In the Krebs cycle, which requires free oxygen, pyruvate is converted into the 2-carbon actetyl group, which becomes attached to a coenzyme forming acetyl coenzyme A.

• 卡尔文循环需要充足的氧气，丙酮酸被氧化成2碳的乙酰基，与一个辅酶结合形成2分子的乙酰辅酶A。