CO2
Summary
CO₂ is carbon dioxide, the fully oxidized carbon end product of fuel breakdown, released in decarboxylation steps such as pyruvate dehydrogenase and the citric acid cycle. In blood it travels mostly as bicarbonate, so a rise in CO₂ lowers pH.
Detail
Meaning: CO₂ is the most oxidized form of carbon in metabolism. It is released by decarboxylations: pyruvate dehydrogenase (1 per pyruvate), then isocitrate dehydrogenase and α-ketoglutarate dehydrogenase (2 per turn of the citric acid cycle). Complete oxidation of one glucose releases 6 CO₂. Yeast fermentation also releases it. In blood, most CO₂ is carried as bicarbonate made by carbonic anhydrase in red blood cells; the rest is dissolved or bound to hemoglobin.
Equation: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻, where each species is a concentration in mol/L and carbonic anhydrase speeds the first step.
On the MCAT: Expect questions that count CO₂ released across glycolysis, pyruvate dehydrogenase and the citric acid cycle, or that give a pathway diagram and ask where carbons are lost. Also expect Le Chatelier reasoning: more CO₂ (hypoventilation, exercise) shifts the equation right and lowers pH, while hyperventilation lowers CO₂ and raises pH. Higher CO₂ and lower pH in active tissue shift the hemoglobin–O₂ curve right (Bohr effect), so hemoglobin releases more O₂.
Common confusion: Students think the CO₂ comes from the O₂ they breathe. Its carbons come from the fuel molecules, released by decarboxylation steps. O₂ is the final electron acceptor at the end of the electron transport chain and is reduced to H₂O.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Gatto, Stryer)
- Campbell Biology
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.