The C4 Pathway

Fig. 1: The C₄ Pathway

The C₄ pathway is designed to efficiently fix CO₂ at low concentrations and plants that use this pathway are known as C₄ plants. These plants fix CO₂ into a four carbon compound (C₄) called oxaloacetate. This occurs in cells called mesophyll cells.

1. CO₂ is fixed to a three-carbon compound called phosphoenolpyruvate to produce the four-carbon compound oxaloacetate. The enzyme catalyzing this reaction, PEP carboxylase, fixes CO₂ very efficiently so the C₄ plants don't need to to have their stomata open as much. The oxaloacetate is then converted to another four-carbon compound called malate in a step requiring the reducing power of NADPH.

3. The malate then exits the mesophyll cells and enters the chloroplasts of specialized cells called bundle sheath cells. Here the four-carbon malate is decarboxylated to produce CO₂, a three-carbon compound called pyruvate, and NADPH. The CO₂ combines with ribulose bisphosphate and goes through the Calvin cycle.

4. The pyruvate re-enters the mesophyll cells, reacts with ATP, and is converted back to phosphoenolpyruvate, the starting compound of the C₄ cycle.

Illustration of The C4 Pathway .jpg by Gary E. Kaiser, Ph.D.
Professor of Microbiology, The Community College of Baltimore County, Catonsville Campus
This work is licensed under a Creative Commons Attribution 4.0 International License.
Based on a work at https://cwoer.ccbcmd.edu/science/microbiology/index_gos.html.

Last updated: Feb., 2021
Please send comments and inquiries to Dr. Gary Kaiser

Fig. 1: The C4 Pathway

Fig. 1: The C₄ Pathway