In Section 9, we learned that oxidative phosphorylation is compartmentalized into mitochondria. Likewise, photosynthesis is sequestered into organelles called chloroplasts, typically 5 mm long. Like a mitochondrion, a chloroplast has an outer membrane and an inner membrane, with an intervening intermembrane space (Figure 22.2). The inner membrane surrounds a space called the stroma, which contains the enzymes that use reducing power and ATP to convert CO₂ into sugar. The stroma is akin to the matrix of the mitochondrion, which is the location of carbon chemistry during respiration. In the stroma are membranous discs called thylakoids, which are stacked to form a granum. Grana are linked by regions of thylakoid membrane called stroma lamellae. The thylakoid membranes separate the thylakoid lumen from the stroma. Thus, chloroplasts have three different membranes (outer, inner, and thylakoid membranes) and three separate spaces (intermembrane, stroma, and thylakoid lumen). The thylakoid membrane and the inner membrane, like the inner mitochondrial membrane, are impermeable to most molecules and ions. The outer membrane of a chloroplast, like that of a mitochondrion, is highly permeable to small molecules and ions. Like the mitochondrial cristae, the thylakoid membranes are the sites of coupled oxidation–reduction reactions that generate the proton-motive force. Plant leaf cells contain between 1 and 100 chloroplasts, depending on the species, cell type, and growth conditions.

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