C4 Plants: Plants that use a supplementary method of CO2 uptake to form a four-carbon sugar compound. Photosynthesis in these plants involves the reaction of CO2 with C3 phosphoenolpyruvate (PEP) to form C4 oxaloacetatic acid (OAA), which is converted into malic acid. Malic acid is then broken down into CO2 (which enters the Calvin Cycle to form sugars and starch) and pyruvic acid (3-carbon molecule), which is then converted back to PEP. C4 plants are well adapted for habitats with high daytime temperatures and intense sunlight. The 12C/13C ratio of C4 plants is distinct and can be determined from masspectrometry. C4 plants have less 12C but more 13C compared to C3 Plants. The 13C in C4 tissue is still less than what naturally occurs in the atmosphere. e.g. Tropical grasses, including crabgrass, corn, sugarcane, sorghum.