Plants obtain the majority of the carbon necessary for building structural molecules from:

• A. glucose absorbed through roots
• B. carbon dioxide (CO_2) in the atmosphere
• C. organic compounds in the soil
• D. carbonates dissolved in water

Answer: (B)

Verified step by step guidance

1. Understand the role of carbon in plants: Carbon is a fundamental element used by plants to build structural molecules like cellulose, which is a major component of their cell walls.
2. Recognize the process of photosynthesis: Plants primarily obtain carbon through the process of photosynthesis, where they use sunlight to convert carbon dioxide (CO₂) from the atmosphere into glucose (C₆H₁₂O₆).
3. Clarify why glucose absorbed through roots is incorrect: Plants do not absorb glucose through their roots. Instead, they synthesize glucose internally during photosynthesis.
4. Explain why organic compounds in the soil are not the primary source: While soil contains organic compounds, plants do not rely on these for their carbon needs. They primarily use carbon dioxide from the air.
5. Discuss why carbonates dissolved in water are not the main source: Carbonates in water may contribute to aquatic plant growth in specific cases, but terrestrial plants primarily depend on atmospheric CO₂ for their carbon needs.