Undergraduate level notes

Biochemical Mechanisms in Plants Variations on C3 photosynthesis in which the drawing down of CO 2 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 CO 2, 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 CO 2 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 (HCO 3 - ) pumps and carbonic anhydrases (to interconvert HCO 3 - and CO 2 ). 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 CO 2 drawdown and fixation; biophysical CCMs actively concentrate CO 2 around RuBisCO by transporting HCO 3 -.