Which component of Earth's atmosphere exists almost entirely as a result of photosynthesis?

• A. Nitrogen (\(\mathrm{N_2}\))
• B. Carbon dioxide (\(\mathrm{CO_2}\))
• C. Argon (\(\mathrm{Ar}\))
• D. Oxygen (\(\mathrm{O_2}\))

Answer: (D)

Verified step by step guidance

1. Understand the role of photosynthesis in Earth's atmosphere: Photosynthesis is a biological process carried out by plants, algae, and cyanobacteria, where light energy is used to convert carbon dioxide (\(\mathrm{CO_2}\)) and water (\(\mathrm{H_2O}\)) into glucose (\(\mathrm{C_6H_{12}O_6}\)) and oxygen (\(\mathrm{O_2}\)).
2. Recognize that oxygen (\(\mathrm{O_2}\)) is a byproduct of photosynthesis: During the light-dependent reactions of photosynthesis, water molecules are split (a process called photolysis), releasing oxygen gas into the atmosphere.
3. Compare the listed atmospheric components: Nitrogen (\(\mathrm{N_2}\)) is primarily introduced through geological processes and biological nitrogen fixation, carbon dioxide (\(\mathrm{CO_2}\)) is released through respiration and combustion, and argon (\(\mathrm{Ar}\)) is inert and originates from radioactive decay. None of these are directly produced by photosynthesis.
4. Identify oxygen (\(\mathrm{O_2}\)) as the correct answer: Oxygen exists in Earth's atmosphere almost entirely due to photosynthesis, as it is continuously replenished by this process.
5. Conclude that photosynthesis is essential for maintaining oxygen levels in the atmosphere, which supports aerobic life forms and contributes to Earth's habitability.