Calvin cycle reductive pentose phosphate cycle C3 photosynthesis GO:0019253 reductive pentose phosphate pathway In the Calvin cycle, the source of the carbon atoms is the simple molecule carbon dioxide. In this extremely important process, carbon dioxide gas is trapped in a form that is useful for many processes. The Calvin cycle brings into living systems the carbon atoms that will become constituents of nucleic acids, proteins, and fats. Photosynthetic organisms are called autotrophs (literally self- feeders) because they can synthesize glucose from carbon dioxide and water, by using sunlight as an energy source, and then recover some of this energy from the synthesized glucose through the glycolytic pathway and aerobic metabolism. Organisms that obtain energy from chemical fuels only are called heterotrophs, which ultimately depend on autotrophs for their fuel. The Calvin cycle also differs from gluconeogenesis in where it takes place in photosynthetic eukaryotes. Whereas gluconeogenesis takes place in the cytoplasm, the Calvin cycle takes place in the stroma of chloroplasts, the photosynthetic organelles. The pentose phosphate pathway is part of central metabolism. This pathway provides a means by which glucose can be oxidized to generate NADPH and is the source of much of the NADPH that is needed for the biosynthesis of many biomolecules, most notably fats. The pentose phosphate pathway and the Calvin cycle have in common several enzymes and intermediates that attest to an evolutionary kinship. Like glycolysis and gluconeogenesis, these pathways are mirror images of one another: the Calvin cycle uses NADPH to reduce carbon dioxide to generate hexoses, whereas the pentose phosphate pathway breaks down glucose into carbon dioxide to generate NADPH. The Calvin cycle is sometimes referred to as the reductive pentose phosphate pathway. [Stryer, Biochemistry, 2002]. pathway UPa:UPA00116 MetaCyc:CALVIN-PWY carbohydrate biosynthesis