Carbon dioxide
All aerobic organisms produce  when they oxidize carbohydrates, fatty acids, and proteins in the mitochondria of cells. The large number of reactions involved are exceedingly complex and not described easily. (They include cellular respiration, anaerobic respiration and photosynthesis). Photoautotrophs (i.e. plants, cyanobacteria) use another reaction: Plants absorb  from the air, and, together with water, react it to form carbohydrates: