Throwing light on photosynthesis

Abstract

Molecular mechanisms of photosynthesisby Robert E. Blankenship, Blackwell Science, 2002. US$49.95/£29.95 (352 pages) ISBN 0 632 04321 0It has long been known that light can be converted into useful chemical energy. The importance of this photosynthetic process has been recognized in different ways throughout history, and was elegantly acknowledged by Julius Robert Mayer in 1845 when he wrote: ‘Nature set herself the task of capturing the light flooding towards the earth and storing this, the most elusive of all forces, by converting it into an immobile force…the plant world constitutes a reservoir in which the fleeting sun rays are fixed and ingeniously stored for future use, a providential measure to which the very existence of the human race is inescapably bound’.In his recently published book, Bob Blankenship replaces Mayer's flowery definition of photosynthesis with ‘a process in which light energy is captured and stored by an organism and the stored energy is used to drive cellular processes’. Behind this simple and rather bland modern definition of photosynthesis lies a vast base of knowledge, encompassing disciplines ranging from photophysics and photochemistry to molecular biology and physiology. The reactions of photosynthesis are initiated by the absorption of quanta by pigments, which occurs in the femtosecond time domain, followed by reactions that proceed in a time-scale stretching to days or even years. Therefore, to write a comprehensive text book on the molecular mechanisms of photosynthesis is a daunting task. Blankenship has taken up this challenge and, in my view, has produced a first class product. His approach is sensitive to the student reader and, with this in mind, he has included a 47-page appendix in which the appropriate basic principles of photophysics are well described. The remaining 257 pages comprise 11 chapters, in which Blakenship leads the reader through a logical progression of photosynthesis. The first three chapters set the scene, giving basic concepts in terms of reactions, where they take place and how they were discovered. This foundation is then built upon with a thorough coverage of structural and spectral properties of the photosynthetic pigments, and of the means by which they are associated with proteins. Over the past few years, X-ray crystallography has provided spectacular insights into how these pigment proteins are arranged, and has also given a structural basis to experimental and theoretical considerations of the energy transfer mechanisms. All this is dealt with rigorously by Blankenship. His account of reaction centre complexes in Chapter 6 deserves equal praise. He could not be more up-to-date in his content, providing the reader, for example, with a discussion of the latest X-ray structures of photosystems (PS) I and II. Chapter 7 is a bit of a hodgepodge. Quite rightly, Blankenship moves on to other components of the photosynthetic electron transport chain. He gives details of the similarities of the bacterial cytochrome bc and chloroplast cytochrome b6f complexes, and discusses the role of diffusable redox cofactors such as plastoquinone, plastocyanin, ferredoxin and NADPH. However, the discussion of turnover of the D1 protein of PSII and chlorophyll fluorescence as a probe of PSII function seems, to me, to be misplaced in this chapter.The conversion of ADP to ATP, the role of proton gradients, and Mitchell's chemiosmotic hypothesis are detailed in a useful and readable way in Chapter 8. In addition, this chapter focuses on speculations about the rotatory ATP synthase mechanism by drawing on the most recent structural information available.It has been traditional to consider the processes of photosynthesis as consisting of two stages: light and dark reactions. Blankenship has continued this tradition and uses Chapter 9 to discuss biochemical details of the dark reactions; namely carbon fixation via C3, C4 and CAM metabolism. He also includes the CO2 concentrating mechanism found in aquatic photosynthetic systems, and the biochemical reactions involved in long-term storage of photosynthate as sucrose and starch.The photosynthetic apparatus, whether it be of anaerobic bacteria or aerobic cyanobacteria, algae and higher plants, is complicated, being composed of many hundreds of different proteins, enzymes and cofactors. How these complex systems assemble themselves and how they evolved are the subjects of the last two chapters, respectively. Blankenship has a keen interest in the evolution of photosynthesis and his chapter on this subject makes good reading. Although there are major gaps in our knowledge, recent advances in gene sequencing and structural analyses of photosynthetic proteins are providing fodder for fascinating and realistic hypotheses; these provide a grand finale for this excellent text book. I will have no hesitation in recommending it to my students and colleagues alike.