Molecular Mechanisms of Photosynthesis. Robert E. Blankenship
Читать онлайн книгу.
purified, and their structures determined. The proteins that make them up to have been identified, and the genes that code for them identified and sequenced. While many questions remain about how the two photosystems interact and how both energy input to them from the antennas and electron flow between them is regulated, the Z scheme is an essential feature of the modern understanding of photosynthesis.
Of course, there are some types of photosynthetic organisms that contain chlorophyll‐like pigments and exhibit light‐dependent growth, but do not evolve oxygen. We encountered them earlier in our discussion of the van Niel formulation for the redox nature of photosynthesis. These anoxygenic (non‐oxygen evolving) photosynthetic organisms do not display any of the enhancement or red drop effects described above that led to the discovery of two photosystems in oxygen‐evolving organisms, which suggests that they contain only a single type of photosystem. Biochemical and genetic studies confirm that these organisms, which are all bacteria, indeed contain only a single type of photosystem. These simpler photosynthetic organisms have been essential for learning about the basic chemical mechanisms of photosynthesis. These organisms are discussed in detail in Chapter 6. They also provide clues to how the complex two‐photosystem version of photosynthesis found in more advanced organisms may have arisen and evolved. This is discussed in Chapter 12.
3.13 ATP formation
So far, we have focused first on the overall process of photosynthesis and then on the discoveries leading up to the discovery of the series formulation of electron transport in oxygen‐evolving photosynthetic organisms, resulting in the reduction of NADP+ to NADPH. There is another critical part to the story, however, involving the light‐dependent formation of ATP and the subsequent utilization of these two products to reduce CO2 to carbohydrates. The discoveries of these processes paralleled the discoveries of the electron transfer processes.
The discovery that chloroplasts could make ATP in a light‐dependent manner was made in 1954, by Daniel Arnon and coworkers at the University of California, Berkeley (Arnon et al., 1954). The idea that chloroplasts could make ATP, in a process called photophosphorylation, initially met with considerable resistance, because it was well known that mitochondria produced large amounts of ATP and, since chloroplasts in many ways drive the mitochondrial reaction in the opposite direction, this initially seemed backward (Arnon, 1984). An analogous discovery of light‐driven ATP formation in non‐oxygen‐evolving purple bacteria was made by Howard Gest and Martin Kamen (1948). The chemiosmotic hypothesis, the theoretical framework for the mechanism of how photon energy is stored in ATP, was provided by the incisive analysis of Peter Mitchell in the 1960s and 1970s, for which he received the Nobel prize in 1978 (Mitchell, 1979). We will discuss the details of the ATP synthesis process in Chapter 8.
3.14 Carbon fixation
At approximately the same time as Arnon was demonstrating photophosphorylation on one side of the Berkeley campus of the University of California, Melvin Calvin, Andrew Benson, and coworkers were working to understand the details of the carbon assimilation process itself on the other side of the campus (Calvin, 1989; Benson, 2002). They elucidated the chemical reactions that convert CO2 and assimilatory power into carbohydrates. These reactions have become known as the Calvin–Benson cycle, and Calvin was awarded the Nobel Prize for chemistry in 1961 in recognition of the brilliant elucidation of this complex set of reactions. He and his coworkers used the newly developed method of radioactive tracers, injecting algae with 14CO2 and then following the path of the radioactivity in the products (Creager, 2013). We will discuss the details of the Calvin–Benson cycle and other aspects of carbon metabolism in Chapter 9.
References
1 Arnon, D. I. (1984) The discovery of photosynthetic phosphorylation. Trends in Biochemical Sciences 9: 1–5.
2 Arnon, D. I., Allen, M. B., and Whatley, F. R. (1954) Photosynthesis by isolated chloroplasts. Nature 174: 394–396.
3 Benson, A.A. (2002) Following the path of carbon in photosynthesis: A personal story. Photosynthesis Research 73: 29–49.
4 Calvin, M. (1989) 40 years of photosynthesis and related activities. Photosynthesis Research 21: 3–16.
5 Creager, A. N. H. (2013) Life Atomic: Radioisotopes in Science and Medicine. Chicago: University of Chicago Press.
6 Duysens, L. M. N. (1989) The discovery of the 2 photosynthetic systems – a personal account. Photosynthesis Research 21: 61–79.
7 Duysens, L. M. N., Amesz, J., and Kamp, B. M. (1961) Two photochemical systems in photosynthesis. Nature 190: 510–511.
8 Emerson, R. and Arnold, W. (1932a) A separation of the reactions in photosynthesis by means of intermittent light. Journal of General Physiology 15: 391–420.
9 Emerson, R. and Arnold, W. (1932b) The photochemical reaction in photosynthesis. Journal of General Physiology 16:191–205.
10 Emerson, R., Chalmers, R., and Cederstrand, C. (1957) Some factors influencing the longwave limit of photosynthesis. Proceedings of the National Academy of Sciences USA 43: 133–143.
11 Gaffron, H. and Wohl, K. (1936) Zur theorie der assimilation. Naturwissenschaften 24: 81–90; 103–107.
12 Gest, H. and Kamen, M. D. (1948) Studies on the phosphorus metabolism of green algae and purple bacteria in relation to photosynthesis. Journal of Biological Chemistry 176: 299–318.
13 Govindjee, S. D. and Björn, L. O. (2017) Evolution of the Z‐scheme of photosynthesis: A perspective. Photosynthesis Research 133: 5–15.
14 Govindjee, S. D., Beatty, J. T., Gest, H., and Allen, J. F., (eds.) (2005) Discoveries in Photosynthesis. Dordrecht: Springer.
15 Hill, R. (1939) Oxygen produced by isolated chloroplasts. Proceedings of the Royal Society of London. Series B 127: 192–210.
16 Hill, J. F. (2012) Early pioneers of photosynthesis research. In: J. J. Eaton‐Rye, B. C. Tripathy, and T. D. Sharkey, (eds.) Photosynthesis. Dordrecht: Springer.
17 Hill, R. and Bendall, F. (1960) Function of the two cytochrome components in chloroplasts: A working hypothesis. Nature 186: 136–137.
18 Jaffe, B. (1976) Crucibles: The Story of Chemistry, 4th Edn. New York: Dover.
19 Magiels, G. (2010) From Sunlight to Insight: Jan Ingen‐Housz, the Discovery of Photosynthesis & Science in the Light of Ecology. Brussels: Academic & Scientific Publishers.
20 Mitchell, P. (1979) Keilin's respiratory chain concept and its chemiosmotic consequences. Science 206: 1148–1159.
21 Myers, J. (1994) The 1932 experiments. Photosynthesis Research 40: 303–310.
22 Myers, J. and French, C. S. (1960) Relationships between time course, chromatic transient, and enhancement phenomena of photosynthesis. Plant Physiology 35: 963–969.
23 Nickelsen, K. (2015) Explaining Photosynthesis: Models of Biochemical Mechanisms, 1840–1960. Dordrecht: Springer.
24 Nickelsen, K. and Govindjee, S. D. (2011) The Maximum Quantum Yield Controversy: Otto Warburg and the “Midwest‐Gang”. Bern: Bern Studies in the History and Philosophy of Science.
25 van Niel, C. B. (1941) The bacterial photosyntheses and their importance for the general problem of photosynthesis. Advances in Enzymology 1: 263–328.
26 Rabinowitch, E. I. (1945) Photosynthesis and Related Processes, Vol. 1. New York: Interscience Publishers.
27 Walker, D. A. (1992) Energy, Plants and Man, 2nd Edn. Brighton: Oxygraphics.
Chapter