Autotrophic and heterotrophic CO₂ fixation.

a) Example of a typical autotrophic CO₂ assimilation cycle (e.g. the Calvin-Benson-Bassham or reductive tricarboxylic acid cycle). ATP and reduction equivalents are provided by photo- or chemosynthesis. b) Heterotrophic CO₂ assimilation can occur via linear pathways from a carbon substrate to certain products. If needed, ATP and/or reduction equivalents are generated from the substrate itself. Depending on the particular substrate-product combination, different amounts of CO₂ can be assimilated.