The Krebs cycle, also known as the citric acid cycle or tricarboxylic acid (TCA) cycle, is a fundamental series of biochemical reactions. It plays a central role in cellular respiration, converting nutrients into adenosine triphosphate (ATP), the primary energy currency of the cell. This cycle integrates the breakdown products of carbohydrates, fats, and proteins to generate molecules that drive further energy production.
Understanding the Krebs Cycle
The Krebs cycle operates within the mitochondrial matrix of eukaryotic cells, a dense solution inside the inner membrane of mitochondria. This location is significant due to mitochondria’s role in energy generation. The cycle is cyclical, where a starting molecule, oxaloacetate, is regenerated at the end of each turn, allowing the process to continue without net consumption.
The purpose of this eight-step cycle is to oxidize carbon atoms, primarily from glucose, fatty acids, and amino acids, into carbon dioxide. As these carbon compounds are broken down, the cycle captures their stored energy in the form of high-energy electron carriers: NADH and FADH2. The Krebs cycle also directly produces a small amount of ATP, or its equivalent GTP, through substrate-level phosphorylation.
Key Reactants
The primary molecule entering the Krebs cycle is Acetyl-CoA. This two-carbon compound is an intermediate formed from the breakdown of glucose through glycolysis and pyruvate oxidation, as well as from the metabolism of fats and proteins. Acetyl-CoA delivers its acetyl group into the cycle, where it undergoes oxidative steps.
To begin the cycle, Acetyl-CoA combines with a four-carbon molecule called oxaloacetate. Oxaloacetate accepts the acetyl group, forming a six-carbon molecule known as citrate. Oxaloacetate is regenerated at the end of each turn, making it a catalytic intermediary. Water (H2O) molecules are also involved in several steps, participating in hydration reactions that prepare intermediates for subsequent transformations.
Primary Products
For each turn of the Krebs cycle, specific molecules are produced. Carbon dioxide (CO2) is released, with two molecules per cycle. This CO2 represents the fully oxidized carbon atoms from the Acetyl-CoA. Its release is a key part of cellular respiration, removing carbon waste products.
The cycle also generates high-energy electron carriers: three molecules of NADH and one molecule of FADH2 per turn. These molecules are reduced forms of nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD), having accepted electrons during the oxidative steps. Additionally, one molecule of ATP (adenosine triphosphate) or GTP (guanosine triphosphate) is directly produced per cycle. This ATP/GTP provides a small, immediate energy yield.
Why These Products Matter
The products of the Krebs cycle are important for the cell’s energy economy, particularly the electron carriers NADH and FADH2. These molecules carry high-energy electrons that are essential for the next major stage of cellular respiration. They transport these electrons to the electron transport chain (ETC), which is embedded in the inner mitochondrial membrane.
In the electron transport chain, the energy from these electrons is harnessed to pump protons across the mitochondrial membrane, creating a proton gradient. This gradient represents stored energy, used by ATP synthase to produce a large quantity of ATP through oxidative phosphorylation. While the Krebs cycle itself produces a modest amount of ATP directly, the NADH and FADH2 generated account for the vast majority of ATP produced during aerobic respiration. The ATP or GTP directly produced within the Krebs cycle can be immediately utilized by the cell for various metabolic needs.