During photosynthesis, carbon is in the form of carbon dioxide (CO2) that is taken up by plants and converted into organic molecules such as glucose.
During respiration, organic molecules such as glucose are broken down into carbon dioxide (CO2) and water (H2O) to release energy. Carbon is in the form of CO2 during respiration.
During consumption (eating), organic molecules such as glucose and other carbohydrates are ingested and broken down into CO2 and H2O during respiration, releasing energy.
During combustion, organic molecules such as fossil fuels (coal, oil, and natural gas) are burned, releasing energy and producing CO2 and other pollutants such as nitrogen oxides and sulfur dioxide. Carbon is in the form of CO2 during combustion.