Site of Photosynthesis: Structure of the Chloroplast

Photosynthesis in higher plants occurs almost entirely within a specialised, double-membrane-bound organelle called the chloroplast, one of a family of related plastids found in plant and algal cells. Chloroplasts are typically oval or discoid in shape and are present in especially large numbers in the mesophyll cells of the leaf -- the leaf's internal tissue that is specifically structured, with its flattened shape and internal air spaces, to maximise the capture of light and the diffusion of CO2 to exactly these cells.

Each chloroplast is bounded by an envelope consisting of two closely apposed membranes, an outer and an inner membrane, which together regulate the movement of ions, small metabolites and other molecules into and out of the organelle. Enclosed within this envelope is a dense, protein-rich fluid matrix called the stroma. The stroma is where the entire biosynthetic (dark) phase of photosynthesis takes place: it contains all the enzymes of the Calvin cycle, most importantly the CO2-fixing enzyme RuBisCO -- widely cited as the single most abundant protein on Earth, precisely because every photosynthesising leaf on the planet contains vast quantities of it. The stroma additionally contains the chloroplast's own small, circular DNA molecule and its own 70S ribosomes (resembling those of bacteria, a genuine reminder of the endosymbiotic origin of the organelle), and often visible starch grains, representing carbohydrate stored temporarily within the chloroplast itself.

Suspended within the stroma is an elaborate internal membrane system that is, in a functional sense, the true engine room of photosynthesis. This membrane is folded into flattened, closed, sac-like structures called thylakoids. In most chloroplasts, many thylakoids are stacked neatly one on top of another, like a pile of coins, into structures called grana (singular: granum); a single chloroplast typically contains many separate grana. Adjacent grana are interconnected by narrower, unstacked membranous tubules called stroma lamellae (or frets), which run through the surrounding stroma so that the entire thylakoid membrane system, despite its complex stacked-and-connected geometry, forms one single continuous compartment, enclosing one continuous internal space called the thylakoid lumen. …