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Types of Carbon Concentrating Mechanism (CCM) - Overview

Types of Carbon Concentrating Mechanism (CCM) - Overview. Undergraduate level notes. Bio chemical Mechanisms in Plants. Variations on C3 photosynthesis in which the drawing down of CO 2 is not directly performed by RuBisCO

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Types of Carbon Concentrating Mechanism (CCM) - Overview

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  1. Types of Carbon Concentrating Mechanism (CCM) - Overview Undergraduate level notes

  2. Biochemical Mechanisms in Plants • Variations on C3 photosynthesis in which the drawing down of CO2 is not directly performed by RuBisCO • Carbon is initially “fixed” as a C4 acid by phosphoenolpyruvatecarboxylase (PEPC), which is delivered to RuBisCO and converted back to CO2, before being fixed as C3 compounds via the CBB cycle. • Two main variants: C4 and CAM (some variants are also found in aquatic plants).

  3. C4 and CAM - overview • C4 photosynthesis is a spatial separation of the drawing down of CO2 and its actual fixation by RuBisCO. • This spatial separation is facilitated by Kranz anatomy. • CAM is a temporal separation of the two processes, facilitated in higher plants by a specific, phased pattern of stomatal opening and closing.

  4. Biophysical Mechanisms • Variants in algae (eukaryotic) and in cyanobacteria (prokaryotic). • Less well characterised than biochemical CCMs in higher plants. • In algae, the main CCM component is the pyrenoidand in cyanobacteria, it is the carboxysome.

  5. The Pyrenoid and Carboxysome • Pyrenoid in algae is where most of the RuBisCO in the chloroplast is found, but is not membrane-bound. • Pyrenoid is part of a CCM comprising a series of bicarbonate (HCO3) pumps and carbonic anhydrases(to interconvert HCO3 and CO2). • Carboxysome is a more clearly defined microcompartmentof cyanobacterial cells and has a protein shell; bicarbonate ions are also delivered by active transport.

  6. Summary • Biochemical vs biophysical mechanisms • Biochemical: C4 and CAM (higher plants and some algae); Biophysical: pyrenoid (algae) and carboxysome (cyanobacteria). • Biochemical CCMs separate CO2 drawdown and fixation; biophysical CCMs actively concentrate CO2 around RuBisCO by transporting HCO3.

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