The starting molecule of the electron transport chain (ETC) is NADH (nicotinamide adenine dinucleotide) and FADH2 (flavin adenine dinucleotide).

Here's why:

* NADH and FADH2 are electron carriers: They are produced during the earlier stages of cellular respiration (glycolysis and the Krebs cycle) and carry high-energy electrons.

* Electron transfer: They donate these electrons to the first protein complex in the ETC, initiating the chain reaction.

Note: While NADH and FADH2 are both important starting molecules, NADH carries more energy and enters the ETC at a higher energy level.