Chloroplast

Chloroplasts are the organelle that makes green plants green, and they are found across green algae and higher plants. Like a mitochondrion, a chloroplast is bounded by two separate membranes, an outer membrane and an inner membrane, with a distinct gap between them called the periplastidial space. The volume enclosed by the inner membrane is filled with a gelatinous, lipoprotein-rich fluid called the stroma. Suspended within that stroma are flattened, interconnected sac-like structures called thylakoids, and the space enclosed inside each thylakoid membrane is called the thylakoid lumen. When many thylakoids stack up on top of one another, like a pile of coins, that stack is called a granum (plural: grana). This architecture directly maps onto the two stages of photosynthesis: light is absorbed and converted into chemical energy specifically within the grana (the light reactions), and that chemical energy is then used out in the surrounding stroma to actually build carbohydrate molecules (the so-called dark reactions). Scattered through the stroma are small, rounded, fat-storing osmophilic granules, alongside the chloroplast's own 70S ribosomes and its own circular, non-histone-associated DNA and RNA - features that mirror the mitochondrion closely and, again, mark the chloroplast as a semi-autonomous organelle that multiplies by its own fission rather than relying entirely on the nuclear genome. The chloroplast's genome is compact but genuinely important, encoding roughly 30 proteins directly involved in photosynthesis, including components of Photosystem I, Photosystem II, the cytochrome bf complex, and ATP synthase; it also encodes one of the subunits of Rubisco, which is separately notable as the single most abundant prot …