The Krebs cycle breaks down fuel from the food you eat and converts it into the energy carriers your cells need to produce ATP, the molecule that powers nearly everything your body does. Each turn of the cycle processes a small two-carbon fragment and produces electron-carrying molecules that go on to generate the bulk of your cellular energy. It also releases carbon dioxide as a waste product, which is part of the CO2 you exhale with every breath.
Also called the citric acid cycle or the TCA cycle, it takes place inside the mitochondria of your cells and sits at the very center of your metabolism. It’s the meeting point where carbohydrates, fats, and proteins all converge to be turned into usable energy.
Where It Fits in Cellular Respiration
Your cells break down glucose (and other fuels) in stages, not all at once. The first stage, glycolysis, happens outside the mitochondria and splits glucose into smaller molecules. Those smaller molecules then get converted into a two-carbon compound called acetyl-CoA, which enters the mitochondria and feeds directly into the Krebs cycle.
The Krebs cycle is the second major stage. It doesn’t produce much ATP on its own. Instead, its real job is loading up electron carriers, molecules called NADH and FADH2, that shuttle high-energy electrons to the third stage: the electron transport chain. That final stage is where the vast majority of ATP gets made. Think of the Krebs cycle as the preparation step that harvests the electrons and hands them off to the machinery that actually generates most of your energy.
What Goes In and What Comes Out
Each turn of the cycle starts when acetyl-CoA (carrying two carbon atoms) joins with a four-carbon molecule called oxaloacetate to form a six-carbon molecule, citrate. Over the next seven chemical steps, those two extra carbons are stripped away and released as two molecules of CO2. The original four-carbon oxaloacetate is regenerated at the end, ready to grab another acetyl-CoA and start the cycle again.
Per single turn of the cycle, the outputs are:
- 3 NADH, electron carriers that each go on to help produce roughly 2.5 ATP in the electron transport chain
- 1 FADH2, another electron carrier worth about 1.5 ATP
- 1 GTP, which is essentially equivalent to 1 ATP
- 2 CO2, released as waste
Since each glucose molecule produces two acetyl-CoA (one from each half of the original sugar), the cycle turns twice per glucose. That means one glucose molecule yields 6 NADH, 2 FADH2, and 2 GTP from the Krebs cycle alone. When those NADH and FADH2 molecules pass their electrons down the electron transport chain, the total contribution from the Krebs cycle accounts for the majority of the roughly 30 to 32 ATP molecules a single glucose can generate.
The Eight Steps of the Cycle
The cycle moves through eight intermediate molecules in sequence: citrate, isocitrate, alpha-ketoglutarate, succinyl-CoA, succinate, fumarate, malate, and finally back to oxaloacetate. You don’t need to memorize them all, but understanding the general flow helps.
The first step combines acetyl-CoA with oxaloacetate to make citrate (which is why the cycle is also called the citric acid cycle). Steps three and four are where the two carbon atoms leave as CO2, and where most of the NADH is generated. Step five is the only point where an energy molecule (GTP) is produced directly. Step six generates the cycle’s lone FADH2. The final step regenerates oxaloacetate so the whole process can repeat.
The rate-limiting step, the reaction that controls how fast the entire cycle spins, is step three: the conversion of isocitrate to alpha-ketoglutarate. When your cells need more energy, this reaction speeds up. When energy is plentiful, it slows down. This is one of the key ways your body matches energy production to demand.
It Processes More Than Just Carbohydrates
The Krebs cycle isn’t exclusive to glucose. It’s the common endpoint for all three macronutrients. Fatty acids are broken down into acetyl-CoA through a process called beta-oxidation and enter the cycle the same way glucose-derived fragments do. In fact, fats produce a large number of acetyl-CoA molecules per fatty acid chain, which is why fat is such an energy-dense fuel.
Amino acids from protein can also enter the cycle, though at different points. Some are converted to acetyl-CoA, while others are transformed into cycle intermediates like alpha-ketoglutarate or oxaloacetate. This flexibility makes the Krebs cycle a metabolic hub, not just an energy pathway. It connects carbohydrate, fat, and protein metabolism into a single integrated system.
Why CO2 Release Matters
The two molecules of CO2 released per turn of the cycle represent the final fate of the carbon atoms that were originally part of the food you ate. This is, quite literally, where your body finishes breaking down organic molecules. The carbon dioxide dissolves into your blood, travels to your lungs, and leaves your body when you exhale. A significant portion of the CO2 in every breath you take was produced in mitochondria during the Krebs cycle (and in the step just before it, when pyruvate is converted to acetyl-CoA).
What Happens When the Cycle Goes Wrong
Because the Krebs cycle is so central to metabolism, defects in its enzymes tend to cause serious problems. Mutations in the genes for different cycle enzymes are linked to a range of conditions, often involving the brain or cancer.
Defects in aconitase, the enzyme responsible for the cycle’s second step, are associated with brain disease and vision loss. Mutations in isocitrate dehydrogenase, which controls the rate-limiting step, have been found in patients with certain brain tumors (gliomas) and a form of blood cancer called acute myeloid leukemia. Mutations in succinate dehydrogenase, the enzyme behind step six, increase susceptibility to rare tumors called paragangliomas and have also been linked to kidney cancer. And defects in fumarase, which handles step seven, can cause severe brain disease in recessive form or increase cancer risk in dominant form, including uterine and kidney tumors.
The connection between cycle enzyme mutations and cancer is an active area of interest. When these enzymes malfunction, certain intermediate molecules can accumulate and interfere with normal cell signaling, pushing cells toward uncontrolled growth. This has reshaped how scientists think about metabolism’s role in cancer, showing it’s not just about energy but about the chemical signals that cycle intermediates send throughout the cell.