Answer:
Glucose. I'm not sure if this is correct. but the answer could be in article attached under explanation
Explanation:
During cellular respiration, a glucose molecule is gradually broken down into carbon dioxide and water. Along the way, some ATP is produced directly in the reactions that transform glucose. Much more ATP, however, is produced later in a process called oxidative phosphorylation. Oxidative phosphorylation is powered by the movement of electrons through the electron transport chain, a series of proteins embedded in the inner membrane of the mitochondrion
These electrons come originally from glucose and are shuttled to the electron transport chain by electron carriers \text{NAD}^+NAD+start text, N, A, D, end text, start superscript, plus, end superscript and \text{FAD}FADstart text, F, A, D, end text, which become \text{NADH}NADHstart text, N, A, D, H, end text and \text{FADH}_2FADH2start text, F, A, D, H, end text, start subscript, 2, end subscript when they gain electrons. To be clear, this is what's happening in the diagram above when it says ++plus \text {NADH}NADHstart text, N, A, D, H, end text or ++plus \text{FADH}_2FADH2start text, F, A, D, H, end text, start subscript, 2, end subscript. The molecule isn't appearing from scratch, it's just being converted to its electron-carrying form:
