C3 Plants: Plants that only use the Calvin-Benson Cycle for fixing CO2 from the air. Photosynthesis in these plants involves the reaction of CO2 with C5 RuBP (ribulose-1,5-biphosphate) to form two C3 phosphoglyceric acid molecules (3PGA) in the Calvin Cycle, making hexose carbohydrates. C3 plants originated during the Mesozoic and Paleozoic eras, predating C4 plants. C3 plants thrive in moderate sunlight and temperature environments. The 12C/13C ratio of C3 plants is unique and can be determined from mass spectrometry. C3 plants have more 12C compared to C4 Plants, and have less 13C in their tissue compared to what naturally occurs in the atmosphere. e.g. Herbaceous plants, cool season grasses, tree leaves, flowers, stems, and fruits.