In the Calvin cycle, what is the process of carbon fixation?

In the Calvin cycle, carbon fixation is the process where carbon dioxide is incorporated into an organic molecule. This occurs when carbon dioxide from the atmosphere is captured by the enzyme ribulose bisphosphate carboxylase/oxygenase (commonly known as Rubisco) and attached to ribulose bisphosphate (RuBP). The resulting unstable six-carbon compound quickly splits into two three-carbon molecules called 3-phosphoglycerate (3-PGA). This initial incorporation of carbon into a stable organic form is crucial for the subsequent stages of the Calvin cycle, which ultimately lead to the synthesis of glucose and other carbohydrates.

The other options describe processes that either do not occur in the Calvin cycle or represent different biochemical reactions. For instance, removing carbon from organic compounds, converting carbon dioxide directly into glucose, or breaking down sugars all involve either degradation or different metabolic pathways, rather than the initial fixation of carbon into organic molecules that defines this phase of the Calvin cycle.