Editing Chloroplast (section)

====Thylakoid composition====

Embedded in the thylakoid membranes are important [[protein complexes]] which carry out the [[light reactions]] of [[photosynthesis]]. [[Photosystem II]] and [[photosystem I]] contain [[light-harvesting complexes]] with [[chlorophyll]] and [[carotenoid]]s that absorb light energy and use it to energize electrons. Molecules in the thylakoid membrane use the energized electrons to pump [[hydrogen ions]] into the thylakoid space, decreasing the [[pH]] and turning it acidic. [[ATP synthase]] is a large protein complex that harnesses the [[concentration gradient]] of the hydrogen ions in the thylakoid space to generate [[Adenosine triphosphate|ATP]] energy as the hydrogen ions flow back out into the stroma—much like a dam turbine.<ref name="Campbell-2009b" />

There are two types of thylakoids—granal thylakoids, which are arranged in grana, and stromal thylakoids, which are in contact with the [[stroma (fluid)|stroma]]. Granal thylakoids are pancake-shaped circular disks about 300–600 nanometers in diameter. Stromal thylakoids are [[helicoid]] sheets that spiral around grana.<ref name="Mustárdy-2008" /> The flat tops and bottoms of granal thylakoids contain only the relatively flat [[photosystem II]] protein complex. This allows them to stack tightly, forming grana with many layers of tightly appressed membrane, called granal membrane, increasing stability and [[surface area]] for light capture.<ref name="Mustárdy-2008" />

In contrast, [[photosystem I]] and [[ATP synthase]] are large protein complexes which jut out into the stroma. They can't fit in the appressed granal membranes, and so are found in the stromal thylakoid membrane—the edges of the granal thylakoid disks and the stromal thylakoids. These large protein complexes may act as spacers between the sheets of stromal thylakoids.<ref name="Mustárdy-2008" />

The number of thylakoids and the total thylakoid area of a chloroplast is influenced by light exposure. Shaded chloroplasts contain larger and more [[Thylakoid|grana]] with more thylakoid membrane area than chloroplasts exposed to bright light, which have smaller and fewer grana and less thylakoid area. Thylakoid extent can change within minutes of light exposure or removal.<ref name="Wise-2007a" />