The C₄ Plants

Numerous plants produce the 4-carbon compound oxaloacetic acid rather than the 3-carbon compound in PGA photosynthesis. Those that produce PGA are called C₃ plants. Those that produce oxaloacetic acid are called C₄ plants.

The oxaloacetic acid molecule is formed when a 3-carbon phosphoenol pyruvate (PEP) unites with carbon dioxide with the help of the enzyme phosphoenol pyruvate carboxylase. The C₄ plants possess two kinds of chloroplasts: large chloroplasts with few grana (which are found in the bundle sheath cells surrounding the veins of a leaf) and smaller, more numerous chloroplasts (which occur in the mesophyll of the leaf). The large chloroplasts have numerous starch grains not present in the smaller chloroplasts. The C₄ plants thrive at much higher temperature ranges than do the C₃ plants, and the rate of photosynthesis in C₄ plants is much faster than that in C₃ plants.