25. Self-Sustaining Chemical        Systems: Living Cells   Previous PageNext Page

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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