Final answer:
The substrate used in the production of ATP by mitochondria is glucose.
Step-by-step explanation:
In an investigation into the production of ATP by mitochondria, the substrate used is glucose. Cellular respiration is a set of metabolic processes that take place within cells to convert nutrients, especially glucose, into energy in the form of adenosine triphosphate (ATP). This energy conversion process occurs in three main stages: glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation (electron transport chain and chemiosmosis).