What is the 4-C compound produced in C4 carbon fixation?

In C4 carbon fixation, the primary 4-carbon compound produced is oxaloacetate. This process occurs in certain plants, known as C4 plants, which have adapted to efficiently carry out photosynthesis in conditions of high light intensity and low carbon dioxide availability.

During the initial phase of C4 photosynthesis, carbon dioxide is fixed into a 3-carbon compound, phosphoenolpyruvate (PEP), by the enzyme PEP carboxylase. This reaction leads to the formation of oxaloacetate, which is then converted into malate or aspartate. These C4 acids are transported to specialized cells where they release carbon dioxide for the Calvin cycle, effectively concentrating CO2 to enhance photosynthetic efficiency.

This differentiation in pathways and products is what distinguishes C4 plants from those utilizing the more common C3 pathway, thereby allowing them to thrive in specific environmental conditions.