Photosynthesis, the chemistry Hank Green breaks down in this Crash Course Biology episode, starts with three raw ingredients: water, carbon dioxide, and sunlight, all pulled together inside a plant's chloroplasts. Hank walks through the light-dependent reactions first, tracing electrons through Photosystem II, the cytochrome complex, ATP synthase, and Photosystem I, the machinery that turns sunlight into chemical energy and splits water for its electrons and oxygen. From there he moves into the light-independent reactions, better known as the Calvin cycle, breaking it into its three phases: carbon fixation, reduction, and regeneration, the steps that build sugar out of carbon dioxide using the energy just captured. Diagrams track each molecule through the cycle rather than leaving the process abstract. The episode treats photosynthesis as a set of interlocking reactions with named parts and a clear sequence, not just a vague description of plants making food, and closes by noting the oxygen this whole system produces as a byproduct other organisms depend on to breathe.