The C3− cycle (a type of dark phase of photosynthesis) begins with RuBP, a five carbon sugar. Enzymes combine six RuBP molecules with CO2 from the atmosphere to form six molecules of an extremely unstable six carbon compound. This compound spontaneously reacts with water to form twelve three-carbon molecules of phosphoglyceric acid (PGA), whose 3-carbons give the C3 cycle its name.