In the previous chapter the functions of hydrogen in the energy processes in the cosmos were covered. It was shown that hydrogen burning during thermo-nuclear reaction on the sun releases great amount of energy, part of which reaches the earth. The paradox is that while hydrogen that plays the main role in processes which supply energy to the earth, it also simultaneously plays a main role in the conversion of this energy. Energy conversion takes place through photosynthesis. Photosynthesis is a process during which organisms synthesize organic chemical compounds, especially carbohydrates, from carbon dioxide using the energy obtained from light [61]. Photosynthesis is an enormously important natural process which creates the possibility for sustainable existence of living organisms. Investigations on photosynthesis have been carried out by many research groups for centuries. The extreme importance of this scientific trend can be only stressed by the fact that 10 (!) Nobel Prizes have been awarded to researchers working on photosynthesis. 4.1 IntroductionIn this chapter the main principles of photosynthesis are elucidated; special attention being paid to the specific role played by hydrogen

which may occur in other systems and materials also and interesting hints and prompts made for implementing the natural design in various man-created devices and technological processes. 4.2 Design of Photosynthetic Systems In green plants and cyanobacteria, photosynthetic reactions occur in chloroplasts which are autonomous cytoplasmic bodies containing all elements necessary for photosynthesis, namely proteins that are organic molecules consisting of amino acids arranged in a linear chain and folded into a globular form (Fig. 4.1) [62]. The proteins that collect light for photosynthesis are embedded within sub-unit cells (organelles) called chloroplasts. Usually a plant cell contains about 10 to 100 chloroplasts. Here, membrane design is implemented by nature. The chloro-plast is enclosed by a membrane composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space between them [62]. Phospholipids are a class of lipids that are a major component of all cell membranes as they can form lipid bi-layers. The membrane architecture is an inalienable feature in liv-ing organisms; it facilitates the creation of proton gradient across the membrane or, in other words, to charge the biological battery whose energy is employed for the synthesis of adenosine triphos-phate (ATP) molecules that will be described below. Very complicated enzyme-protein complexes, in which all chlorophyll cells are concentrated and the primary act of the light energy conversion occurs, are embedded into three membranes of ~50 Å thickness (Fig. 4.1). The first membrane is a smooth outer membrane which is freely permeable to molecules. The second one is a smooth inner membrane which contains many transporters: integral membrane proteins that regulate the passage in and out of the chloroplast of small molecules like sugars proteins synthesized in the cytoplasm of the cell but used within the chloroplast. Besides the outer and the inner membranes the chloroplast has also a system of thylakoid membranes. A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria [62]. The thylakoid membranes enclose a lumen: a system of vesicles (that may all be interconnected).