Why is the process referred to as C3 carbon fixation?

The process is referred to as C3 carbon fixation because it produces three-carbon (3-C) compounds as the first stable products of the photosynthetic process. Specifically, during the Calvin cycle, carbon dioxide is fixed by a molecule called ribulose bisphosphate (RuBP), resulting in the immediate formation of a three-carbon intermediate called 3-phosphoglycerate (3-PGA). This step is what distinguishes C3 photosynthesis from other types, like C4 and CAM, which have different initial products and adaptations for carbon fixation.

In this context, the reference to C3 underscores the specific nature of the carbon compounds involved in the initial fixation step, highlighting the significance of the 3-carbon compounds in the photosynthetic pathway that characterizes this method.