Photosynthetic pathways

Sternberg, L.O., DeNiro, M.J. and Johnson, H.B. 1984 Isotope ratios of cellulose from plants having different photosynthetic pathways. Plant Physiology 74 557-561. 

Osmond, C.B., Winter, K. Powles, S.B. (1979). Adaptive significance of CO cycling in leaves of plants with different photosynthetic pathways. In Stress Physiology, ed. N.C. Turner and P.J. Kramer. New York Academic Press. 

The outcome of the photosynthesis processes is ultimately similar in all green plants carbon dioxide from the atmosphere is taken up by the plants, where it reacts with water to form carbohydrates and oxygen the carbohydrates are assimilated by the plants while the oxygen is released to the atmosphere (see Textbox 53). Extensive studies have shown that the conversion of carbon dioxide and water into carbohydrates in different plants may follow, however, one of three different photosynthetic pathways, which are usually referred to as the C3, C4, and CAM paths. Each type of plant follows just one of these three pathways. 

Ehleringer, J.R. and Monson, R.K. (1993). Evolutionary and ecological aspects of photosynthetic pathway variation. Annual Review of Ecology and Systematics 24 411 439. 

Figure 36 A panoramic view of the photosynthetic pathway in green plants...

The present discussion is only concerned with the structure/redox capacity of the site responsible for the oxidation of water. The starting point is the evidence that the photosynthetic pathway is triggered by photooxidation of the chlorophylls in photosystem II. The need for chlorophylls to recover the electrons lost in photooxidation (in order to regenerate their ability to absorb light) induces water to undergo oxidation, according to ... 

In coastal areas, measurements of 513C in bulk organic matter can help identify the origins of organic material in sediments. In general, material produced using the dominant C3 photosynthetic pathway has a value of 513C around -27 %o for terrestrial matter and around -20 %o for marine matter (Deines, 1980). The interpretation of such results is complicated because some plants use the C4 photosynthetic pathway, which... 

C3 photosynthetic pathway Also known as the Calvin cycle. A series of enzymatically mediated photosynthetic reactions during which C02 is reduced to 3-phosphoglyceraldehyde and the C02 acceptor, ribulose 1,5-bisphosphosphate, is regenerated. 

C4 photosynthetic pathway The set of reactions through which C02 is fixed to a compound known as phosphoenolpyruvate (PEP) to yield ox-aloacetate, a four-carbon compound. 

CAM (Crassulacean Acid Metabolism) photosynthetic pathway A variant of the C4 pathway phosphoenolpyruvate fixes C02 in C4 compounds at night, and then, the fixed C02 is transferred to the ribulose bisphosphate of the Calvin cycle within the same cell during the day. Characteristic of most succulent plants, such as cacti. 

Atmospheric CO2 first moves through the stomata, dissolves into leaf water and enters the outer layer of photosynthetic cells, the mesophyll cell. Mesophyll CO2 is directly converted by the enzyme ribulose biphosphate carboxylase/oxygenase ( Rubisco ) to a six carbon molecule that is then cleaved into two molecules of phosphoglycerate (PGA), each with three carbon atoms (plants using this photosynthetic pathway are therefore called C3 plants). Most PGA is recycled to make ribulose biphosphate, but some is used to make carbohydrates. Free exchange between external and mesophyll CO2 makes the carbon fixation process less efficient, which causes the observed large C-depletions of C3 plants. 

There are three major photosynthetic pathways Cg, C4 and CAM. Please see glossary for further detail. (Appendix C). 

The plastocyanins are found in plant chloroplasts and other photosynthetic organisms, and act as membrane-bound electron carriers between photosystems II and I in the photosynthetic pathway of higher plants, green algae and some blue-green algae. 

More recently, the carbon stable isotope ratio test (SIRA) has become an easy method to detect adulteration with cane and corn syrup (Carro et al, 1980). Because maple trees are C3 plants with a somewhat different photosynthetic pathway for carbon fixation, the ratio of 13C/12C in the sugar produced is different than cane or com. Maple has a 813C of approximately —24.5, whereas com and cane are closer to a 813C of —8 to —12. Thus, even a small addition of cane or corn syrup is readily detectable. Because beets are also C3 plants, the SIRA test is not able to detect adulteration with beet sugar. Improvement of the SIRA method is possible using malic acid as an internal standard (Tremblay and Paquin, 2007). 

Figure 8-15. Carboxylase reactions and locations for the three photosynthetic pathways (a) C3, (b) C4, and (c) Crassulacean acid metabolism (CAM). The reactions for C3 and C4 plants occur during the daytime. The indicated decarboxylations of C4 acids occur in the cytosol of bundle sheath cells for C4 plants and the cytosol of mesophyll cells for CAM plants.

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