utilized to yield ATP because it can undergo glycolysis, whereas substrates Y and Z enter metabolic pathways that require the regeneration of coenzymes by oxidative phosphorylation.
The RQ of substrate Y is lower than that of substrate X because the oxidation of lipids yields more molecules of CO2\text{CO}_2CO2 per molecule of O2\text{O}_2O2 consumed.
If a tissue has a measured RQ of 1.201.201.20, this indicates that it is aerobically respiring a mixture of substrate Y and substrate Z.
