In the second stage of photosynthesis, what I call the dark reactions, the plant utilizes ATP and NADPH from the light reactions to capture carbon from the air and to convert it into usable sugars. It's called the dark reactions not because it happens at night, actually, they usually happen during the day. But because the reactions themselves do not directly use the light, like the reactions we learned about in the previous section. Carbon is the basis of all organic matter on Earth, sugars, proteins, DNA, fatty acids. All the building blocks of life contains chains of carbon atoms. The dark reactions take carbon dioxide from the atmosphere, and turn it into sugar, a molecule that our bodies, and also plants knows how to utilize. Turning atmospheric carbon dioxide into sugar takes a lot of energy, and the dark reactions utilize the molecules made in the light reactions to power this process. The energy carrier ATP moves energy from the light-dependent reactions in the thylakoids to the dark reactions which occur in the stroma of the chloroplasts. ATP fuels the assembly of sugar molecules. Let's see what needs to happen to convert carbon dioxide into sugar. This process is called carbon fixation. In the series of chemical reactions called the Calvin cycle, a single carbon atom from CO2 is first incorporated into a three carbon molecule. And from there, it becomes a six carbon sugar molecule, which we all know as glucose. The combining of carbon dioxide into long carbon chains, there is not a spontaneous process. It needs a lot of energy. This energy comes from the ATP. During the dark reactions, 18 molecules of ATP transfer their energy and are converted back to ADP to make one molecule of glucose. The ADP is then used again in the light reactions to renew the ATP. A big difference between carbon dioxide and sugar is that carbon dioxide has no hydrogen atoms in it while sugars contain multiple hydrogens. The source of these hydrogens is the NADPH. In the process of converting carbon dioxide to sugar, protons from NADPH are transferred to the sugar, leaving NADP+, which is then ready to be recharged by the light reactions. All in all, 12 molecules of NADPH are needed to make one molecule of glucose. In a nutshell, photosynthesis is summarized in the following slide. In the light reactions, light powers an electric current which results in one, the production of NADPH. Two, a proton gradient which powers the formation of ATP. And three, the splitting of water into protons, electrons and oxygen. The dark reactions utilize the ATP and NADPH to fix atmospheric carbon dioxide into sugars. A waste product of this process is ADP and NADP+ which is then recycled through the light reactions. The cycling of ATP to ADP and back to ATP, and NADPH to NAD+ back to NADPH links the dark and the light reactions. Sugar can be further converted to other molecules that build plant cells such as carbohydrates, proteins and lipids.