Editing Cellular respiration (section)

===Citric acid cycle===
{{Main|Citric acid cycle}}
The [[citric acid cycle]] is also called the ''Krebs cycle'' or the ''tricarboxylic acid cycle''. When oxygen is present, [[acetyl-CoA]] is produced from the pyruvate molecules created from glycolysis. Once [[acetyl-CoA]] is formed, aerobic or anaerobic respiration can occur. When oxygen is present, the mitochondria will undergo aerobic respiration which leads to the Krebs cycle. However, if oxygen is not present, [[fermentation]] of the pyruvate molecule will occur. In the presence of oxygen, when acetyl-CoA is produced, the molecule then enters the citric acid cycle (Krebs cycle) inside the mitochondrial matrix, and is oxidized to [[Carbon dioxide|CO<sub>2</sub>]] while at the same time reducing [[Nicotinamide adenine dinucleotide|NAD]] to [[NADH]]. NADH can be used by the [[electron transport chain]] to create further [[Adenosine triphosphate|ATP]] as part of oxidative phosphorylation. To fully oxidize the equivalent of one glucose molecule, two acetyl-CoA must be metabolized by the Krebs cycle. Two low-energy [[cellular waste product|waste products]], H<sub>2</sub>O and CO<sub>2</sub>, are created during this cycle.<ref name="Caspi three">{{cite web |url=https://biocyc.org/META/NEW-IMAGE?type=PATHWAY&object=PWY66-398 |title=Pathway: TCA cycle III (animals) |author=R. Caspi |publisher=MetaCyc Metabolic Pathway Database |date=2012-11-14 |access-date=2022-06-20 }}</ref><ref name="Caspi one">{{cite web |url=https://biocyc.org/META/NEW-IMAGE?type=PATHWAY&object=TCA |title=Pathway: TCA cycle I (prokaryotic) |author=R. Caspi |publisher=MetaCyc Metabolic Pathway Database |date=2011-12-19 |access-date=2022-06-20 }}</ref>

The citric acid cycle is an 8-step process involving 18 different enzymes and co-enzymes. During the cycle, acetyl-CoA (2 carbons) + [[Oxaloacetic acid|oxaloacetate]] (4 carbons) yields [[citrate]] (6 carbons), which is rearranged to a more reactive form called [[isocitrate]] (6 carbons). Isocitrate is modified to become [[Α-Ketoglutaric acid|α-ketoglutarate]] (5 carbons), [[succinyl-CoA]], [[Succinic acid|succinate]], [[fumarate]], [[malate]] and, finally, [[oxaloacetate]].<ref>{{Citation |last=Haddad |first=Aida |title=Biochemistry, Citric Acid Cycle |date=2025 |work=StatPearls |url=https://www.ncbi.nlm.nih.gov/books/NBK541072/ |access-date=2025-02-01 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=31082116 |last2=Mohiuddin |first2=Shamim S.}}</ref>

The net gain from one cycle is 3 NADH and 1 FADH<sub>2</sub> as hydrogen (proton plus electron) carrying compounds and 1 high-energy [[Guanosine triphosphate|GTP]], which may subsequently be used to produce ATP. Thus, the total yield from 1 glucose molecule (2 pyruvate molecules) is 6 NADH, 2 FADH<sub>2</sub>, and 2 ATP.<ref name="Caspi three"/><ref name="Caspi one"/><ref name=Mannion/>{{rp|90–91}}