Photosynthesis is the process by which green plants, some green algae, and some bacteria, through the use of solar energy convert carbon dioxide into energy-rich molecules called carbohydrates (e.g., glucose, fructose, or polysaccharides). In most photosynthetic organisms, light energy is absorbed by pigments within the cells called antenna complexes that are then transferred to chlorophyll molecules within the cells. The chlorophyll molecules absorb light energy and transfer it to reaction centers where chemical reactions occur that produce a large amount of energy-rich molecules that feed the cells and enable them to grow. It is generally accepted that the first step in photosynthesis is the absorption of light energy by pigments in the photosynthetic organism that are then transferred to molecular oxygen (O2) and starch (C6H10O5) through a series of enzyme-mediated reactions (Hoffman, 1956; Kewley, 1960; McCree, 1968).
Over the years, researchers have tried to identify the exact mechanism of energy transduction in photosynthesis and have proposed two fundamentally distinct mechanisms (Figure 1) (Doyle and Van Orden, 2013). The first mechanism involves the direct conversion of light energy into chemical energy. In this mechanism, also known as the “direct mechanism”, the absorbed light energy is directly transduced into chemical energy, and ultimately, into cellular ATP and NADH, via the action of photosynthetic pigments located in the photosynthetic membranes. The second mechanism involves the transfer of excitation energy from one compound to another, a process that is subsequently used to produce cellular ATP, NADH, and/or other reduced molecules. In this mechanism, also known as the “indirect mechanism”, light energy is indirectly transduced into chemical energy by way of a series of electron transport steps that occur within the thylakoid membranes. The two fundamentally distinct mechanisms of energy transduction are not exclusive and may occur simultaneously.
Can Light Energy Be Converted Into Electrical Energy Without Going Through The Wires?
The biophotonic revolution seeks to capture the energy of visible light and to use it for practical applications. Visible light energy can be converted into electrical energy using a device known as a photoconductor. When light hits the photoconductor, some of its energy is absorbed by electrons in the form of photogenerated electron-hole pairs. This creates an electrical field that can be used to turn mechanical energy into digital data (electronically switching on a light bulb, or turning a turbine to generate electricity, for example).
Like photosynthesis, the process of converting light into electrical energy works through a pigmentation system called the “photosynthetic apparatus”. Many biological molecules, such as porphyrins, chlorophylls, and carotenoids, are used as photoconductors in nature. The first step in energy conversion is the absorption of light energy by these molecules. In the next step, the energy from the absorbed light is transferred to higher-energy molecules in the form of an electron. Finally, an electron transfer agent, located in the thylakoid membranes of the cell, removes the extra electron from the higher-energy molecules, which results in the generation of a net positive charge on the molecule.
Because of its ability to absorb light energy, convert it to electrical energy, and be incorporated into biomolecules, porphyrin-based molecules are of particular interest in the context of energy biotechnology (Hoffman, 1956; Kewley, 1960; McCree, 1968; Van Orden and Doyle, 2013). For example, it was shown that protoporphyrin IX molecules can be used to convert sunlight into electricity (Jung et al., 2017). Thus, porphyrin-based molecules are ideal candidates for the development of new bio-inspired solar-energy-harvesting technologies.
What About The “Dark Reaction” In Photosynthesis?
While the above discussion has focused on the “bright reaction” in which light energy is converted to glucose and oxygen through a series of enzyme-mediated reactions, there is another series of reactions that occurs simultaneously but that is not as well studied (Figure 1). Specifically, in the dark reaction, carbon dioxide is reduced to carbon monoxide and acetyl coenzyme A (acetyl-CoA), which is then converted to methane (CH4) and ethanol (CH3CHOHCH3) through a series of enzyme-mediated reactions. As noted above, photosynthesis is a process by which green plants, some green algae, and some bacteria utilize solar energy to convert carbon dioxide into energy-rich molecules called carbohydrates. It is important to understand that in addition to carbon dioxide, several other inorganic molecules, such as nitrates, phosphates, and sulfates, are reduced to inorganic molecules during photosynthesis (Hoffman, 1956; Kewley, 1960; McCree, 1968; Van Orden and Doyle, 2013). There is evidence that these inorganic compounds play an essential role in the process (Berg, 1960; McCree, 1968; Van Orden and Doyle, 2013). While the exact function of these inorganic molecules is currently unknown, it is clear that they are reduced to simple inorganic molecules (e.g., H+, HCO3-, PO43-, and SO42-) during the process of photosynthesis (Doyle and Van Orden, 2013).
It is also possible that some, or all, of the inorganic compounds that are reduced during photosynthesis play a role in the electron transport chain that occurs in the thylakoid membranes, similar to the way phosphate groups in ATP play a role in the function of the ATP synthase (McCree, 1968; Schramm et al., 1970). Further research is needed to determine the exact role that these inorganic molecules play in the process of photosynthesis. However, what is known is that these compounds are necessary for the proper function of the photosynthetic apparatus and for the production of carbohydrates through the “bright reaction” (Hoffman, 1956; Kewley, 1960; McCree, 1968). Therefore, if inorganic compounds are removed from the environment during photosynthesis, carbohydrates cannot be efficiently produced, resulting in the cells’ inability to grow (Doyle and Van Orden, 2013).
Photosynthetic Membranes Are Of Together With Other Membranes In The Cell
Photosynthetic membranes are of particular interest in the context of energy biotechnology because they can be used to efficiently absorb light energy and convert it to chemical energy. Like other membranes in the cell, photosynthetic membranes are composed of two layers: a hydrophobic interior and a hydrophilic exterior that are connected by lipid molecules (Figure 2). The lipid molecules in the two layers are arranged in such a way that allows for the transfer of energy from the hydrophobic interior layers to the hydrophilic exterior layers, which in turn, facilitates the transfer of energy to other molecules (Jung et al., 2017; Yossi and Mark, 2017). Thus, it is clear that photosynthetic membranes are specialized in such a way that allows them to efficiently absorb light energy and convert it into chemical energy. This is important because, as discussed above, if the absorption of light energy is not efficient, then the production of carbohydrates will be severely impaired or even inhibited.
Another important function of photosynthetic membranes is the prevention of “photo-damage” to the cell. Specifically, it was noted that the light-harvesting chlorophyll-protein complexes (LHCPs) in the thylakoid membranes of the chloroplasts have a dual function: they act as light-sensors and photo-catalysts (McClune and Linden, 1969; Sankar et al., 1991). The LHCPs act as sensitive photo-receptors that can detect light energy and trigger a photochemical response through the coordinated action of a group of enzymes called the “light-harvesting complex”. These enzymes in turn, act as photo-catalysts that can promote the conversion of atmospheric carbon dioxide into energy-rich, reduced molecules, such as carbohydrates (C6H10O5), which in turn, provide the cell with energy (Kewley, 1960; McCree, 1968).