Carbon fixation refers to the chemical transformation of simple, inorganic compounds of carbon into more complex forms of organic matter. Examples of the simple compounds include carbon dioxide (CO₂) and bicarbonate (HCO₃ ^-), while the more complex forms include calcite (CaCO ₃) and the organic matter of organisms.

In ecosystems, carbon is fixed by autotrophs, which are organisms that utilize an external energy source to drive the synthesis of CO₂ and water (H₂O) into simple sugars. Usually, sunlight is the energy source for the fixation reactions, which is referred to as photosynthesis, and the organisms as photoautotrophs. Examples of photoautotrophs include plants, algae, and blue-green algae. Expressed simply, the photosynthetic reaction is:

(1) CO₂ + 2H₂O ^light CH ₂O + 2O₂ + H₂O.

In reaction 1, the term CH₂O refers to a carbohydrate, which is a primary product of photosynthesis. The molecular oxygen (O ₂) is a "waste" product...