The citric acid cycle is a central stage of cellular respiration, the process cells use to extract usable energy from nutrients. Also called the Krebs cycle or tricarboxylic acid (TCA) cycle, it takes place primarily in the mitochondria of eukaryotic cells and processes carbon compounds derived from carbohydrates, fats, and proteins.
Despite its name, the cycle does not directly produce most of the cell’s ATP, the molecule commonly used to power cellular work. Instead, its main job is to transfer energy into electron carriers—NADH and FADH₂—that deliver high-energy electrons to the electron transport chain. The resulting electron flow drives the production of large amounts of ATP.
Understanding the cycle is much easier when it is viewed as a sequence of chemical transformations rather than as a list of eight molecules to memorize.
Where the citric acid cycle fits into cellular respiration
Cellular respiration can be divided into several interconnected stages.
Glycolysis occurs in the cytoplasm and splits one glucose molecule into two molecules of pyruvate. Pyruvate then enters the mitochondrion in eukaryotic cells and is converted into acetyl-CoA, producing NADH and releasing carbon dioxide. This conversion is sometimes called the pyruvate oxidation or link reaction.
The citric acid cycle then oxidizes the acetyl group of acetyl-CoA. The cycle captures much of the released energy in NADH and FADH₂.
Finally, the electron transport chain uses those electron carriers to establish a proton gradient across the inner mitochondrial membrane. ATP synthase uses that gradient to make ATP in a process called oxidative phosphorylation.
So the cycle is not an isolated pathway. It is a metabolic hub between the breakdown of food molecules and the machinery that produces most of the ATP during aerobic respiration.
The cycle begins with acetyl-CoA
Before the cycle starts, pyruvate produced by glycolysis is converted to acetyl-CoA when oxygen is available to support continued aerobic metabolism.
A molecule of pyruvate contains three carbon atoms. During its conversion to acetyl-CoA, one carbon is released as carbon dioxide. The remaining two-carbon acetyl group is attached to coenzyme A, forming acetyl-CoA. At the same time, NAD⁺ accepts electrons and becomes NADH.
Acetyl-CoA then supplies the two-carbon unit that enters the citric acid cycle.
Importantly, the carbon dioxide released during this preparatory reaction and during the cycle does not represent the oxygen we breathe being directly attached to carbon from glucose. Oxygen is used later as the final electron acceptor in the electron transport chain. The carbon dioxide comes from the oxidation of carbon-containing metabolic intermediates.
Step 1: Acetyl-CoA combines with oxaloacetate
The cycle begins when the two-carbon acetyl group from acetyl-CoA combines with oxaloacetate, a four-carbon molecule.
The reaction produces citrate, a six-carbon compound, and releases coenzyme A.
This reaction is catalyzed by the enzyme citrate synthase.
Oxaloacetate is crucial because it is regenerated at the end of the cycle. That regeneration allows another acetyl-CoA molecule to enter, making the pathway a cycle rather than a simple linear chain of reactions.
Step 2: Citrate is rearranged
Citrate is converted into isocitrate, another six-carbon molecule with its atoms arranged differently.
The enzyme aconitase catalyzes this rearrangement.
Although this step does not directly generate ATP or an electron carrier, it prepares the molecule for the oxidation reactions that follow.
Step 3: Isocitrate is oxidized
Isocitrate undergoes oxidation, meaning it loses electrons. NAD⁺ accepts those electrons and is reduced to NADH.
At the same time, one carbon is removed from the molecule and released as carbon dioxide.
The six-carbon isocitrate therefore becomes a five-carbon compound called α-ketoglutarate (alpha-ketoglutarate).
This is the first step of the cycle that produces both NADH and carbon dioxide.
Step 4: α-Ketoglutarate is oxidized
The five-carbon α-ketoglutarate undergoes another oxidative decarboxylation. In other words, it is oxidized while losing a carbon as carbon dioxide.
NAD⁺ again accepts electrons, producing another NADH.
