Citric acid cycle
The citric acid cycle is a central metabolic pathway that oxidizes acetyl-CoA, releases carbon dioxide, and produces electron carriers for ATP production.
What the citric acid cycle is
The citric acid cycle is a sequence of enzyme-catalyzed reactions that sits near the center of cellular metabolism. It takes the acetyl group carried by acetyl-CoA, removes high-energy electrons, and releases carbon dioxide as carbon atoms are oxidized.
Why it is a cycle
The pathway begins when acetyl-CoA joins with oxaloacetate to form citrate. Through later steps, citrate is rearranged, oxidized, and shortened until oxaloacetate is regenerated. Because the starting acceptor is restored, another acetyl-CoA can enter and the route can continue.
Where it fits after glycolysis
Glycolysis breaks glucose into pyruvate. In aerobic eukaryotic cells, pyruvate can be transported into mitochondria and converted to acetyl-CoA before entering the citric acid cycle. The cycle therefore links sugar breakdown to the electron transport chain and broader energy metabolism.
What the cycle produces
A turn of the cycle produces reduced electron carriers, especially NADH and FADH2, plus a small amount of ATP or GTP depending on the cell type. It also releases carbon dioxide. The cycle does not make most of the ATP directly; it prepares electrons for later ATP production.
Feeding the electron transport chain
NADH and FADH2 carry electrons from the cycle to the electron transport chain. As those electrons move through membrane proteins, cells can build a proton gradient that ATP synthase uses to make ATP. This is why the cycle is closely tied to oxidative phosphorylation.
A metabolic crossroads
The citric acid cycle is not only a breakdown route. Several intermediates can be drawn off for amino acid synthesis, heme production, fatty-acid metabolism, and other biosynthetic work. Cells refill the cycle with anaplerotic reactions when intermediates are removed.
Regulation and the oxygen link
The cycle is regulated by energy demand and by the availability of substrates, oxidized electron carriers, and pathway products. Oxygen is not consumed in the cycle itself, but aerobic cells depend on oxygen at the end of the electron transport chain to keep NADH and FADH2 recycling back to NAD+ and FAD.
Why it matters
The citric acid cycle helps explain how cells extract energy from carbohydrates, fats, and some amino acids while also supplying building blocks for growth. It matters in physiology, microbiology, biochemistry, inherited metabolic disorders, cancer metabolism, exercise, and biotechnology.