The X in the diagram mentions the electron transportation. This process happens during cellular respiration.
The third and last phase in aerobic cell respiration is electron transportation and oxidative phosphorylation. In eukaryotic cells, the mitochondrial membrane contains a collection of protein molecules and electron transport system molecules that make up the electron transport chain. The "highly energetic" electrons produced by the cycle of citric acid are transferred to oxygen through a sequence of processes. As h+ ions are pushed out of matrix of the mitochondria and into the cell membranes space, an electrical and chemical gradient is created from across mitochondrial inner membrane. In the end, oxidative phosphorylation—the method by which cell enzymes oxidize nutrients—produces ATP. The energy generated by the electron transport chain is used by the protein ATP synthase to phosphorylate (add a phosphate group to a molecule) ADP into ATP. The majority of ATP is produced during the oxidative phosphorylation and electron transport chain phases of cellular respiration.
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