Cells obtain their energy from their environment by oxidation of carbohydrates, proteins and triglycerides to CO
O. The energy from these macronutrients is transformed into ATP in 4 different stages. Stage 1 - hydrolysis of macromolecules into monosaccharides, fatty acids, amino acids. Stage 2 - oxidation into Acetyl Co A. Stage 3 - Acetyl Co A oxidation into Tricarboxylic acid (TCA) cycle → Co2. Stage 4 - reduction NAD → NADH, FAD → FADH2, phosphorylation ATP H2O and ATP.