Photosynthesis is one of the most fascinating biological processes: it is of fundamental ecological and evolutionary importance and involves some of the most complex biochemical systems. In Molecular Mechanisms Blankenship describes not just the photosynthetic pathways but the detailed structure of the molecules involved and the kinetics and energetics of their interactions. He also relates the mechanisms of photosynthesis to its origin, evolution, and broader significance.

A basic description of photosynthesis is followed by a quick survey of photosynthetic organisms and organelles and a very short history of the study of photosynthesis. Successive chapters then survey pigments, antenna complexes and energy transfer, reaction centres, electron transfer pathways, chemiosmotic coupling to ATP synthesis, carbon metabolism, and the regulation and assembly of these systems. A final chapter covers the origin and evolution of photosynthesis (and the multiple endosymbiotic events involved) and of the metabolic pathways and reaction centres involved.

There's way too much in this to summarise, but here are a few facts about carbon metabolism that grabbed my attention. Rubisco is a key enzyme in the Calvin cycle, catalysing the step where carbon dioxide is fixed. Its low specificity and efficiency mean lots of it is required: "as much as 50% of the soluble protein in a leaf is rubisco" and it is "very likely the most abundant protein on Earth". Photorespiration is an unwanted alternative reaction in which rubisco reacts with oxygen instead of carbon dioxide; recovering from this imposes significant energetic costs. Various methods have evolved for avoiding photorespiration: C4 plants concentrate carbon dioxide in the vicinity of rubisco, while CAM (Crassulacean Acid Metabolism) plants collect carbon dioxide at night for use during the day (usually to reduce water loss) — the limited storage capacity explains why cactuses grow so slowly.

The more detailed explanations are based on the best studied organisms — purple bacteria and cyanobacteria feature prominently — but alternatives and variation across taxa are also covered. Many disciplines are involved in understanding photosynthesis, but Blankenship explains any specialised material as he goes — and a sixty page appendix covers some background physics and chemistry, looking at light, thermodynamics, kinematics, spectroscopy and quantum mechanics. Molecular Mechanisms of Photosynthesis should be accessible to anyone with basic undergraduate biology and chemistry — and there's plenty of motivation for the heavier-going material.

October 2004

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