Introduction

Abstract

Photosynthesis is (still) defined as the assimilation of C0₂ in the light to form carbohydrates and oxygen. However, as A. Pirson has pointed out already in his introduction to the volumes on photosynthesis of the first edition of the Encyclopedia, this definition may change, if the primary products of the light reactions (and photosynthesis proper) should prove to be used up to a large extent not only in carbohydrate biosynthesis, but also in other metabolic pathways. The shifting of emphasis in photosynthesis research to the characterization of these primary products of the light reactions, the mechanisms and kinetics of their formation and turnover, as anticipated in the first edition of the Encyclopedia, has indeed occurred. The theory of van Niel served as a guide for the very successful early photosynthetic research period. Even though it is no longer tenable that the water-splitting reaction is common to all photosynthetic organisms, plants and bacteria, his principle concept of the homology of the primary light reactions in bacterial and plant photosynthesis and of carbon metabolism in photo- and chemosynthesis has in fact been strengthened.