Chloroplasts are the sites of photosynthesis in eucaryotes (right).
In purple photosynthetic bacteria the light-trapping pigments are
found in folded pockets or vesicles in the outer membrane. In blue-green
algae these vesicles are enlarged, flattened, and stacked, with
adjacent vesicles sometimes fused or connected. In chloroplasts
this development of structure is continued. The individual vesicles,
called thylakoids, are stacked like pennies into grana, with extensive
connections by hollow membrane tubules from one stacked granum to
the next. Light stimulates the growth and development of grana in
the chloroplast, just as it does the photosynthetic vesicles in
bacteria.
The light reactions of photosynthesis take place in Type-I and Type-II
pigment centers in the thylakoid membranes, and electrontransport
chains from Photocenter II to I and from Photocenter I to NAD
also are found in the thylakoid membrane surface. The dark reactions
of carbohydrate synthesis occur in the chloroplast matrix between
grana. The organization resembles that of mitochondria and bacteria:
glucose degradation or synthesis in the interior matrix of an organelle,
and electron- transport chains-flavoproteins, quinones, cytochromes,
and copper proteins-at the inner surface of the surrounding membrane.
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