The Krebs cycle is a series of eight chemical reactions that your cells use to break down food molecules and extract energy. Also called the citric acid cycle, it’s the central hub of your metabolism, the process that converts the calories you eat into the fuel your cells actually run on. It takes place inside structures called mitochondria, often described as the powerhouses of your cells.
How the Krebs Cycle Fits Into Energy Production
Your body breaks down food in stages. First, glucose (from carbohydrates) gets split in half through a process called glycolysis, which happens outside the mitochondria. That splitting produces a molecule called pyruvate. Before the Krebs cycle can begin, pyruvate enters the mitochondria and gets converted into a smaller molecule called acetyl-CoA, releasing one molecule of carbon dioxide in the process. Acetyl-CoA is the actual fuel that enters the Krebs cycle.
Fats and proteins can also feed into the cycle. Fatty acids get broken down into acetyl-CoA through a separate process, and certain amino acids from protein can enter at various points. This is why the Krebs cycle is sometimes called a metabolic hub: nearly everything you eat eventually passes through it.
What Happens in the Eight Steps
The cycle begins when acetyl-CoA (a two-carbon molecule) combines with oxaloacetate (a four-carbon molecule) to form citrate, a six-carbon compound. This is where the name “citric acid cycle” comes from. From there, citrate is gradually rearranged and broken down through a sequence of chemical transformations. It passes through intermediates including isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, and malate before regenerating oxaloacetate, which loops back to grab another acetyl-CoA and start again.
At two specific steps, carbon atoms are stripped off and released as carbon dioxide. This is, quite literally, why you breathe out CO2. The carbon in your exhaled breath originated from the food molecules being disassembled in this cycle.
Energy Produced Per Turn
Each turn of the Krebs cycle doesn’t produce much ATP (your cell’s energy currency) directly. One turn generates just one molecule of GTP, which is essentially equivalent to one ATP. The real energy payoff comes from the cycle’s other products: three molecules of NADH and one molecule of FADH2. These are electron carriers, molecules loaded with high-energy electrons that get passed along to the next stage of energy production, the electron transport chain, where the bulk of ATP is made.
Since each glucose molecule produces two acetyl-CoA molecules, the cycle turns twice per glucose. That means one glucose molecule yields six NADH, two FADH2, and two GTP from the Krebs cycle alone. When those electron carriers are processed in the electron transport chain, the total ATP yield from a single glucose molecule (across all stages of metabolism) reaches roughly 30 to 38 ATP.
Why It Needs Oxygen (Indirectly)
The Krebs cycle itself doesn’t use oxygen directly. None of the eight reactions require an oxygen molecule. However, the cycle depends entirely on oxygen being available downstream. Here’s why: the NADH and FADH2 produced by the cycle must hand off their electrons to the electron transport chain, which uses oxygen as the final electron acceptor. Without oxygen, those carriers can’t be recycled back to their empty forms (NAD+ and FAD), and without those empty carriers, the Krebs cycle grinds to a halt. This is why the cycle only runs during aerobic (oxygen-using) metabolism.
How Your Body Regulates the Cycle
Your cells don’t run the Krebs cycle at full speed all the time. Three key enzymes act as control points: citrate synthase (the first step), isocitrate dehydrogenase, and alpha-ketoglutarate dehydrogenase. All three respond to the ratio of NAD+ to NADH in the mitochondria, which itself reflects how much ATP the cell currently has. When energy is abundant, NADH levels are high, and these enzymes slow down. When the cell is burning through ATP, NAD+ levels rise and the cycle speeds up.
Calcium ions also play a regulatory role. They increase the activity of several cycle enzymes, which helps explain why the cycle ramps up during muscle contraction, when calcium floods into muscle cells to trigger movement.
The Krebs Cycle During Exercise
During physical activity, your muscles demand far more ATP than at rest, and the Krebs cycle is a major part of meeting that demand. When you exercise at moderate intensity, your breathing and heart rate increase to deliver more oxygen to your muscles. Within a few minutes, your aerobic metabolism reaches a steady state where the Krebs cycle and electron transport chain supply the majority of energy your muscles need.
During high-intensity exercise, your muscles burn through carbohydrates preferentially because the rate of ATP production from carbs is about twice as fast as from fat. Pyruvate from glucose breakdown floods into the mitochondria and gets converted to acetyl-CoA, spinning the cycle faster. At very high intensities, when oxygen delivery can’t keep pace, your muscles increasingly rely on anaerobic pathways (producing lactic acid) while the aerobic system works at its ceiling.
Beyond Energy: Building Blocks for the Body
The Krebs cycle isn’t just about burning fuel. Several of its intermediates get pulled out and used as raw materials for building other molecules your body needs. Alpha-ketoglutarate and oxaloacetate, for example, serve as starting points for making certain amino acids. Citrate can be exported from the mitochondria and used to build fatty acids for cell membranes and energy storage. Succinyl-CoA is a precursor for making heme, the molecule in red blood cells that carries oxygen.
When intermediates are siphoned off for these purposes, they must be replaced to keep the cycle turning. Your cells handle this through “anaplerotic” reactions, which is just a technical way of saying refill reactions. The most common one converts pyruvate directly into oxaloacetate, topping off the cycle so it can keep running even while donating its components to other processes.
Who Discovered It
The cycle is named after Hans Adolf Krebs, a German-born British biochemist who mapped out the pathway in 1937. He received the Nobel Prize in Physiology or Medicine in 1953 for the discovery, sharing the prize with Fritz Lipmann, who identified coenzyme A, the molecule that delivers acetyl groups into the cycle. Krebs built on earlier work identifying individual reactions and was the first to recognize that they formed a continuous loop rather than a linear pathway.