Unit 13 1 UNIT 13 PART A: THE LIGHT-DEPENDENT REACTIONS OF PHOTOSYNTHESIS PART B: THE CARBON-ASSIMILATION REACTIONS OF PHOTOSYNTHESIS PART C: GLYCOGEN SYNTHESIS AND DEGRADATION PART A: THE LIGHT- DEPENDENT REACTIONS OF PHOTOSYNTHESIS Photosynthetic organisms trap solar energy and form ATP and NADPH which they use as energy sources to make carbohydrates and other organic compounds from CO 2 and H 2 O. Asignment: Nelson & Cox pp. 742 ? 749, 752 - 764. 1. Distinguish betwen the light-dependent and the carbon-asimilation reactions of photosynthesis (p. 742). 2. Draw a simple diagram of a chloroplast. (Se Figs. 19-45, p. 743, and 19-63, p. 759). Label the following: inner membrane, outer membrane, thylakoid membrane, grana, stroma, and lumen. Unit 13 2 3. Light absorption a. Name and describe the molecules that absorb light (pp. 745 - 746). b. Use Fig. 19-48 (p. 746) to explain how chloroplasts maximize the eficiency of absorption of visible light. 4. Photosystems a. What is a photosystem (p. 747)? b. What is a reaction center (p. 747)? c. What is the mechanism by which light drives electron transfer (pp. 748 - 749)? 5. Photoelectron transfer chain: The electron transfer chain of chloroplasts is shown in Fig. 19-56 (p. 752). a. Why is the diagram caled the Z scheme (p. 753)? b. On the diagram, note any electron flow that requires an input of energy as wel as any that occurs pasively. c. On the Z scheme diagram note the following: photosystem 1, photosystem 2, NADP + /NADPH, cytochrome b 6 f complex, plastocyanin, plastoquinone, oxygen evolving complex. d. Compare the cytochrome b 6 f complex to the cytochrome bc 1 complex in the mitochondrion (p. 756). e. What molecules in the photosynthetic electron transport chain are equivalent to ubiquinone and cytochrome c in the mitochondrial chain (p. 756)? f. Name the proces that occurs in the cytochrome b 6 f complex and explain why it is important (p. 756). Unit 13 3 e. On your Z scheme diagram include cyclic electron flow. Why is cyclic electron flow sometimes advantageous to the plant (p. 756)? f. What is the function of the oxygen-evolving complex (pp. 756 - 758)? 6. Chemiosmotic model of photophosphorylation a. Discuss the arangement of the photoelectron transfer chain in the thylakoid membrane (Fig. 19-63, p. 759). b. Where do the protons acumulate? Which reactions contribute to this acumulation (Fig. 19-63, p. 759)? Note the Q-cycle shown in Fig. 19-59 (p. 755) is not supposed to be diferent from the mitochondrial Q-cycle in Fig. 19-12 (p. 717). c. Include ATP synthase in your diagram of the chloroplast indicating the arangement of CF O and CF 1 . Show where ATP and NADPH are produced (Fig. 19-63, p. 759). d. ATP synthase in chloroplasts has recently been reported to have 14 c subunits, giving H+/ATP= 14/3= 4.66. There is no extra proton from ATP transport because ATP is used in the stroma where it is made by F 1 . Thus the P/2e- ratio of non-cyclic photophosphorylation is 6/4.66= 1.29 which is very close to what has been measured. 7. Compare oxidative phosphorylation and photophosphorylation with respect to: a. The original donor of electrons b. The ultimate aceptor of electrons c. The direction of the proton gradient d. The orientation of the F 1 -F o complex e. The need for a closed membrane compartment Unit 13 4 PART B: CO 2 FIXATION: THE CARBON-ASSIMILATION REACTIONS OF PHOTOSYNTHESIS Photosynthetic organisms can make carbohydrates from CO 2 and water. They synthesize glucose, sucrose and other carbohydrates by reducing CO 2 at the expense of energy furnished by ATP and NADPH generated in photosynthetic electron transfer. The first stage in the fixation of CO 2 is its condensation with a five carbon aceptor, ribulose 1,5-bisphosphate to form two molecules of 3-phosphoglycerate. The reactions that result in CO 2 fixation make up a cyclic pathway in which key intermediates are constantly regenerated. The pathway is often caled the Calvin cycle. Asignment: Nelson & Cox, pp. 773 - 784. 