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-.