What is the ultimate source of carbon for plants?

The ultimate source of carbon for plants is carbon dioxide in the air. During photosynthesis, plants take in carbon dioxide from the atmosphere through small openings in their leaves known as stomata. This carbon dioxide is then used in the Calvin cycle, which is a critical part of photosynthesis, to synthesize glucose and other carbohydrates.

Plants convert the carbon from carbon dioxide into organic compounds, which serve not only as their own food but also as energy sources for other organisms within the ecosystem. Although soil nutrients and organic matter can provide other essential elements and compounds for plant growth, the carbon atoms that form the backbone of organic molecules in plants originate from atmospheric carbon dioxide. Glucose in the roots is a product of this process rather than a source of carbon. Thus, carbon dioxide is ultimately essential for the establishment of the carbon compounds that are foundational to plant life and the life that depends on them.