9.3: Anatomy of Chloroplasts
In plants, photosynthesis takes place within the leaf’s dense mesophyll cell layers—where the highest number of chloroplasts is contained. Scattered throughout these specialized, double-membrane organelles are another set of compartments—fluid-filled membranous sacs called thylakoids, which are interconnected and form into multiple stacks called grana. On the outer edges of each granum —embedded within the thylakoid membranes—are multi-protein complexes, such as the photosystems. These structures contain the antenna proteins, bound with numerous pigment molecules like chlorophylls, to absorb light and begin the first stage of light-dependent reactions. Meanwhile, the second stage—the Calvin cycle—takes place in the stroma, the aqueous cavity outside of the thylakoid’s lipid bilayer. With both processes working together, plants produce their own food—thanks to the biochemical factories found in the chloroplast!