In the context of introduction to photosynthesis, the carbon that plants use to build sugars during photosynthesis comes primarily from which source?

• A. Mineral nutrients (e.g., nitrate and phosphate) absorbed from the soil
• B. Carbon dioxide ($C_1{O}_2$) in the atmosphere
• C. Oxygen gas ($O_2$) released during the light reactions
• D. Organic carbon compounds taken up directly through the roots from humus

Answer: (B)

Verified step by step guidance

1. Understand that photosynthesis is the process by which plants convert light energy into chemical energy, producing sugars as a form of stored energy.
2. Recognize that the carbon atoms in the sugars produced during photosynthesis must come from an external source, as plants cannot create carbon atoms themselves.
3. Identify the main sources of carbon available to plants: mineral nutrients from the soil (such as nitrate and phosphate), oxygen gas (O\_2) in the atmosphere, organic carbon compounds in the soil, and carbon dioxide (CO\_2) in the atmosphere.
4. Recall that mineral nutrients like nitrate and phosphate provide nitrogen and phosphorus, not carbon, and oxygen gas is a byproduct of photosynthesis, not a carbon source.
5. Conclude that the carbon used to build sugars during photosynthesis primarily comes from carbon dioxide (CO\_2) in the atmosphere, which plants take in through their leaves and incorporate into organic molecules during the Calvin cycle.