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Why do Plants and Algae need Carbon Concentrating Mechanisms (CCMs)?

Why do Plants and Algae need Carbon Concentrating Mechanisms (CCMs)?. Undergraduate level notes. 1 – Limitations of RuBisCO. Slow enzyme: catalytic turnover of ca. 3 molecules s -1 . 3.8 billion years old – evolved before atmosphere was oxygenated, therefore non-specific for CO 2 .

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Why do Plants and Algae need Carbon Concentrating Mechanisms (CCMs)?

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  1. Why do Plants and Algae need Carbon Concentrating Mechanisms (CCMs)? Undergraduate level notes

  2. 1– Limitations of RuBisCO • Slow enzyme: catalytic turnover of ca.3 molecules s-1. • 3.8 billion years old – evolved before atmosphere was oxygenated, therefore non-specific for CO2. • Oxygenase activity competes with carboxylase activity (increasingly at high temperature); generates 1 molecule 3-phosphoglycerate (3-PGA) ( CBB cycle) and 1 molecule of 2-phosphoglycolate (2-PG). Photorespiration is process by which 2-PG converted to 3-PGA (see next)

  3. 2 –Photorespiration • 2-PG  2 molecules glycolate in chloroplast • glycolate  glyoxylate in peroxisome, accompanied by production of H2O2 (dissociates to O2 + 2H2O, by catalase) • glyoxylate  glycine (by incorporation of 2 glutamate and release of 2 2-oxoglutarate) • 2 mol glycine  1 mol serine in mitochondria by glycinedecarboxylase, releasing NH4+ which returns to chloroplast, NH4+ + glutamate  glutamine (glutamine synthase). (cont.  )

  4. Photorespiration (continued) • Serine converted ultimately to glycerate (via hydroxypyruvate) in peroxisome. • glycerate  3-PGA in chloroplast. • Overall process consumes 2 molecules ATP (glutamine synthase step and glycerate  3-PGA step) and 2 reducing equivalents, per O2. • (Taiz and Zeiger, 2010, fig. 8.8 and table 8.2 for full details)

  5. 3 – Unavailability of CO2 • Atmospheric CO2 : O2 = 0.04% : 21% • Soluble CO2 concentration = ca.10μM (at ambient temperature and pressure) ≈ Km of RuBisCO for CO2. • Therefore RuBisCO can only ever operate at about half Vmax.

  6. 4 – CO2 in an aquatic environment • Diffuses 10,000 times slower than in air. • Slow to equilibrate between water and air, resulting in CO2 depleted waters. • Equilibrates with HCO3-, on a pH dependent basis – the pH of seawater (7.2) results in relatively low CO2availability.

  7. Summary – Why do Plants and Algae need photosynthetic turbochargers? • RuBisCO is old, slow and non-specific. • Photorespiration (to retrieve the carbon lost through RuBisCO’soxygenase activity) is wasteful. • CO2 is in low availability both in the atmosphere and in water. • CCMs go some way towards overcoming these limitations

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