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Carbon Concentrating Mechanisms (CCM): A Comprehensive Overview

Learn about various types of CCM including C4 and CAM in plants, biophysical mechanisms in algae and cyanobacteria, and the significance of Pyrenoid and Carboxysome structures. Understand the biochemical and biophysical processes involved in CO2 fixation and concentration.

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Carbon Concentrating Mechanisms (CCM): A Comprehensive Overview

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

  2. BiochemicalMechanisms 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 phosphoenolpyruvate carboxylase (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. BiophysicalMechanisms • 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 pyrenoid and in cyanobacteria, it is the carboxysome.

  5. The Pyrenoid and Carboxysome • Pyrenoidin algae is where most of the RuBisCO in the chloroplast is found, but is not membrane-bound. • Pyrenoidis part of a CCM comprising a series of bicarbonate (HCO3-) pumps and carbonic anhydrases (to interconvert HCO3-and CO2). • Carboxysomeis a more clearly defined microcompartment of 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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