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Unformatted text preview: Bioc 460 - Dr. Miesfeld Fall 2008 1 of 12 pages Figure 1. Lecture 29 - Photophosphorylation Key Concepts- Overview of photosynthesis and carbon fixation - Chlorophyll molecules convert light energy to redox energy - The Z scheme of photosynthetic electron transport - Protein complexes in the photosynthetic ETS Key Concept Question in Photosynthesis: How do photosynthetic reaction centers convert light energy into redox energy? Biochemical Applications of photosynthesis: DCMU (dichlorophenyl dimethylurea) is a broad spectrum herbicide that functions by blocking electron flow through photosystem II and is used to reduce weeds in non-crop areas. Another herbicide, paraquat, prevents reduction of NADP + by accepting electrons from intermediate reductants in photosystem I. Paraquat was used extensively in the 1980s as an aerial herbicide to destroy illegal fields of marijuana and coca plants in North and South America, however its use was discontinued because smoking paraquat-contaminated plants causes lung damage. Overview of photosynthesis and carbon fixation Figure 1 shows where the photosynthetic electron transport system and Calvin cycle (carbon fixation) fit into the metabolic forest. Together, these two processes convert sunlight energy into chemical energy (ATP, NADPH, triose phosphate), and in the process, oxidize H 2 O to form O 2 . The photosynthetic electron transport system is often referred to as the light reactions of photosynthesis and consists of two photosystems (PSI and PSII), whereas, the Calvin cycle has been called the dark reactions . However, as you will see in lecture 30, the term "dark reactions" can be misleading because the Calvin cycle is most active in the light when ATP and NADPH levels are high. In the dark, plants actually depend on newly synthesized carbohydrate (glycerladehyde-3P) as a metabolic fuel for mitochondrial respiration just like any other aerobic organism. Light excitation of Photosystems I and II results in oxygen evolution from the splitting H 2 O, and the generation of chemical energy in the form of ATP and NADPH. Plants use this chemical energy (ATP and NADPH) to convert CO 2 into sugars (carbon fixation) via the Calvin cycle which takes place in the stroma. We will describe the Calvin cycle and carbon fixation in lecture 30. The Bioc 460 - Dr. Miesfeld Fall 2008 2 of 12 pages Figure 2. Figure 3. combined reactions of photosynthetic electron transport system and carbon fixation can be written as: H 2 O + CO 2---(light energy)---> O 2 + (CH 2 O) in which (CH 2 O) represents hexose sugars such as starch and sucrose. Importantly, even though this is written as a balanced equation, O 2 generation is the result of H 2 O oxidation, whereas, the CO 2 is used to synthesize carbohydrate. As you will see shortly, it requires the oxidation of two H 2 O to generate one O 2 . Moreover, since six CO 2 molecules are required for the synthesis of each molecule of glucose by the Calvin cycle, this reaction can be rewritten as:...
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This note was uploaded on 05/05/2011 for the course BIOL 400 taught by Professor Dr.biology during the Spring '11 term at University of Tennessee.
- Spring '11