Carbon dioxide
Explanation
The atmosphere is composed of a mixture of gases, some of which are constant while others are variable. These gases play distinct roles in sustaining the biosphere and regulating the Earth's temperature. Photosynthesis is the primary biological process through which autotrophs convert light energy into chemical energy.Analysis of Options:
- Carbon dioxide ($CO_2$): Although it constitutes a small fraction (approximately 0.036%) of the atmosphere, it is biologically crucial. Green plants absorb atmospheric carbon dioxide to manufacture food (glucose) in the presence of sunlight and chlorophyll. It is the specific reactant used as the carbon source in photosynthesis.
- Oxygen ($O_2$): Oxygen is utilized by living organisms for respiration and is essential for combustion. In the context of photosynthesis, oxygen is a by-product released by plants, not the gas utilized.
- Nitrogen ($N_2$): Nitrogen is the most abundant gas in the atmosphere (78%). However, plants cannot utilize atmospheric nitrogen directly; it must be fixed into soil nitrates by bacteria or other means before plants can absorb it for protein synthesis.
- Argon ($Ar$): Argon is an inert noble gas constituting roughly 0.93% of the atmosphere. It is chemically inactive and plays no role in biological processes like photosynthesis.
Key Takeaway: Carbon dioxide is the essential atmospheric raw material for photosynthesis, while oxygen is the vital gas for respiration.