Explain the process of carbon fixation in C4 plants, including the roles of mesophyll and bundle sheath cells.

In C4 plants, the Calvin cycle is carried out in a two-step process involving two types of cells: mesophyll cells and bundle sheath cells. Carbon dioxide is first fixed in the mesophyll cells by the enzyme PEP carboxylase, which has a high affinity for $\text{CO}_2$ and does not bind $\text{O}_2$. This forms a 4-carbon compound, typically oxaloacetate.

The 4-carbon compound is then converted to malate or aspartate and transported to the bundle sheath cells. Inside the bundle sheath cells, the 4-carbon compound is decarboxylated, releasing $\text{CO}_2$. This creates a high concentration of $\text{CO}_2$ around RuBisCO, allowing the Calvin cycle to proceed efficiently with minimal photorespiration. The $\text{CO}_2$ is then fixed by RuBisCO into sugars via the standard Calvin cycle.