lecture 10 - CO2 Concentrating Mechanisms In Plants And...

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CO 2 Concentrating Mechanisms In Plants And Algae
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Section II Diversity of vascular plants, photosynthesis and respiration Session 7 Bryophytes, Vascular Plants Session 8 Flowering Plants Session 9 Photosynthesis Session 10 C4 photosynthesis and Respiration Session 11 Mineral nutrition Session 12 Plant tissues
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CO 2 Concentrating Mechanisms Purpose: To concentrate CO 2 around the site of Rubisco to minimize photorespiration and to CO 2 saturate Rubisco Three important types: 1) Bicarbonate concentration in aquatic systems (primarily by algae) 2) C 4 photosynthesis 3) Crassulacean Acid Metabolism (CAM) photosynthesis
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CO 2 Concentration in a Green Algae ( Chlamydomonas ) Rubisco HCO 3 - HCO 3 - HCO 3 - HCO 3 - ATP ADP + P i H + H + H + CO 2 Chloroplast Pyrenoid CA Thylakoids Flagella Starch sheath Rubisco is localized in the pyrenoid with the enzyme carbonic anhydrase (CA). HCO 3 + enter the pyrenoid where it is converted to CO 2 by CA. Bicarbonate is actively accumulated in the cell from the surrounding water by the action of an ATPase. CA = carbonic anhydrase H + O 2 HCO 3 - CO 2 + H 2 O Cyclic photophosphorylation provides ATP for the bicarbonate pump
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Bicarbonate Accumulation In water, CO 2 is in equilibrium with bicarbonate (HCO 3 - ). Because it is charged, bicarbonate can be pumped across membranes and accumulated inside cells. This occurs by coupling HCO 3 - pumping to a proton gradient generated by a proton ATPase in the plasmalemma. Once accumulated in the cell, bicarbonate is transported to a special compartment where Rubisco is localized, there the bicarbonate is converted back to CO 2 by carbonic anhydrase . In cyanobacteria (blue-green algae) Rubisco is in the carboxysome , a protein-bounded compartment In eukaryotic algae, Rubisco is in the pyrenoid , a starch bounded compartment. The shells of these compartment restrict CO 2 efflux, so CO 2 levels are elevated tenfold or more above the outside.
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C 4 Photosynthesis
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Leaf Anatomy C 3 leaf cross section Mesophyll cells Bundle sheath cells C 4 leaf cross section Mesophyll cells
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A Schematic of C 4 Photosynthesis CO 2 HCO 3 - PEP CO 2 PCR cycle sugars xylem phloem Export OAA C 4 acid DC C 3 acid Pyruvate ATP AMP PP i 2 P i Cytosol Mesophyll Tissue Bundle Sheath Tissue PPDK RUBISCO RUBISCO RUBISCO Pco 2 ~150 μbar 2 ~ 1500 μbar CHL PEPC DC: decarboxylating enzyme PEPC: PEP carboxylase PPDK: pyruvate phosphate dikinase
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Kranz Anatomy Mesophyll cells Bundle sheath cells C 4 leaf cross section “Kranz anatomy”
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Kranz Anatomy (courtesy of Athena McKown) Another view of Kranz anatomy from the genus Flaveria
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C 4 Photosynthesis Where a PEP carboxylation and regeneration cycle is used to pump CO 2 from an outer photosynthetic cell layer (the mesophyll) to an inner layer (the bundle sheath) where Rubisco is localized (Raven Fig. 7-22). Because the bundle sheath wall resists CO 2 efflux, CO 2 levels can be elevated 10x over atmospheric levels.
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lecture 10 - CO2 Concentrating Mechanisms In Plants And...

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