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Chapter 10 Notes Continued

Chapter 10 Notes Continued. Part 2: The light-independent reactions. A.k.a “dark reactions” or “Calvin cycle” or “C 3 cycle”; occurs in the stroma of the chloroplast. ATP and NADPH produced by the light reactions are used in the Calvin cycle to reduce carbon dioxide to sugar.

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Chapter 10 Notes Continued

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  1. Chapter 10 Notes Continued

  2. Part 2: The light-independent reactions • A.k.a “dark reactions” or “Calvin cycle” or “C3 cycle”; occurs in the stroma of the chloroplast. • ATP and NADPH produced by the light reactions are used in the Calvin cycle to reduce carbon dioxide to sugar. • ATP is the energy source, while NADPH is the reducing agent that adds high-energy electrons to form sugar. • For the Calvin cycle to synthesize one molecule of sugar (glyceraldehyde 3-phosphate or G3P), three molecules of CO2 must enter the cycle.

  3. Calvin Cycle Phase 1 • Carbon Fixation: • CO2 is attached to a five-carbon sugar, ribulose biphosphate (RuBP); catalyzed by the enzyme RuBP carboxylase (rubisco). • Product is an unstable six-carbon molecule that immediately splits into two molecules of 3-phosphoglycerate (3-PG). • 3 CO2 + 3 RuBP  3 unstable 6-C  6 3-PG

  4. Calvin Cycle Phase 2 • Reduction: • Step 1: ATP hydrolysis is coupled with the reduction of 3-phosphoglycerate to 1,3 -diphosphoglycerate. • 6 3-PG + 6 ATP  6 1,3-DPGA + 6 ADP • Step 2: NADPH donates electrons to DPGA, forming 3-carbon “phosphoglyceraldehyde” (PGAL) or “glyceraldehyde 3-phosphate” (G3P). • 6 DPGA + 6 NADPH  6 G3P + 6 NADP + 6 Pi • One of the six G3P can be used to assemble glucose, cellulose, starch, fatty acids, and amino acids. Other five will continue in the cycle.

  5. Calvin Cycle Phase 3 • Regeneration of RuBP: • The carbon skeletons of five G3P molecules are rearranged into three RuBP molecules. • These reactions use 3 ATP molecules. • For the net gain of one G3P molecule, the Calvin cycle uses the products of the light reactions: • • 9 ATP molecules • • 6 NADPH molecules • Calvin cycle uses 18 ATP and 12 NADPH molecules to produce one glucose molecule.

  6. Photorespiration • In plants, a metabolic pathway that consumes oxygen, produces carbon dioxide, produces no ATP and decreases photosynthetic output. • Occurs when the O2 concentration in the leaf’s air spaces is higher than CO2 concentration. • Photorespiration occurs in hot, dry, sunny weather when plants close their stomata to prevent dehydration. • Photosynthesis then depletes available carbon dioxide and increases oxygen within the leaf air spaces. • If photorespiration could be reduced in some agricultural plants, crop yields and food supplies would increase. • Certain species of plants which live in hot arid climates have developed alternate pathways of carbon fixation; C4 and CAM plants.

  7. C4 Pathway • The Calvin cycle occurs in most plants and produces a three-carbon compound in the first stage; important C3 plants include rice, wheat, and soybeans. • C4 plants have a few steps occurring before the Calvin cycle which incorporate carbon dioxide into four-carbon compounds; includes corn, sugarcane, and important agricultural grasses. • CO2 + PEP  oxaloacetic acid  malic acid  CO2 • Malic acid can be stored in the leaf and converted back to CO2 when needed.

  8. CAM Pathway • Crassulacean Acid Metabolism: • Found in succulent plants adapted to very arid conditions; plants open their stomata primarily at night and close them during the day (opposite of most plants). • This conserves water, but prevents CO2 from entering the leaves. • When stomata are open at night, CO2 is taken up and incorporated into a variety of organic acids. • These acids are stored in vacuoles of mesophyll cells until morning, when the stomata close. • During daytime, CO2 is released from the acids and converted to sugar as the light reactions and the Calvin cycle continue.

  9. A good website • http://www.geosciences.unl.edu/~dbennett/physiology.html

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