To fix carbon dioxide into sugar molecules in the process of photosynthesis, chloroplasts use an enzyme called RuBisCO. RuBisCO has a problem—it has trouble distinguishing between carbon dioxide and oxygen, so at high oxygen concentrations, RuBisCO starts accidentally adding oxygen to sugar precursors. This has the end result of ATP energy being wasted and being released, all with no sugar being produced. This is a big problem, since O is produced by the initial light reactions of photosynthesis, causing issues down the line in the Calvin cycle which uses RuBisCO.