describing the Tricarboxylic Acid Cycle (Krebs' cycle):

1. Isocitrate undergoes oxidative decarboxylation to form $\alpha$-ketoglutaric acid (producing NADH).
2. Succinic acid undergoes oxidation to form Fumarate (producing FADH$_2$).
3. Malic acid undergoes oxidation to form Oxaloacetic acid (producing NADH).
4. The conversion of Succinyl-CoA to Succinic acid involves substrate-level phosphorylation where a molecule of GTP is synthesised. This step does not involve oxidation (removal of hydrogen) of the substrate.