Biochem 2 - Ch. 21 notes - CHAPTER 21 PHOTOSYNTHESIS...

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CHAPTER 21: PHOTOSYNTHESIS Introduction 1% of sunlight from sun is absorbed by plants and converted to chemical energy. The chemicals formed are used by members of the food chain. The energy is used to fix CO 2 , with the evolution of O 2 . The overall reaction is 6 CO 2 + 6 H 2 O generates one hexose, C 6 H 12 O 6 + 6 O 2 . About 10 11 tons of CO 2 fixed per year. This process is the reverse of glycolysis, and requires energy. Light energy is also used to fix N 2 and assimilate sulfur-containing compounds. General aspects of photosynthesis Can be done by simple prokaryotes to large trees. There are several common features. Photosynthesis occurs in membranes. In photosynthetic bacteria, the membranes are the major components of the cytoplasm. In plants, the membranes are in large organelles called chloroplasts ( Fig 21.1 and 21.2 ). Chloroplasts are plastids, which are a group of related organelles. The inner membrane system is called the thylakoid membranes, which are flattened discs or sacs. There are three membrane-bound aqueous regions: stroma, intermembrane space (between inner and outer membrane), and interior of thylakoid membranes (thylakoid space or lumen). Chloroplasts contain DNA, RNA, and ribosomes. However, some functions are encoded by the nucleus. Light and dark reactions ( Fig 21.4 ) Illuminated chloroplasts without CO 2 will generate oxygen, NADPH, and ATP (light reactions). These reactions require the thylakoid membrane. Once illuminated, they can be placed in dark and fix CO 2 (dark reaction). These reactions occur in the stroma. The NADPH and ATP are used for CO 2 fixation. Water is the electron source for NADPH and ATP synthesis. See equation 21.2 on p 633. In bacteria, the electron source may also be H 2 S (photosynthetic green and purple sulfur bacteria), isopropanol, or other oxidizable (reduced) compounds. Light must provide the energy for NADPH synthesis, which must exceed 219 kJ/mol. 1
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Chlorophyll Magnesium-containing substituted tetrapyrroles ( Fig 21.5a ). Similar to heme, but differing with respect to the metal ion, a long-chain alcohol (phytol), and a fifth five-membered ring. They are good light absorbers because of delocalized π electrons above and below planar rings. Visible light absorption promotes an electron to a higher orbital, which can be transferred to another compound. This is a reduction reaction. The absorption of chlorophylls a and b differ ( Fig 21.5b ), which along with accessory light- harvesting reactions ( Fig 21.6 ) permit absorption of a variety of wavelengths. Note the conjugated double bond system. Carotenoids also destroy reactive oxygen species. Leaf colors during autumn are due to persistence of some of these pigments. The excited electron has four fates (
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This note was uploaded on 04/06/2012 for the course BIOL 3362 taught by Professor Candas during the Spring '09 term at University of Texas at Dallas, Richardson.

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Biochem 2 - Ch. 21 notes - CHAPTER 21 PHOTOSYNTHESIS...

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