The remaining four-carbon compound is attached to coenzyme A, forming succinyl-CoA.
This reaction is catalyzed by the α-ketoglutarate dehydrogenase complex.
At this point, two carbons have been released as carbon dioxide since the acetyl group entered the cycle, although the individual carbon atoms released as CO₂ do not necessarily correspond directly to the two carbons that most recently entered the cycle.
Step 5: Succinyl-CoA produces GTP or ATP
Succinyl-CoA is converted into succinate, a four-carbon compound.
The energy released by breaking the high-energy bond involving coenzyme A is used to generate GTP in many animal cells. GTP is closely related to ATP and can readily be converted to ATP. In some tissues or organisms, the corresponding reaction produces ATP directly.
This is an example of substrate-level phosphorylation: an enzyme transfers energy from a metabolic intermediate to form a phosphorylated nucleotide rather than relying on the electron transport chain.
Only a small amount of ATP-equivalent energy is produced this way during the cycle. The larger energy payoff comes from NADH and FADH₂ later donating electrons to the respiratory chain.
Step 6: Succinate is oxidized
Succinate is oxidized to fumarate.
Here, the electron carrier FAD accepts the electrons and hydrogen associated with the reaction, becoming FADH₂.
The enzyme responsible, succinate dehydrogenase, has an unusual role: it participates directly in both the citric acid cycle and the electron transport chain. It is also known as Complex II of the respiratory chain and is embedded in the inner mitochondrial membrane.
Because FADH₂ enters the electron transport chain at a different point from NADH, it ultimately supports less ATP production than an equivalent amount of NADH.
Step 7: Fumarate is hydrated
Fumarate reacts with water to form malate, another four-carbon compound.
This reaction, catalyzed by fumarase, does not directly produce ATP, NADH, or FADH₂.
Its purpose is to create the chemical arrangement needed for the final oxidation step of the cycle.
Step 8: Malate is oxidized and oxaloacetate is regenerated
Malate is oxidized to oxaloacetate.
NAD⁺ accepts the electrons, producing a third NADH for each turn of the cycle.
The regeneration of oxaloacetate completes the cycle. The newly formed oxaloacetate can combine with another acetyl-CoA molecule, beginning another round.
The pathway can therefore continue as long as acetyl-CoA, the necessary enzymes, and suitable electron acceptors are available.
What one turn of the cycle produces
One molecule of acetyl-CoA entering the citric acid cycle produces:
| Product | Amount per acetyl-CoA |
|---|---|
| NADH | 3 |
| FADH₂ | 1 |
| GTP or ATP | 1 |
| CO₂ | 2 |
The cycle itself therefore produces only one ATP-equivalent molecule directly. Its greater importance comes from the four reduced electron carriers it generates: three NADH molecules and one FADH₂ molecule.
Each glucose molecule produces two pyruvate molecules through glycolysis, and each pyruvate can yield one acetyl-CoA. Consequently, the citric acid cycle turns twice for each glucose molecule that is completely processed through aerobic respiration.
Why NADH and FADH₂ matter more than ATP production in the cycle
NADH and FADH₂ function as energy-rich electron carriers.
During the cycle’s oxidation reactions, electrons are removed from carbon-containing molecules and transferred to NAD⁺ or FAD. These carriers then deliver the electrons to the electron transport chain.
As electrons move through the respiratory chain, their energy is used to pump protons across the inner mitochondrial membrane. This creates an electrochemical gradient. Protons then flow back through ATP synthase, which uses the stored energy to phosphorylate ADP and produce ATP.
This is why the citric acid cycle is often described as indirectly generating most of the ATP associated with aerobic cellular respiration. The cycle captures chemical energy in electron carriers; oxidative phosphorylation converts much of that captured energy into ATP.
