The citric acid cycle, also known as the Krebs cycle or TCA cycle, is a central series of biochemical reactions that generates energy in cells. It processes molecules from food breakdown to create energy carriers. It is an integral component of cellular respiration, fundamental for aerobic life. Operating within the mitochondria of eukaryotic cells, it oxidizes carbon compounds.
The Essential Inputs
The citric acid cycle relies on specific molecules to begin and continue its reactions. Acetyl-CoA, a two-carbon compound from carbohydrate, fat, and protein breakdown, is the primary fuel molecule entering the cycle. It combines with oxaloacetate, a four-carbon molecule that is the starting point and is regenerated at the end of each cycle.
Other reactants include nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD), which serve as electron carriers. They pick up high-energy electrons during the cycle, transforming into their reduced forms, NADH and FADH2. Water (H2O) is also consumed, contributing to chemical transformations. Additionally, guanosine diphosphate (GDP) or adenosine diphosphate (ADP) are used, converting to guanosine triphosphate (GTP) or adenosine triphosphate (ATP) in one direct energy-producing step.
Where Reactants Originate
Acetyl-CoA, the main entry molecule, originates from the breakdown of macromolecules. Glucose, a carbohydrate, is broken down through glycolysis into pyruvate in the cytoplasm. Pyruvate then moves into the mitochondria, converting into Acetyl-CoA via oxidative decarboxylation, releasing carbon dioxide.
Fats, specifically fatty acids, are another source of Acetyl-CoA through beta-oxidation. During beta-oxidation, fatty acids break down into two-carbon Acetyl-CoA units, producing NADH and FADH2. Proteins also contribute, as certain amino acids convert into Acetyl-CoA or other intermediates after their amino groups are removed.
Oxaloacetate, the four-carbon molecule that initiates the cycle, is regenerated at the conclusion of each turn, ensuring continuous operation. Beyond its regeneration within the cycle, oxaloacetate can also form from other metabolic pathways, such as certain amino acids or gluconeogenesis. NAD+ and FAD are regenerated primarily in the electron transport chain, where they release their carried electrons, highlighting the interconnectedness of these energy-producing pathways.
How Reactants Drive Energy Production
Processing reactants within the citric acid cycle primarily generates high-energy electron carriers. The cycle produces three NADH and one FADH2 for each turn. These reduced electron carriers then move to the electron transport chain, the final stage of cellular respiration, where their stored energy produces a substantial amount of ATP, the cell’s main energy currency.
A small amount of energy is also produced directly within the cycle through substrate-level phosphorylation, resulting in one GTP (readily converted to ATP) per cycle. As Acetyl-CoA is processed, its carbon atoms are released as two molecules of carbon dioxide, a waste product. This complete oxidation of carbon atoms from original fuel molecules is a key outcome. Overall, the citric acid cycle acts as a preparatory stage, extracting energy in a form that efficiently converts into ATP, contributing significantly to the cell’s total energy supply.