Initial and rapid fixation of CO2 within mesophyll cells results in the formation of a four-carbon compound which is then pumped to bundle sheath cells for decarboxylation and subsequent incorporation into the PCR cycle in that tissue. This neat division of labour hinges on specialised anatomy and has even resulted in evolution of distinct classes of chloroplasts in mesophyll compared with bundle sheath cells. Three biochemical variants of C4 photosynthesis (termed subtypes) are known to have evolved from C3 progenitor and in all cases with a recurring theme where the C4 cycle of mesophyll cells is complemented by a PCR cycle in bundle sheath cells, where Rubisco is exclusively localised. In effect, a biochemical ‘pump’ concentrates CO2 at Rubisco sites in bundle sheath cells thereby sustaining faster net rates of CO2 incorporation and virtually eliminating photorespiration. For this overall mechanism to have evolved, a complex combination of cell specialisation and differential gene expression was necessary. Figure 2.3a shows a low-magnification electron micrograph of a C4 leaf related to a generalised scheme for the C4 pathway.
By contrast, evolution of a new photosynthetic pathway (C4) has occurred independently and on many occasions in diverse taxa over 25 to 30 million years as CO2 levels declined. Despite its complexity, C4 photosynthesis evolved more than 60 independent times in 19 distantly related flowering families. About 50% of C4 species are grasses (Poaceae) with ~18 distinct origins distributed over 370 genera and ~4600 species (Sage et al. 2011). The oldest identifiable fossils with pronounced bundle sheath layers are seven million years old, although necessary metabolic pathways could have evolved earlier, prior to this adaptation in anatomy. C4 plants are known to differ from C3 plants in their discrimination against atmospheric 13CO2, and shifts in the stable carbon isotope signature of soil carbonate layers that reflect emergence of C4 plants have been dated at 7.5 million years bp. Modern evidence from molecular phylogeny places the origin of the main C4 taxa at 25-30 million years ago (Christin et al. 2009). By inference, C4 photosynthesis evolved in response to a significant decline in atmospheric CO2 concentration, from 1500–3000 µL L–1 to about 300 µL L–1. By evolving a CO2-concentrating mechanism, C4 plants presented their Rubisco with an elevated partial pressure of CO2 despite lower atmospheric CO2. As a consequence, photorespiration was inhibited, maximum photosynthetic rates increased and energetic costs reduced.
what goes into the photosynthesis reaction - carbon dioxide …
Rubisco is characterised by its low affinity for its productive substrate, CO2 and slow catalytic turnover rate (i.e., 1-3 cycles per sec). Importantly, Rubisco reacts with O2 (photorespiration), and this culminates in loss of CO2 and energy. In C3 plants, photorespiration can drain more than 25% of fixed CO2 under non-stressful conditions. The ratio of photorespiration to photosynthesis increases with increasing temperature and decreasing intercellular CO2 such as occurs when stomatal conductance is reduced under water stress. C3 plants compensate for Rubisco’s inefficiencies by (i) opening their stomata to increase CO2 diffusion into chloroplasts, which increases water loss and lowers leaf-level water use efficiency, WUE; and (ii) investing up to 50% of leaf nitrogen in Rubisco, which lowers their leaf-level nitrogen use efficiency, NUE.
This tutorial introduces the carbon cycle
What advantages did all this offer plants over plants that fix CO2 directly by the PCR cycle — that is, using CO2 diffusing directly from air (and distinguished as C3 plants by virtue of their initial three-carbon fixation product phosphoglycerate). As Section 2.2 explains, concentrating CO2 in bundle sheath cells eliminates photorespiration. This, in turn, gives C4 plants distinct advantages in terms of growth and survival, especially at higher temperatures and under strong light. This can be seen most graphically in the distribution of grass species in Australia. In Tasmania, as well as the cooler and wetter southern-most tips of the continent, C4 species are in the minority. However, going north there is a rapid transition and for most of the continent most or all of the grass species are C4.
08/01/2018 · Photosynthesis vs
By 1970 we had a reasonably good understanding of how C4 photosynthesis worked in species like maize and sugar cane (see Section 2.2 for details), and suggested that the reactions unique to C4 photosynthesis might function to concentrate CO2 in the bundle sheath cells of C4 leaves, acting essentially as a CO2 pump. Later, we obtained direct experimental evidence that CO2 was indeed concentrated about 10- to 20-fold in these cells in the light.