C3 and C4 photosynthetic pathways
Plants have the ability to grown and survive in a number of different environments. They can adapt from hot and dry to frigid temperatures. In order to adapt to various environmental conditions plants have to adapt and develop mechanisms to survive different conditions. One mechanism of adaptation involves the mode of fixing carbon dioxide. Carbon fixation is the process of incorporating inorganic Carbon to an organic form. Photosynthesis is the process of carbon fixation used by the plants. C3 plants usually are found in cold, temperate climate and are most widely distributed worldwide. They use environmental carbon-dioxide (CO2) and incorporate it into the Calvin cycle. The CO2 reacts with ribulose 1,5 bisphosphate to form 3-phospho glycerate (Gowik &Westhoff, 2011). However, C3 plants suffer from some disadvantages making them unsuitable to grow and thrive under hot and dry conditions. C4 plants have evolved to overcome this deficiency. These plants grow especially well in dry and hot temperature. C4 plants grow with a high rate of growth and photosynthesis. CO is fixed as phosphenolpyruvate to form oxaloacetate a 4 carbon molecule. The enzyme catalyzing this first reaction of C4 carbon fixation cannot react with oxygen and avoids photorespiration a problem associated with C3 plants. There are two kinds of cells present in the C4 plant leaves including the mesophyll cells and bundle-sheath cells. This characteristic anatomy of C4 plants is Kranz anatomy and is used to recognize the C4 plants. C4 plants eventually use the Calvin cycle for CO2 fixation by reacting it with ribulose 1,5 bisphosphate. This step for carbon fixation, as a result, is preceded by three to four reactions in the C4 plants. This allows them to grow and survive arid and warm temperatures. C3 plants predate C4 plants (figure 1 represents the schematic of C3 and C4 photosynthetic mechanism).
C3 plants account for 90% of all kinds of plants found on earth. The assimilation of CO2 to biomolecules occurs by the process of carbon fixation. It involves three stages. The first step involves the incorporation of CO2 with a 5- Carbon sugar, ribulose 1,5- bisphosphate. The CO2 is covalently attached to Ribulose 1,5 bisphosphate by the enzyme Ribulose 1,5 bisphosphate carboxylase (RUBISCO). The product of this reaction is two molecules of 3, phosphoglycerate. Rubisco is the most prevalent enzyme in the biosphere (Gowik & Westhoff, 2011). The second stage involves a 2 step conversion of 3 phosphoglycerate to Glyceraldehyde 3-Phosphate. The first step involves the transfer of a phosphoryl moiety from the ATP molecule to 3-phosphoglycerate, forming 1,3 bisphospho glycerate. The next reaction is catalyzed by the enzyme glyceraldehyde 3-phosphate dehydrogenase. The NADPH molecule functions as a cofactor and donates electrons to form Glyceraldehyde 3-phosphate (Nelson & Cox 2008). The molecule Glyceraldehyde 3-phosphate undergoes isomerization to Dihydroxy acetone phosphate catalyzed by the enzyme glyceraldehyde 3 phosphate dehydrogenase. The triose phosphate molecule can be either converted to starch for later use or converted to sucrose (Dey & Harborne 1997). The triose phosphate is also broken down by the process of glycolysis to provide additional energy. The third and final stage of the carbon assimilation in the C3 plants occurs to regenerate the ribulose 1,5-bisphosphate (Neslon & Cox 2008).
A rearrangement in the carbon skeleton of the glyceraldehyde 3-phosphate and dihydroxy acetone phosphate occurs with the formation of various three, four, five, six and seven carbon sugar molecules. The process of carbon fixation and photosynthesis occurs in the mesophyll cells, in the C plants. The C3 plants utilize atmospheric CO2 that is derived following opening of stomata. However, stomata opening also leads to a high amount of water loss by transpiration. The second drawback associated with the C3 plants is the process of photorespiration decrease efficiency of photosynthesis. Furthermore, RUBISCO has a greater affinity for O2 than for CO2. Loss of O2 occurs, which makes C3 plants’ inefficient (Rawsthorne 1992).
The C4 plants have evolved to compensate for the deficiencies faced by the C3 plants. The bindle-sheath and mesophyll cells in the leaves of C4 plants are involved in photosynthesis. The C4 plants also utilize Calvin cycle; however, a few reactions precede the CO2 fixation by Calvin cycle. The assimilation of CO2 occurs by the Hatch Slack pathway by 4 carbon intermediate (Ehleringer, Cerling, & Helliker, 1997). The First reaction involves condensation of CO2 with Phosphenol pyruvate to form a 4-C sugar called Oxaloacetate that is catalyzed by phosphoenolpyruvate carboxylase. The oxaloacetate gets reduced to malate by the enzyme malate dehydrogenase. The malate molecules enter the bundle sheath cells. CO2 is released from the malate by the enzyme malic enzyme resulting in the formation of pyruvate. The CO2 that is released in the bundle sheath cells undergoes the Calvin cycle to react with Ribulose 1,5 bisphosphate and further on. Pyruvate is returned to mesophyll cells to carry on the cycle. Since the bundle sheath cells do not have oxygen, there is no scope for photorespiration (Ehleringer, Cerling, & Helliker, 1997). The enzyme involved with carbon fixation, PEP carboxylase, has a very high affinity for CO2 that allows photosynthesis to occur at low CO2 concentration and requires little opening of stomata. This prevents water loss from the C4 plants. This allows the C4 plants to grown and survive in dry climate. The evolution of the C4 plants is an energy expensive process as it utilizes more ATP molecules than C3 plants (Rawsthorne, 1992).
The C3 plants have existed for a long time and evolution of C4 plants occurred to adapt to higher temperatures and environmental dryness. C4 plants offer a more efficient photosynthetic capacity and water utilization than the C plants. Studies are also being carried to incorporate the C4 photosynthetic machinery into C3 plant to provide for the growing food demand in the world. (Gowik & Westhoff 2011).
Figure 1. A schematic representation of C3 and C4 photosynthesis. (Wang, Guo, Li & Zhang, 2011)
References
Dey, P.M., & Harborne, J.B. Plant Biochemistry Book (1997) Academic Press.
Ehleringer, J. R., Cerling, T. E., & Helliker, B. R. (1997). C4 photosynthesis, atmospheric CO2, and climate. Oecologia, 112(3), 285-299.
Gowik, U., & Westhoff, P. (2011). The path from C3 to C4 photosynthesis. Plant Physiology, 155(1), 56-63.
Lehninger Principles of Biochemistry Book (Jun 2008) by Albert Lehninger, David L. Nelson, Michael M. Cox. Worth Publishers 3rd Ed.
Rawsthorne S (1992). Towards an understanding of C3-C4 photosynthesis. Essays Biochem. 27: 135-46.
Wang, C., Guo, L., Li, Y., & Wang, Z. (2012). Systematic comparison of C3 and C4 plants based on metabolic network analysis. BMC systems biology, 6(Suppl 2), S9.
