Figure 2.3. C4 photosynthesis is an evolutionary development where specialised mesophyll cells initially fix CO2 from the air into 4-carbon acids which are transported to the site of the PCR cycle in the bundle sheath. The bundle sheath cells are relatively impermeable to CO2, so that when the CO2 is released here from the 4-carbon acids, it builds up to high levels. The C4 photosynthetic mechanism is a biochemical CO2 pump. The pathway shown here is overlayed on a micrograph of a C4 leaf, showing bundle sheath and mesophyll cells. Rubisco and the other PCR enzymes are in the bundle sheath cells while phosphoenolpyruvate (PEP) carboxylase is part of the CO2 pump in the mesophyll cells. In C4 plants, after radioactive labelling, 14C appears first in a 4-carbon acid, rather than in 3-PGA. Scale bar = 10 µm. (Original drawings courtesy M.D. Hatch).
The C4 pathway (Figure 2.3) is ‘a unique blend of modified biochemistry, anatomy and ultra-structure’ (Hatch 1987). The classical C4 syndrome in most terrestrial plants consists of two photosynthetic cycles (C3 (or PCR) and C4) operating across two photosynthetic cell types (mesophyll and bundle sheath), which are arranged in concentric layers around the vascular bundle, also known as the kranz anatomy.
why is photosynthesis referred to as a biochemical pathway?
By analogy with Calvin’s biochemical definition of the C3 pathway at Berkeley in the 1950s, the C4 pathway was also delineated with radioactively labelled CO2 (see Feature Essay 2.1). Significantly, and unlike C3 plants, 3-PGA is not the first compound to be labelled after a 14C pulse (Figure 2.3b). Specialised mesophyll cells carry out the initial steps of CO2 fixation utilising the enzyme phosphoenolpyruvate (PEP) carboxylase. The product of CO2 fixation, oxaloacetate, is a four-carbon organic acid, hence the designation ‘C4’ photosynthesis (or colloquially, C4 plant). A form of this four-carbon acid, either malate or aspartate depending on the C4 subtype, migrates to the bundle sheath cells which contain Rubisco and the PCR cycle. In the bundle sheath cells, CO2 is removed from the four-carbon acid by a specific decarboxylase and a three-carbon product returns to the mesophyll to be recycled to PEP for the carboxylation reaction. Thus, label first appears in the four-carbon acid after 14C feeding, followed by 3-PGA and, finally, in sucrose and starch (Figure 2.1b).
synthesize compounds from non-living matter by photosynthesis.
Although CAM and C4 photosynthesis share common enzyme machineries, the physiological bases of spatially-separated and time-separated CCMs are very different and involve complex suites of distinctive regulatory processes ranging from allosteric modulation of enzyme activities, through cell and organelle membrane metabolite transport systems, to long-term responses to stress. The resulting metabolism is rarely at steady state. It is thus helpful to reference the principal biochemical interacting components of CAM to the CO2 exchange patterns and the pool sizes of acidity and carbohydrates in the archetypical Kalanchoë daigremontiana as outlined in Figure 2.33.
Biology - Georgia Virtual School > Home
Despite much diversity in life form and biochemical process, all of the photosynthetic pathways focus upon a single enzyme which is by far the most abundant protein on earth, namely ribulose-1,5-bisphosphate carboxylase/oxygenase, or Rubisco (Figure 2.1a). Localised in the stroma of chloroplasts, this enzyme enables the primary catalytic step in photosynthetic carbon reduction (or PCR cycle) in all green plants and algae. Although Rubisco has been highly conserved throughout evolutionary history, this enzyme is surprisingly inefficient with a slow catalytic turnover (Vcmax), a poor specificity for CO2 as opposed to O2 (Sc/o), and a propensity for catalytic misfiring resulting in the production of catalytic inhibitors. This combination severely restricts photosynthetic performance of C3 plants under current ambient conditions of 20% O2 and 0.039% CO2 (390 μL L-1). Furthermore, Rubisco has a requirement for its own activating enzyme, Rubisco activase, which removes inhibitors from the catalytic sites to allow further catalysis. Accordingly, and in response to CO2 limitation, C4, C3-C4 intermediate, CAM and SAM variants have evolved with metabolic concentrating devices which enhance Rubisco performance (Section 2.2).