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Photosynthesis part II – carbon fixation

Photosynthesis part II – carbon fixation. C3 photosynthesis: Calvin cycle with Rubisco C4 photosynthesis: a mechanism to concentrate CO2 for the Calvin cycle. Products of light reactions are used for carbon fixation. Thylakoid membranes. Chloroplast stroma.

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Photosynthesis part II – carbon fixation

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  1. Photosynthesis part II – carbon fixation • C3 photosynthesis: Calvin cycle with Rubisco • C4 photosynthesis: a mechanism to concentrate CO2 for the Calvin cycle

  2. Products of light reactions are used for carbon fixation Thylakoid membranes Chloroplast stroma

  3. Calvin and Benson used radioactive 14CO2 to investigate carbon fixation • 14CO2 was injected into culture of Chlorella (green alga). After 30 s, algal cells were boiled in ethanol and extract analyzed by chromatography and autoradiography. Calvin’s apparatus Photo by Dr. James A Bassham http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CalvinCycle.html

  4. The earliest compound to be labeled with 14CO2 is 3-phosphoglycerate The dark spots show the radioactive compounds produced after 10 secs (left) and 2 minutes (right) of photosynthesis by the green alga Scenedesmus. Photo by Dr. James A. Bassham http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CalvinCycle.html

  5. Calvin, Benson and Bassham's conclusions and hypotheses • 3PG (3-phosphoglycerate: PGA) is first stable molecule to be labeled, at carboxyl group • A cyclic process, because other carbon atoms of 3PG are also labeled at longer times • But no 2-carbon compound could be found as acceptor for CO2 • Hypothesize that 3PG reduced to G3P (glyceraldehyde-3-phosphate) using ATP & NADPH

  6. Reduction of 3PG (acid) to G3P (aldehyde/sugar) requires ATP and NADPH Purves et al. Life 6th ed. 3PG is labeled with 14CO2 in both light and dark, but G3P is labeled only in light (light reactions produce ATP and NADPH

  7. Ribulose bisphosphate (RuBP) is the CO2 acceptor molecule CO2 + RuBP  [6-carbon intermediate]  2 x 3PG The enzyme rubisco (RuBP carboxylase/oxygenase) may be the most abundant enzyme on Earth!

  8. Calvin-Benson-Bassham cycle ATP is required also for regeneration of RuBP.

  9. Metabolic interactions in plants

  10. Rubisco, key enzyme in carbon fixation • Source of most biological carbon on Earth • Problem: can oxygenate RuBP (photorespiration) • O2 competes with CO2, especially at higher temperature (>28 deg. C), low CO2, high O2 • produces glycolate (2-carbon compound) • glycolate exits chloroplast, oxidized in peroxisomes, metabolized to CO2 in mitochondria • undoes carbon fixation, with no ATP synthesis

  11. C4 photosynthesis is a CO2 concentration mechanism that reduces photorespiration • In C4 plants, 14CO2 labels a 4-carbon product, instead of 3PG • C4 plants (corn, sugar cane, crabgrass, etc.) thrive in dry, hot environments • plants close stomata to limit water vapor loss, thus limit CO2 availability • C4 plants take up CO2 via PEP carboxylase, which has higher affinity for CO2 and no oxygenase reaction

  12. C4 plants concentrate CO2 for Rubisco Mesophyll cells fix CO2 efficiently to 4-carbon malate. Malate shipped to bundle sheath cells decarboxylates to deliver CO2 to Rubisco. Energy cost = 1 ATP per CO2

  13. C4 vs C3 • C4 minimizes loss of organic carbon from oxygenase activity of Rubisco • C4 plants grow better in hot, dry climates • C3 plants grow better in cool, moist climates • C3 plants more sensitive to atmospheric CO2 concentrations

  14. Photosynthesis and respiration 6 CO2 + 12 H2O  C6H12O6 + 6 H2O + 6 O2 • Carbon and oxygen cycles • Chemiosmotic ATP synthesis • Organelles with endosymbiotic origins

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