1. Using Fig. 20-4 (p. 775) as a guide, identify the thre Stages in carbon dioxide asimilation. a. Stage 1: Using structures, write the net equation for the reaction catalyzed by ribulose 1,5- bisphosphate carboxylase (rubisco; do not be concerned about intermediates). b. Stage 2: Conversion of 3-phosphoglycerate to glyceraldehyde 3- phosphate (p. 778). Show where the products of the light reactions are used. c. Stage 3: Regeneration of Ribulose 1,5-bisphosphate. Name the types of enzymes that catalyze the reactions shown in Fig. 20-10 (p. 780) that convert glyceraldehyde 3-phosphate into Ribulose 1,5-bisphosphate. What other pathway that you learned previously involved these types of enzymes? 2. Use Fig. 20-9 (p. 779) to discuss the two alternative fates of glyceraldehyde 3-phosphate. Unit 13 5 3. Discuss the stoichiometry of CO 2 asimilation in the Calvin cycle (Fig. 20-14, p. 782). PART C: GLYCOGEN SYNTHESIS AND EGRADATION The principle storage form of glucose in vertebrates is glycogen. In this unit you wil learn how glycogen is broken down and synthesized and how these proceses are regulated to control the level of glucose circulating in the blood. Asignment: Nelson & Cox, review pp. 223 - 225 (stop at "Multiple phosphorylations."), pp. 423 - 431, 594 - 601, 605. 1. What is the function of glycogen in mamals? In what tisues does it occur (p. 594)? 2. Glycogen degradation (pp. 595 - 596). a. Using structures, write a balanced chemical equation for the reaction catalyzed by glycogen phosphorylase (Fig. 15-25, p. 595). b. Explain why glycogen phosphorolysis is energeticaly more eficient than hydrolysis (that occurs during digestion in the intestine; pp. 595 - 596. Think about this in terms of the product of glycogen phosphorolysis and what would be required to produce a similar molecule using glycolysis). Unit 13 6 3. Glycogen Synthesis (pp. 596 - 601) a. List several reasons why sugar nucleotides are suitable substrates for biosynthetic reactions (pp. 598 - 599). b. Using structures, write a balanced chemical equation for the reaction that generates a sugar nucleotide (Fig. 15-29, p. 600). Name the other product of the reaction and discuss why it is important. c. Using structures, write a balanced chemical equation for the reaction catalyzed by glycogen synthase (Fig. 15-30 , p. 600). Note: It is important that you generalize from this example of glycogen synthesis. Glycogen represents but one of hundreds of polysacharides that exist in nature. Al polysacharide synthesis follows the same general patern: conversion of a monosacharide 1-P to a nucleoside diphosphate sugar (NDP - sugar) followed by transfer of the sugar to the growing end of a polysacharide chain. d. Discuss the biological significance of the branched structure in glycogen (p. 601). 4. Glycogen metabolism is regulated by covalent modification (review pp. 223 - 225). a. Write a balanced equation for the reaction catalyzed by a kinase (pp. 224 - 225). 1) What protein side chains are involved in this reaction? 2) What molecule serves as the phosphoryl donor? b. Write a balanced equation for the reaction catalyzed by a phosphatase (pp. 224 - 225). c. Discuss how phosphate addition / removal causes conformational changes that alter enzyme activity (pp. 224 - 225). Unit 13 7 5. In your body, hormones regulate covalent modification (pp. 423 - 431, 605). a. Discuss the epinephrine signal transduction pathway including the role of each of the following (Fig. 12-4, p. 424): 1) G protein-coupled receptor (p. 423) 2) Adenylyl cyclase (pp. 426 - 427) 3) Protein Kinase A (PKA; pp. 427 - 428) 4) Target enzymes (glycogen phosphorylase and glycogen synthase - Note that the bifunctional enzyme that regulates the level of fructose 2,6-bisphosphate in the cel is also regulated via this cascade [Fig. 15-17, p. 588]) When glycogen phosphorylase (p. 428) and glycogen synthase (p. 605) are phosphorylated, are they active or inactive? b. Use Fig. 12-7 (p. 429) to ilustrate the principle of amplification within the signal transduction cascade. Point out each step that results in signal amplification. c. After hormone stimulation ceases, discuss how the pathway inactivates. 1) How is the receptor protein desensitized (pp. 430 - 431)? 2) How does the G protein inactivate (Fig. 12-5, p. 428)? 3) How is adenylyl cyclase inactivated? 4) Name the enzyme that degrades residual cAMP in the cel (p. 430). 5) How is PKA inactivated (Fig. 12-6, p. 428)? 6) How is the activity of the target enzymes reversed (p. 430)? Jim Blankenship Microsoft Word - U13_F08.doc
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