Given a scientific question, generate a correct answer to it.

Ex Input:
What model postulates that electrons orbit the nucleus at fixed energy levels?

Ex Output:
bohr

Ex Input:
Electrons in covalent compounds are shared between the two atoms, unlike the case in what type of bonds?

Ex Output:
ionic bonds

Ex Input:
Enzymes, proteins, electron carriers, and pumps that play roles in glycolysis, the citric acid cycle, and the electron transport chain tend to catalyze reactions that are what?

Ex Output:
non-reversible