What is the major source of free oxygen (e O_2) in the Earth's atmosphere?

• A. Decomposition of organic matter
• B. Respiration by animals
• C. Photosynthetic splitting of water during the light reactions of photosynthesis
• D. Volcanic outgassing

Answer: (C)

Verified step by step guidance

1. Understand the context: Free oxygen (O₂) in the Earth's atmosphere primarily originates from biological processes, specifically photosynthesis. This process occurs in plants, algae, and cyanobacteria.
2. Recall the light reactions of photosynthesis: During these reactions, light energy is absorbed by chlorophyll and used to split water molecules (H₂O) into oxygen gas (O₂), protons (H⁺), and electrons. This process is called photolysis.
3. Recognize the role of photolysis: The splitting of water molecules during the light reactions releases oxygen gas (O₂) as a byproduct, which is then released into the atmosphere.
4. Eliminate incorrect options: Decomposition of organic matter and respiration by animals consume oxygen rather than produce it. Volcanic outgassing releases gases like CO₂ and sulfur compounds but is not a major source of free oxygen.
5. Conclude that the major source of free oxygen in the Earth's atmosphere is the photosynthetic splitting of water during the light reactions of photosynthesis.