O2
Summary
O₂ is molecular oxygen, the final electron acceptor of the electron transport chain, where it is reduced to H₂O. Without it the chain stalls, ATP yield drops, and cells shift toward glycolysis and fermentation. Blood carries it on hemoglobin, whose affinity changes with pH, CO₂ and temperature.
Detail
Meaning: In aerobic respiration, Complex IV (cytochrome c oxidase) passes electrons to O₂, which takes up protons to form H₂O. This keeps electrons moving, sustains the proton gradient that drives ATP synthase, and regenerates NAD⁺ and FAD. In blood, most O₂ travels bound to hemoglobin (four per molecule, cooperative binding), with a small fraction dissolved in plasma.
Equation: O₂ + 4H⁺ + 4e⁻ → 2H₂O, where H⁺ is a proton and e⁻ an electron; the coefficients are mole ratios (mol).
On the MCAT: Passages describe an inhibitor or a lack of O₂ and ask what follows: carriers stay reduced, the proton gradient falls, ATP synthesis drops, and lactate or ethanol fermentation rises. Other questions give a hemoglobin saturation curve. It is sigmoidal. Higher CO₂, lower pH, higher temperature or more 2,3-BPG shift it right, so affinity falls and tissues receive more O₂. Fetal hemoglobin sits to the left of adult hemoglobin, and myoglobin has a hyperbolic curve with higher affinity.
Common confusion: Students think the O₂ consumed turns into the CO₂ exhaled. The CO₂ comes from pyruvate dehydrogenase and the citric acid cycle, while the oxygen atoms from O₂ end up in water. O₂ does not make ATP directly; it lets the chain keep running.
Sources
- Campbell Biology
- Lehninger Principles of Biochemistry
- Biology (Raven)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.