In the presence of oxygen, __________________ enters the mitochondria from the cytoplasm. One ____________________ is removed from each pyruvate. The remaining __________________ attach to ___________________, creating _____________ which then enters the ____________________. In the absence of oxygen, pyruvate receive ___________________ from __________________, creating ________________. _________________  can then be converted back into _________ in the _____________. This is called the ___________ cycle.