Final answer:
A chemical inhibitor such as cyanide or oligomycin can inhibit mitochondrial respiration, allowing for the measurement of nonmitochondrial respiration.
Step-by-step explanation:
The substance that shuts down mitochondrial respiration and enables the calculation of nonmitochondrial respiration driven by processes outside the mitochondria is often a chemical inhibitor. Chemical inhibitors such as cyanide or oligomycin can shut down the Electron Transport Chain (ETC) or ATP synthase, respectively, within the mitochondria. This halts oxidative phosphorylation and allows scientists to measure the respiration that takes place outside of mitochondria, which is essential to distinguish between mitochondria-dependent and independent respiration rates. When mitochondrial respiration is inhibited, the production of ATP can no longer occur in this fashion, and as a result, any subsequent ATP production would be through alternative pathways, such as fermentation, which occurs in the cytosol and does not contribute to the proton gradient used by ATP synthase.