Site of Photosynthesis

Chloroplasts are the dedicated organelle of photosynthesis, and both of the process's two main phases - the energy-yielding light reaction and the carbon-fixing dark reaction - take place inside them. A chloroplast is a double-membrane-bounded organelle, discoid or lens-shaped, typically 4-10 micrometres in diameter and about 1-3 micrometres thick; each of its two boundary membranes is a standard unit membrane, and the two are separated by a gap of about 100-200 Angstrom. Inside this envelope sits the stroma, a colloidal, protein-rich matrix that houses the enzymes of the dark reaction (including RuBisCO) along with 70S ribosomes, circular DNA, and starch grains - features that give the chloroplast semi-autonomous status and support the endosymbiotic hypothesis that chloroplasts descend from free-living photosynthetic bacteria engulfed by an ancestral eukaryotic cell. Suspended within the stroma is a separate, sac-like membrane system called the thylakoid (or lamella); these flattened sacs stack one above another like a pile of coins to form a granum (plural grana), and a typical chloroplast contains 40-80 grana, each granum built from 5-30 individual thylakoid discs roughly 0.25-0.8 micrometres across. Thylakoids that sit within a granum are called grana lamellae, while thinner connecting membranes that run between separate grana, outside any stack, are called stroma lamellae (or fret membranes). The two photosystems are not evenly distributed across this membrane system: Pigment System II sits on the inner thylakoid membrane surface facing the thylakoid lumen, while Pigment System I sits on the outer membrane surface facing the stroma - so grana lamellae, which have both faces available, carry both PS I and PS II, while stroma lamellae, exposed only to the stroma, carry PS I alone. By dry-weight composition, a chloroplast is roughly 30- …