How does CO2 function in the carbon cycle?

The role of carbon dioxide (CO2) in the carbon cycle is fundamentally linked to its use as a substrate in photosynthesis. During photosynthesis, plants, algae, and some bacteria take in CO2 from the atmosphere and, using sunlight and water, convert it into glucose and oxygen. This process not only supports the growth and energy needs of the plant itself but also serves as the foundational source of energy for the entire food web.

Photosynthesis effectively transforms CO2, which is often considered a greenhouse gas, into organic compounds that are vital for life on Earth. As plants grow, they capture carbon from the atmosphere, thus contributing to the regulation of CO2 levels while also sequestering carbon in their biomass. This process is essential for maintaining the balance of the carbon cycle, linking the atmosphere, biosphere, and lithosphere.

In contrast, other options do not accurately describe CO2's primary function in the carbon cycle. While it is true that CO2 is a waste product of respiration, this is not its role in the broader cycle but rather a consequence of cellular metabolism. It does not provide energy for plant growth directly; rather, plants convert it into energy through the process of photosynthesis. Furthermore, CO2's connection to