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How does oxidative phosphorylation differ from the tricarboxylic acid cycle?

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Fluorescent confocal image of HeLa cells with green microtubules outlining cell shapes, red mitochondria, and blue nuclei. — mitochondrion Mitochondria (red) are found throughout the cytoplasm of almost all eukaryotic cells (cell nucleus is shown in blue; cytoskeleton is shown in yellow).

The tricarboxylic acid (TCA) cycle and oxidative phosphorylation are distinct, sequential stages of cellular respiration that occur within the mitochondrion.

The Tricarboxylic Acid (TCA) Cycle

The TCA cycle, also known as the Krebs or citric acid cycle, is a series of eight enzymatic reactions occurring in the mitochondrial matrix. Its primary role is the catabolism of organic fuel molecules, such as acetyl-CoA, into carbon dioxide. While it produces a small amount of ATP directly, its main function is to capture high-energy electrons by reducing NAD+ to NADH and FAD to FADH2.123

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of respiration, where the chemical energy captured in NADH and FADH2 is converted into ATP. This process involves two main components:23
* Electron Transport Chain: Electrons from NADH and FADH2 are passed through a series of proteins, eventually reducing oxygen to form water. This flow of electrons pumps hydrogen ions across the inner mitochondrial membrane, creating an electrochemical gradient.23
* ATP Synthesis: The energy stored in this gradient (the "transmembrane battery") is used by the enzyme ATP synthase to generate ATP from ADP and inorganic phosphate.2