Q: Given scientific fact as input, generate the question from this fact such that it can be answered from the input.
Hydrocarbons are combined with oxygen in a series of enzymatic steps to product water, carbon dioxide, and energy, which is stored in the form of reactive molecules.
A:
Hydrocarbons are combined with oxygen in a series of enzymatic steps to product water, carbon dioxide, and energy, which is stored in the form of what?