What happens to most of the oxygen a leaf makes while it is photosynthesising?
Explanation
Photosynthesis produces far more oxygen than the plant has any use for, so nearly all of it diffuses out of the leaf into the surrounding air as waste. The gas the reaction takes in as a raw material is a different one; a leaf has no store in which it could hold this gas back until dark; and the roots receive no delivery of it either, because the oxygen around their cells comes from the air trapped between the soil particles.