Photosynthesis: Site & Pigments
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Photosynthesis occurs in green leaves and other green parts, within chloroplasts in the walls of mesophyll cells of leaves, optimizing the capture of incident light.
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Chloroplasts contain a membranous system consisting of grana, stroma lamellae, and matrix stroma.
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Each granum is a group of membrane-bound sacs called thylakoids (lamellae), which contain leaf pigments.
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The membrane system traps light energy to synthesize ATP and NADPH during light reactions.
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In the stroma, enzymatic reactions synthesize sugar, forming starch during dark reactions (carbon reactions), which are light-dependent despite the name.
Pigments Involved in Photosynthesis
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Pigments absorb light at specific wavelengths. Chromatography reveals the following leaf pigments:
- Chlorophyll a (bright or blue-green in chromatogram)
- Chlorophyll b (yellow-green)
- Xanthophylls (yellow)
- Carotenoids (yellow to yellow-orange)
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Chlorophyll b, xanthophylls, and carotenoids are accessory pigments.
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Functions of accessory pigments:
- Absorb light at different wavelengths and transfer energy to chlorophyll a.
- Protect chlorophyll a from photo-oxidation.
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The absorption spectrum and action spectrum coincide closely, showing maximum photosynthesis in the blue and red regions of the spectrum, with chlorophyll a as the chief pigment.
Photosystems
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Pigments are organized into two photosystems: Photosystem I (PSI) and Photosystem II (PSII), named in order of discovery.
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Each photosystem includes chlorophyll a and accessory pigments bound by proteins.
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All pigments except one chlorophyll a molecule form a light-harvesting complex (LHC or antennae).
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A single chlorophyll a acts as the reaction center.
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In PS I, the reaction center absorbs light at 700 nm, called P700.
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In PS II, the reaction center absorbs light at 680 nm, called P680.