Embed Size (px)
Citation preview
Metabolic Minimap
Photosynthesis in Chloroplast: Revised, Designed by Donald NicholsonCOMMENTARY
Received for publication, June 20, 2003
Peter Rich‡
From the Department of Biology, University College of London, London WCIE 6BT, United Kingdom
The process of light-driven photosynthesis that is cou-pled to carbon fixation is the key process that sustains thebalance of life on earth. It regenerates the oxygen that isconsumed in respiration and produces the carbohydratefoodstuffs that fuel the respiratory chains by “fixation” ofcarbon dioxide. The complex series of reactions that areinvolved have been elucidated in considerable detailthrough a vast range of biochemical, biophysical, andphysiological studies. Reactions are conventionally di-vided into the “light” reactions that produce NADPH andATP and the “dark” reactions that use these products todrive the fixation of carbon dioxide into sugars and othercarbohydrates. The overall reaction scheme is a corner-stone in courses of biochemistry and cell biology andillustrates an exquisite balance of diverse reactions toprovide a sustained and integrated biochemical process.
The acquisition of a clear overview of the process is aconsiderable challenge to students of the subject, andthe “Photosynthesis in Chloroplast” minimap of DonaldNicholson§ (Fig. 1) is an admirable attempt to provide sucha simplified and accessible account. He shows in sche-matic form how the thylakoid membrane-embedded pho-tosystems and cytochrome bf complex, together withplastocyanin and ferredoxin, provide a pathway of linearelectron flow from water to NADP�, resulting in oxygenand NADPH production, and shows how this is linked tothe movement of protons from the stroma to the lumen tocreate a pH gradient across the thylakoid membrane. He
illustrates what is often a difficult concept for students,namely the operation of a Q-cycle mechanism within thecytochrome bf complex that results in additional protontranslocations across the membrane, resulting in greaterefficiency of energy conservation. He goes on to show howthe protons flow back across the membrane through therotary machinery of the ATP synthase to drive the gener-ation of ATP from ADP and inorganic phosphate. Inte-grated in the diagram is an indication of the associatedpathway of cyclic electron transfer around photosystem Ithat results, in addition, proton transfers that are not linkedto NADPH production reduction or oxygen evolution, andwhich can provide additional ATP when required to main-tain the ratio of ATP/NADPH that is needed to provide thecorrect balance for sustained operation of the “dark”reactions.
These “dark” reactions are usefully summarized in theform of the classical Calvin cycle, and the diagram illus-trates how CO2 is “fixed” by combination with the 5-car-bon ribulose-1,5-bis-phosphate to produce two 3-carboncompounds followed by the regeneration of the 5-carboncompound with the net production of glucose. This followsstandard accounts that can be found in most textbooks,but the pathway is summarized usefully here together withthe enzymes that catalyze the component reactions, andwith an attempt to summarize the stoichiometries ofNADPH and ATP used in relation to carbon productsformed.
Overall, therefore, this minimap provides a succinct andsimplified overview of the process of photosynthetic car-bon fixation that should be useful to new students of thisformidable subject.
‡ To whom correspondence should be addressed. E-mail:[email protected].
§ School of Biochemistry and Molecular Biology, University ofLeeds, Leeds LS2 9J2, United Kingdom. E-mail: [email protected].
© 2003 by The International Union of Biochemistry and Molecular Biology BIOCHEMISTRY AND MOLECULAR BIOLOGY EDUCATIONPrinted in U.S.A. Vol. 31, No. 5, pp. 281–282, 2003
This paper is available on line at http://www.bambed.org 281
FIG. 1. Photosynthesis in chloroplast.
282 BAMBED, Vol. 31, No. 5, pp. 281–282, 2003