The exact ATP yield is not a fixed whole number because it depends on how reducing equivalents enter the mitochondrion, how efficiently the proton gradient is used, and other cellular conditions. A commonly cited overall yield for complete aerobic oxidation of one glucose molecule in eukaryotic cells is roughly 30–32 ATP, with the citric acid cycle contributing primarily through its production of NADH and FADH₂ rather than through direct ATP formation.
The cycle is also a metabolic crossroads
The citric acid cycle does more than extract energy.
Its intermediates are used to build other molecules. For example, citrate can contribute carbon for fatty-acid synthesis, while α-ketoglutarate and oxaloacetate can participate in the production of amino acids. Other intermediates can feed into pathways involved in synthesizing compounds needed by the cell.
This means that the cycle must balance two competing demands: it needs to continue oxidizing acetyl-CoA for energy while also supplying intermediates for biosynthesis.
When an intermediate is removed for another purpose, cells can replenish the cycle through anaplerotic reactions. Conversely, when intermediates accumulate, they can be diverted into other metabolic pathways.
The cycle is therefore better understood as part of a flexible metabolic network than as a simple circular conveyor belt.
What happens when oxygen is unavailable?
The citric acid cycle does not use molecular oxygen directly in any of its eight main reactions. Nevertheless, in typical animal cells it cannot continue indefinitely without oxygen.
The reason is indirect. The NADH and FADH₂ produced by the cycle must be reoxidized to NAD⁺ and FAD so that they can accept more electrons. Under aerobic conditions, the electron transport chain accomplishes this by ultimately transferring electrons to oxygen, forming water.
Without sufficient oxygen, electron transport slows or stops. NADH then accumulates while the supply of NAD⁺ falls. Several oxidative reactions, including key reactions associated with the citric acid cycle, can no longer proceed normally.
Cells that lack adequate oxygen can instead rely more heavily on pathways such as fermentation, which regenerate NAD⁺ from NADH and allow glycolysis to continue. Fermentation does not, however, provide the same overall energy yield as complete aerobic oxidation.
Why the cycle is called a cycle
The name can be confusing because acetyl-CoA enters the pathway but is not simply transformed into a molecule that leaves at the end.
The key is oxaloacetate. It participates at the beginning by combining with acetyl-CoA to form citrate, passes through a series of transformations, and is regenerated at the end.
The two-carbon acetyl group is progressively oxidized as the pathway proceeds. Its carbon atoms ultimately contribute to carbon dioxide, while the energy associated with oxidation is captured primarily in NADH and FADH₂.
The regeneration of oxaloacetate is what allows the process to repeat.
A useful way to remember the eight intermediates
The eight principal intermediates, in order, are:
Citrate → isocitrate → α-ketoglutarate → succinyl-CoA → succinate → fumarate → malate → oxaloacetate
Rather than memorizing the names as an isolated sequence, it is more useful to recognize the major events attached to them:
- Citrate formation: acetyl-CoA enters.
- Isocitrate oxidation: NADH and CO₂ are produced.
- α-Ketoglutarate oxidation: NADH and CO₂ are produced.
- Succinyl-CoA conversion: GTP or ATP is produced.
- Succinate oxidation: FADH₂ is produced.
- Fumarate hydration: water is added.
- Malate oxidation: NADH is produced.
- Oxaloacetate regeneration: the cycle is ready to begin again.
The pattern reveals the central purpose of the pathway: oxidize carbon compounds and capture their energy in electron carriers while regenerating the molecule required for another turn.
The bigger picture
The citric acid cycle sits at the center of aerobic metabolism because it connects the breakdown of carbohydrates, fats, and many amino acids with the electron transport chain and ATP production.
Its eight reactions do not simply “make energy.” They progressively oxidize acetyl-derived carbon, release carbon dioxide, generate a small amount of ATP-equivalent energy directly, and—most importantly—load NADH and FADH₂ with high-energy electrons. Those carriers then power oxidative phosphorylation, where most of the ATP associated with aerobic respiration is produced.
That combination of energy extraction, electron transfer, and metabolic flexibility makes the citric acid cycle one of the central pathways of cellular metabolism.
