Slide 1

Calvin Cycle Organisms capture and store free energy for use in biological processes

Slide 2

Where does the Calvin Cycle take place? Stroma of the chloroplast the fluid filled area outside of the thylakoid membrane

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How does CO 2 enter the Calvin Cycle? CO2 enters through the stomata microscopic pores in leaves Once in the leaf the CO2 diffuses into mesophyll cells where it can enter the chloroplast Within the chloroplast carbon fixation takes place

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Fig. 10-3a 5 m Mesophyll cell Stomata CO 2 O2O2 Chloroplast Mesophyll Vein Leaf cross section

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What occurs during carbon fixation? Carbon dioxide joins a five-carbon molecule called ribulose bisphophate (RuBP) This reactions is catalyzed by RuBP carboxylase, aka Ribisco Ribisco the most abundant enzyme in nature This enzyme often takes up 50% of the total chloroplast protein content Ribisco is a slow only catalyzing 3 molecules of substrate per second (compared to 1,000 per second) Unstable 6 carbon compound is formed which splits to form 2 three carbon molecules of PGA (phosphoglycerate)

Slide 6

How is PGA turned into sugar? Each molecule of PGA is systematically reduced by enzyme action. NADPH provides the hydrogen atoms and ATP provides the energy for these reactions to occur. (NADPH and ATP from Light Reactions) PGAL (phosphoglyceraldehyde), also called G3P (glyceraldehyde-3-phosphate) is the final product of the Calvin Cycle G3P can be exported to the cytoplasm and combined to form fructose-6-phosphate and glucose 1-phosphate. Fructose and glucose can join to form sucrose

Slide 7

How does the Calvin Cycle get back to 5-C RuBP? For every 3 molecules of carbon dioxide fixed, 6 molecules of G3P are formed Only 1 of the G3P exits the cycle The other five G3P (3C) molecules are used to regenerate 3 molecules of RuPB (5C) using ATP from the Light Reactions

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Fig. 10-18-3 Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P ATP 6 6 ADP P P 6 1,3-Bisphosphoglycerate 6 P P 6 6 6 NADP + NADPH i Phase 2: Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar) Glucose and other organic compounds Calvin Cycle 3 3 ADP ATP 5 P Phase 3: Regeneration of the CO 2 acceptor (RuBP) G3P

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Alternative Carbon Fixation Mechanisms Organisms use feedback mechanisms to maintain their internal environments and respond to external environmental changes

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Why do plants need alternative mechanisms for carbon fixation? Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves H 2 O but also limits photosynthesis The closing of stomata reduces access to CO 2 and causes O 2 to build up These conditions favor a seemingly wasteful process called photorespiration

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What is photorespiration? In most plants (C 3 plants), initial fixation of CO 2, via rubisco, forms a three-carbon compound In photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin cycle Photorespiration consumes O 2 and organic fuel and releases CO 2 without producing ATP or sugar

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How do C4 plants avoid photorespiration? C 4 plants minimize the cost of photorespiration by incorporating CO 2 into four-carbon compounds in mesophyll cells This step requires the enzyme PEP carboxylase PEP carboxylase has a higher affinity for CO 2 than rubisco does; it can fix CO 2 even when CO 2 concentrations are low These four-carbon compounds are exported to bundle-sheath cells, where they release CO 2 that is then used in the Calvin cycle

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Fig. 10-19 C 4 leaf anatomy Mesophyll cell Photosynthetic cells of C 4 plant leaf Bundle- sheath cell Vein (vascular tissue) Stoma The C 4 pathway Mesophyll cell CO 2 PEP carboxylase Oxaloacetate (4C) Malate (4C) PEP (3C) ADP ATP Pyruvate (3C) CO 2 Bundle- sheath cell Calvin Cycle Sugar Vascular tissue

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How do CAM plants avoid photorespiration? Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon CAM plants open their stomata at night, incorporating CO 2 into organic acids Stomata close during the day, and CO 2 is released from organic acids and used in the Calvin cycle

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Fig. 10-20 CO 2 Sugarcane Mesophyll cell CO 2 C4C4 Bundle- sheath cell Organic acids release CO 2 to Calvin cycle CO 2 incorporated into four-carbon organic acids (carbon fixation) Pineapple Night Day CAM Sugar Calvin Cycle Calvin Cycle Organic acid (a) Spatial separation of steps (b) Temporal separation of steps CO 2 1 2

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Review The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O 2 in our atmosphere

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Fig. 10-21 Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain CO 2 NADP + ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2OH2O O2O2