The chloroplast is the organelle where photosynthesis occurs in photosynthetic eukaryotes. The organelle is surrounded by a double membrane. Inside the inner membrane is a complex mix of enzymes and water. This is called stroma and is important as the site of the dark reactions, more properly called the Calvin cycle.Embedded in the stroma is a complex network of stacked sacs. Each stack is called a granum and each of the flattened sacs which make up the granum is called a thylakoid. Each thylakoid has a series of photosystems and associated proteins. The photosystems contain chlorophyll and other pigments and all these associated structures in the thylakoid membrane are the site for the light reactions in which light energy is converted to chemical energy needed for the Calvin cycle in the dark reaction.As the light reactions proceed, the inside of the thlyakoid develops a high concentration of hydrogen ions, and this is important for the production of ATP by the chloroplast.
By the way, chloroplasts and related organelles, called plastids are believed to have arisen as free living bacteria that became symbiotic with the ancestors of photosynthetic eukaryotes.
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