Which term describes an organism that requires a constant supply of oxygen in order to live?

• A. Aerobic
• B. Anaerobic
• C. Pathogen
• D. Endospore

Answer: (A)

Oxygen use in metabolism defines how an organism gets energy. An organism that requires a constant supply of oxygen is described as aerobic. That means it relies on aerobic respiration, where oxygen serves as the final electron acceptor in the electron transport chain, allowing electrons to flow and a large amount of ATP to be produced from each glucose molecule. Without oxygen, these cells can’t efficiently generate ATP through the main energy pathway, so they either switch to less efficient methods or cannot survive.

The other terms refer to different concepts. Anaerobic describes organisms that don’t require oxygen and can grow with little or no oxygen. Pathogen is about whether an organism causes disease, not its oxygen needs. An endospore is a dormant, tough form some bacteria form to survive harsh conditions, not about whether they require oxygen.