Suggest a reason why life on earth might not continue without photosynthesis.

• A. Lack of oxygen
• B. Lack of food (energy)
• C. Excess CO2
• D. Plants die

Answer: (A)

Photosynthesis not only builds the base of the food chain by creating organic matter, it also continuously adds oxygen to the atmosphere. The energy that most organisms rely on comes from cellular respiration, a process that uses oxygen to extract energy efficiently from glucose. If photosynthesis stopped, oxygen would gradually be used up in respiration and other chemical processes, and the atmosphere would become low in oxygen.

With little oxygen available, most organisms that rely on aerobic respiration would struggle to generate enough energy to survive. Even if some life could cope without oxygen, the many organisms that form the bulk of Earth’s ecosystems—and the energy demands of multicellular life—would be severely impacted. So the lack of oxygen is the strongest reason life as we know it might not continue without photosynthesis.