Final answer:
The first step in the electron transport chain initiated by NADH is the donation of electrons to flavin mononucleotide (FMN), which begins the series of redox reactions leading to ATP production via oxidative phosphorylation.
Step-by-step explanation:
The first step in the electron transport chain starting from NADH is the transfer of electrons to the first molecule in the chain, which is flavin mononucleotide (FMN). NADH donates two electrons to FMN, reducing it to FMNH₂. These high-energy electrons are subsequently transferred through a series of redox reactions to other electron carriers, including iron-sulfur clusters and coenzyme-Q (ubiquinone). The ultimate electron acceptor in the chain is molecular oxygen, which is reduced to form water. This transfer of electrons facilitates the pumping of protons from the mitochondrial matrix across the inner membrane, thereby generating the proton gradient used to produce ATP by oxidative phosphorylation.