Why do Plants and Algae need Carbon Concentrating
Why do Plants and Algae need Carbon Concentrating Mechanisms (CCMs)? Undergraduate level notes
1 – Limitations of Ru. Bis. CO �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. �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)
2 – Photorespiration � 2 -PG 2 molecules glycolate in chloroplast �glycolate glyoxylate in peroxisome, accompanied by production of H 2 O 2 (dissociates to O 2 + 2 H 2 O, by catalase) �glyoxylate glycine (by incorporation of 2 glutamate and release of 2 2 -oxoglutarate) � 2 mol glycine 1 mol serine in mitochondria by glycine decarboxylase, releasing NH 4+ which returns to chloroplast, NH 4+ + glutamate glutamine (glutamine synthase). (cont. )
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 O 2. � (Taiz and Zeiger, 2010, fig. 8. 8 and table 8. 2 for full details)
3 – Unavailability of CO 2 �Atmospheric CO 2 : O 2 = 0. 04% : 21% �Soluble CO 2 concentration = ca. 10μM (at ambient temperature and pressure) ≈ Km of Ru. Bis. CO for CO 2. �Therefore Ru. Bis. CO can only ever operate at about half Vmax.
4 – CO 2 in an aquatic environment �Diffuses 10, 000 times slower than in air. �Slow to equilibrate between water and air, resulting in CO 2 depleted waters. �Equilibrates with HCO 3 -, on a p. H dependent basis – the p. H of seawater (7. 2) results in relatively low CO 2 availability.
Summary – Why do Plants and Algae need photosynthetic turbochargers? �Ru. Bis. CO is old, slow and non-specific. �Photorespiration (to retrieve the carbon lost through Ru. Bis. CO’s oxygenase activity) is wasteful. �CO 2 is in low availability both in the atmosphere and in water. �CCMs go some way towards overcoming these limitations
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