Photosynthesis in Nature

Several researchers have considered photosynthesis in nature as a model for electrochemical conversion of sunlight to chemical energy [8]. Photosynthesis is discussed in several textbooks [37-40]. The topic of photosynthesis and photoelectrochemistry is discussed in Section 7.5. 

In contrast to a conventional p-n-junction-type solar cell, the mechanism of the DSSC does not involve a charge-recombination process between electrons and holes because electrons are injected from the dye photosensitizers into the semiconductor, and holes are not formed in the valence band of the semiconductor. In addition, electron transport takes place in the Ti02 film, which is separated from the photon absorption sites (i.e., the photosensitizers) thus, effective charge separation is expected. This photon-to-current conversion mechanism of the DSSC is similar to that for photosynthesis in nature, where chlorophyll functions as the photosensitizer and electron transport occurs in the membrane. 

Long SP, Humphries S, Falkowski PG. 1994. Photoinhibition of photosynthesis in nature. Anna Rev Plant Physiol Plant Mol Biol 45 633-662. 

Morris, I., and W. Skea. 1978. Products of photosynthesis in natural populations of marine phytoplankton from the Gulf of Maine. Marine Biology 47 303-312. 

C.A. Marwood, R.E.H. Smith, K.R. Solomon, M.N. Charlton, B.M. Greenberg (1999). Intact and photodified polycyclic aromatic hydrocarbons inhibit photosynthesis in natural assemblages of Lake Erie phytoplankton exposed to solar radiation. Ecotoxicol. Environ. Saf., 44, 322-327. 

Redox-mediated reactions are very common in biochemistry. More specifically, photo-induced redox-mediated reactions form the basis of photosynthesis in nature. 

The problem with respect to photosynthesis in nature is not the underlaying biochemistry to explain the shape of this curve, but to understanding the observed large differences in capacity of different plant types which exist even within the same habitat (Fig. 2). The photosynthetic rates of coniferous needles and sun leaves of herbaceous plants may differ by factor 4 in favor of the herbaceous plant, but obviously the conifer will dominate the habitat. 

In carefully arranged experiments the solar energy conversion efficiency of plants may reach 5%> However, photosynthesis in nature is generally not so effective. This is due to the fact that photosynthesis in nature proceeds often under more or less unfavorable conditions. 

The finding by Buchanan et a/. that RuDP carboxylase activity is stimulated by F6P and inhibited by FDP suggested that these two intermediates of the carbon cycle could regulate carboxylase activity during photosynthesis. Activation of RuDP carboxylase by F6P would solve a serious problem, namely the low affinity of RuDP carboxylase for CO2. This difficulty is particularly noteworthy because photosynthesis in nature operates at low partial pressures of CO2. To overcome this limitation the enzyme would require a concentration of CO2 about 100-fold greater than that normally present in air. 

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

Rainwater and snowmelt water are primary factors determining the very nature of the terrestrial carbon cycle, with photosynthesis acting as the primary exchange mechanism from the atmosphere. Bicarbonate is the most prevalent ion in natural surface waters (rivers and lakes), which are extremely important in the carbon cycle, accoxmting for 90% of the carbon flux between the land surface and oceans (Holmen, Chapter 11). In addition, bicarbonate is a major component of soil water and a contributor to its natural acid-base balance. The carbonate equilibrium controls the pH of most natural waters, and high concentrations of bicarbonate provide a pH buffer in many systems. Other acid-base reactions (discussed in Chapter 16), particularly in the atmosphere, also influence pH (in both natural and polluted systems) but are generally less important than the carbonate system on a global basis. 

The possible effects of increased atmospheric CO2 on photosynthesis are reviewed by Goud-riaan and Ajtay (1979) and Rosenberg (1981). Increasing CO2 in a controlled environment (i.e., greenhouse) increases the assimilation rate of some plants, however, the anthropogenic fertilization of the atmosphere with CO2 is probably unable to induce much of this effect since most plants in natural ecosystems are growth limited by other environmental factors, notably light, temperature, water, and nutrients. 

The photoprotective role of carotenoids is demonstrated in plant mutants that cannot synthesize essential leaf carotenoids. These mutants are lethal in nature since without carotenoids, chlorophylls degrade, their leaves are white in color, and photosynthesis cannot occur. Generally, the carotenoids are effective for visible light but have no effects in ultraviolet, gamma, or x-radiation. The reactions are listed as follows ... 

Photochemical fixation of carbon dioxide is a function of green plants and some bacteria in nature in the form of photosynthesis. All living organisms on the Earth are indebted directly or indirectly to photosynthesis. Thus, many attempts have been made to simulate the photosynthetic system and make artificial systems, although to date very little success has been achieved. 

Metal complexes of the porphyrins have been studied for many years. Such attention is not surprising, since particular derivatives play a central role in photosynthesis, dioxygen transport and storage as well as other fundamental processes such as electron transfer (Smith, 1975 Dolphin, 1978-9). Indeed, there are few compounds found in nature which can compare with the diversity of biochemical functions exhibited by the porphyrins. 

---