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Unformatted text preview: 0B X X X X X X I I I I I I . . . P P P h h h o o o t t t o o o s s s y y y n n n t t t h h h e e e s s s i i i s s s : : : L L L i i i g g g h h h t t t R R R e e e a a a c c c t t i i i o o o n n n s s s Fig. 10-2 (a) Plants (c) Unicel ular protist 10 m 1.5 m 40 m (d) Cyanobacteria (e) Purple sulfur bacteria (b) Multicel ular alga Arguably the most important anabolic pathway, photosynthesis reduces simple compounds (e.g. CO2) into more complex molecules (glucose). The reduction reactions are endergonic (as contrasted with exergonic oxidations); the energy source is the light. 1B C C C h h h l l l o o o r r r o o o p p p l l l a a a s s s t t t s s s A specialized chromoplast of plant cells contains chlorophyll as a light-capturing pigment. 6B S S S t t t r r r u u u c c c t t t u u u r r r e e e Like nuclei and mitochondria, chloroplasts are surrounded by double membranes. The cytoplasm of chloroplasts is called stroma. A third membrane system similar to ER makes thylakoids, flattened vesicles whose membrane contains photosynthetic enzymes. Thylakoids are continuous throughout the cell, but they are either separate in the stroma (stromal thylakoids) or are arranged in stacks forming grana. The two kinds of thylakoids are adapted to different light levels: stromal thylakoids are bathed in the stroma, which contains more CO2, making them more efficient whenever CO2 levels are low, whereas grana thylakoids are better adapted for low levels of light. 7B F F F u u u n n n c c c t t t i i i o o o n n n The only function of chloroplasts is photosynthesis, conversion of carbon dioxide into glucose. 2 n 2 2 2 O O) (CH O H CO + + Photosynthesis is oxidative respiration in reverse: CO2 was a product of respiration, but it is a substrate for photosynthesis; O2 was a substrate, but here it is a (by-) product; electrons were derived from the organic compounds and end up in H2O; in photosynthesis electrons are derived from water and end up in complex organic compounds; water was produced in oxidative respiration; in photosynthesis water is split. There are also similarities; as in oxidative phosphorylation, there are dehydrogenation reactions, electron transport, and chemiosmosis....
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