Overview of Primary Processes of Photosynthesis

This introductory chapter describes the general principles of photosynthetic solar energy exploitation as the unique Gibbs energy source of living matter. The overall process can be subdivided into two sequences: (a) light-driven reactions, referred to as “Primary Processes of Photosynthesis”, which lead to the formation of “energy rich” bound hydrogen (TH₂) and ATP, and (b) fixation of primordial carbon, nitrogen and sulfur compounds (where CO₂ transformation into carbohydrates is by far the most abundant process) with TH₂ as reductant and ATP as driving force for the endergonic reactions.

This chapter focuses on the functional and structural organization of the “Primary Processes of Photosynthesis”. After a brief discussion of the underlying principles of harvesting of light and its transformation into electrochemical Gibbs energy by radical pair formation in pigment–protein complexes, the two types of reaction centers (type I and type II) and the two different modes of photosynthesis in anoxygenic (non-oxygen evolving) and oxygenic (oxygen evolving) organisms are outlined. It is emphasized that anoxygenic (bacterial) photosynthesis is energetically driven by only one type of reaction center (RC) (either RC I or RC II) while in oxygenic photosynthesis both types of RCs act in series, leading to electron transport from water to NADP⁺.

Anisotropic incorporation of the functional complexes into membranes leads to vectorial electron transfer and spatial separation of oxidant- and reductant-induced reactions concomitant with the formation of a transmembrane electrochemical potential difference for protons, providing the driving force for ATP synthesis.

The general architecture of the photosynthetic apparatus and its structural organization within different membrane systems of anoxygenic and oxygenic organisms are described. Finally principles and strategies for construction of biometric systems are briefly discussed.