UPSC Civil Services 2011 Prelims General Studies Paper I

Photosynthesis by plants removes about 120 billion tons of carbon from the air per year, but plant decomposition returns about the same amount. When the organic matter is oxidized through respiration, the reverse of photosynthesis takes place. Respiration releases CO‌2 into the atmosphere. Respiration and photosynthesis occur at nearly equal rates over one year. Volcanic eruptions and metamorphism release gases into the atmosphere. Volcanic gases are primarily water vapor, carbon dioxide and sulphur dioxide.