How is CO2 transported to the bundle sheath cells in C4 plants?

In C4 plants, the transportation of carbon dioxide (CO2) to the bundle sheath cells occurs primarily as part of a 4-carbon compound known as malate. This process is a crucial aspect of the C4 photosynthetic pathway, which allows these plants to efficiently handle CO2 and minimize water loss, particularly in high-light and high-temperature environments.

Here's how it works: when CO2 enters the mesophyll cells, it is initially fixed onto a 3-carbon compound, phosphoenolpyruvate (PEP), by the enzyme PEP carboxylase. This reaction results in the formation of a 4-carbon compound, typically oxaloacetate, which is then converted into malate or other similar compounds. The malate is transported from the mesophyll cells into the bundle sheath cells, where it is decarboxylated to release CO2 for the Calvin cycle. This efficient transport mechanism enhances the concentration of CO2 around the enzyme RuBisCO, promoting photosynthesis while reducing photorespiration.

Understanding this specific pathway is essential for grasping how C4 plants have adapted to their environments and how they maintain efficiency in carbon fixation compared to C3 plants.