Recall that catabolism—the breakdown of complex molecules into simpler units—is the reverse of anabolism—the construction of complex molecules from simpler units. Chapter 9 described several energy-releasing catabolic pathways. The energy stored in chemical bonds in almost all organisms (with the exception of those living near deep-sea vents) ultimately comes from the sun. Photosynthesis (literally, “synthesis from light”) is an anabolic process by which the energy of sunlight is captured and used to convert carbon dioxide (CO₂) into more complex carbon-containing compounds.