Movement of molecular oxygen (O2) and carbon dioxide (CO2) across the cell membrane Choose one: A. occurs only unidirectionally, such that oxygen is allowed to enter and carbon dioxide (waste) is able to leave. B. only occurs when the O and C atoms are part of some other molecule (e. g., a sugar or amino acid). C. requires energy in the form of ATP or the proton motive force. D. occurs passively, but membrane transporters are required to facilitate their movement. E. occurs by passive diffusion.