Types of Carbon Concentrating Mechanism (CCM) - Overview

Undergraduate level notes

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

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).

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.

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 pyrenoid and

in cyanobacteria, it is the carboxysome.

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 microcompartment of cyanobacterial cells and has a protein shell; bicarbonate ions are also delivered by active transport.

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

-.