The chloroplast, a plant organelle, is the site of photosynthesis in higher plants and algae. Like mitochondria, chloroplasts carry their own DNA to code for some of their proteins, as well as the ribosomes necessary for translation of the appropriate mRNAs. Chloroplasts may have evolved from cyanobacteria, which have membrane structures like chloroplast membranes.

The internal structure of chloroplasts (Figure 17.4c) resembles that of the mitochondrion (see here). Note the presence of an outer, relatively permeable membrane and an inner membrane that is selectively permeable. The stroma of the chloroplast is analogous to the mitochondrial matrix. Immersed in the stroma are many flat, saclike membrane structures called thylakois which are stacked like coins. The stacks are called grana. Grana are irregularly interconnected by thylakoid extensions called stroma lamellae. The thylakoid membrane encloses the lumen (or interior) of the thylakoid.

There are analogies in structure and role between the mitochondrial matrix (Figure 15.2a) and the chloroplast stroma and also between the inner membrane of the mitochondrion and the thylakoid membrane of the chloroplast. Absorption of light and all of the light reactions occur within or on the thylakoid membranes. ATP and NADPH produced by these reactions are released into the surrounding stroma, where the synthetic dark reactions occur.