Everything about C4 Carbon Fixation totally explained
C4 carbon fixation is one of three biochemical mechanisms, along with
C3 and
CAM photosynthesis, functioning in land
plants to "fix"
carbon dioxide (binding the gaseous molecules to dissolved compounds inside the plant) for
sugar production through
photosynthesis. Along with CAM photosynthesis, C
4 fixation is considered an advancement over the simpler and more ancient C
3 carbon fixation mechanism operating in most plants. Both mechanisms overcome the tendency of
RuBisCO (the first enzyme in the
Calvin cycle) to
photorespire, or waste energy by using oxygen to break down carbon compounds to CO
2. However C
4 fixation requires more energy input than C
3 in the form of
ATP. C
4 plants separate rubisco from atmospheric oxygen, fixing carbon in the
mesophyll cells and using
oxaloacetate and
malate to ferry the fixed carbon to rubisco and the rest of the Calvin cycle enzymes isolated in the bundle-sheath cells. The intermediate compounds both contain four carbon atoms, hence the name C
4.
The pathway
The C
4 pathway was discovered by
M. D. Hatch and C. R. Slack, in Australia, in 1966, so it's sometimes called the Hatch-Slack pathway.
In
C3 plants, the first step in the
light-independent reactions of photosynthesis involves the fixation of CO
2 by the enzyme
RuBisCo into
3-phosphoglycerate. However, due to the dual
carboxylase /
oxygenase activity of
RuBisCo, an amount of the substrate is oxidized rather than carboxylated resulting in loss of substrate and consumption of energy, in what is known as
photorespiration.
In order to bypass the
photorespiration pathway, C
4 plants have developed a mechanism to efficiently deliver CO
2 to the
RuBisCO enzyme. They utilize their specific leaf anatomy where chloroplasts exist not only in the
mesophyll cells in the outer part of their leaves but in the
bundle sheath cells as well. Instead of direct fixation in the
Calvin cycle, CO
2 is converted to a 4-carbon
organic acid which has the ability to regenerate CO
2 in the chloroplasts of the bundle sheath cells. Bundle sheath cells can then utilize this CO
2 to generate carbohydrates by the conventional
C3 pathway.
The first step in the pathway is the conversion of
pyruvate to PEP by the enzyme pyruvate-phosphate dikinase (
pyruvate, orthophosphate dikinase); this reaction requires inorganic phosphate and
ATP plus
pyruvate, giving
phosphoenolpyruvate,
AMP, and PPi (inorganic pyrophosphate) as products. The next step is the fixation of CO
2 by the enzyme
phosphoenolpyruvate carboxylase. Both of these steps occur in the mesophyll cells:
» pyruvate + Pi + ATP → PEP + AMP + PPi
PEP carboxylase + PEP + CO
2 → oxaloacetate
PEP carboxylase has a lower
Km for CO
2—and hence higher affinity—than Rubisco. Furthermore, O
2 is a very poor substrate for this enzyme. Thus, at relatively low concentrations of CO
2, most CO
2 will be fixed by this pathway.
The product is usually converted to
malate, a simple
organic compound that's transported to the bundle-sheath cells surrounding a nearby
vein, where it's decarboxylated to release CO
2, which enters
Calvin cycle. The decarboxylation leaves
pyruvate, which is transported back to the
mesophyll cell.
Since every CO
2 molecule has to be fixed twice, the C
4 pathway is more energy-consuming than the C
3 pathway. The C
3 pathway requires 18 ATP for the synthesis of one molecule of glucose while the C
4 pathway requires 30 ATP. But since otherwise tropical plants lose more than half of photosynthetic carbon in
photorespiration, the C
4 pathway is an adaptive mechanism for minimizing the loss.
There are several variants of this pathway:
- The 4-carbon acid transported from mesophyll cells may be malate as above, or may be aspartate.
- The 3-carbon acid transported back from bundle-sheath cells may be pyruvate as above, or alanine.
- The enzyme which catalyses decarboxylation in bundle-sheath cells differs. In maize and sugarcane, the enzyme is NADP-malic enzyme, in millet, it's NAD-malic enzyme, and in Panicum maximum it's PEP carboxykinase.
C4 Leaf Anatomy
The C
4 plants possess a characteristic
leaf anatomy. Their vascular bundles are surrounded by two rings of cells. The inner ring, called Bundle Sheath Cells, contain
starch-rich
chloroplasts
lacking grana which differ from those in
mesophyll cells present as the outer ring. Hence, the chloroplasts are called dimorphic. This peculiar anatomy is called
Kranz Anatomy (Kranz-Crown/Halo). The primary function of the Kranz is to provide a site in which carbon dioxide can be concentrated around RuBisCO, thus reducing photorespiration. In order to facilitate the maintenance of a significantly higher carbon dioxide concentration in the bundle sheath compared to the mesophyll, the boundary layer of the Kranz has a low conductance to carbon dioxide, a property which may be enhanced by the presence of suberin.
Although most C
4 plants exhibit Kranz anatomy, there are a number of species which operate a limited C
4 cycle without any distinct bundle sheath tissue.
Suaeda aralocaspica (formerly known as
Borszczowia aralocaspica),
Bienertia cycloptera and
Bienertia sinuspersici (all
chenopods) are terrestrial plants which inhabit dry, salty depressions in the deserts of south-east Asia. These plants have been shown to operate single-cell C
4 carbon dioxide concentrating mechanisms which are unique amongst the known C
4 mechanisms. Although the cytology of both species differ slightly, the basic principle is that fluid filled vacuoles are employed to divide the cell into to separate areas. Carboxylation enzymes in the cytosol can therefore be kept separate from decarboxylase enzymes and RuBisCo in the chloroplasts, and a diffusive barrier can be established between the chloroplasts (which contain RuBisCO) and the cytosol. This enables a bundle-sheath type area and a mesophyll type area to be established within a single cell. Although this does allow a limited C
3 cycle to operate, it's relatively inefficient, with much leakage of CO2 from around RuBisCO occurring. There is also evidence for the non-Kranz aquatic macrophyte Hydrilla verticillata exhibiting inducible C
4 photosynthesis under warm conditions, although the mechanism by which CO2 leakage from around RuBisCO is minimised is currently uncertain.
The Evolution and Advantages of the C4 Pathway
C
4 plants have a competitive advantage over plants possessing the more common
C3 carbon fixation pathway under conditions of
drought, high
temperatures and
nitrogen or
carbon dioxide limitation. 97% of the water taken up by plants is lost through transpiration, Today they represent about 5% of Earth's plant biomass and 1% of its known plant species. However, they account for around 30% of terrestrial carbon fixation.
[ These species are concentrated in the tropics (below latitudes of 45°) where the high air temperature contributes to higher possible levels of oxygenase activity by RuBisCO, which increases rates of photorespiration in C3 plants.]
Further Information
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