The energy for ATP synthesis comes from organic molecules (such as carbohydrates), or from sunlight, or from inorganic electron donors. We can classify organisms according to their source of energy and organic carbon:
heterotrophs – get energy and organic carbon from metabolism of pre-existing organic compounds (food)photoautotrophs – use energy from sunlight to make ATP and their own organic carbon compounds from carbon dioxide chemoautotrophs – use energy from inorganic chemicals to make ATP and their own organic carbon compounds from carbon dioxideMetabolic pathways carry out reactions that capture energy from these various sources (organic compounds, sunlight or chemicals) and couple them to synthesis of ATP from ADP.
