Photosynthesis: Light Reactions

Abstract

In plants and cyanobacteria the ‘light reactions’ of photosynthesis use light energy to generate reducing power in the form of nicotinamide–adenine dinucleotide phosphate (NADPH) and metabolic energy as adenosine triphosphate (ATP), which are subsequently used in the ’dark reactions’ to drive the reduction of carbon dioxide and synthesis of carbohydrate. The light reactions, as conventionally defined, are actually a complex series of processes which include not only reactions in which light participates directly, but also closely associated reactions that indirectly depend on light.

Keywords: light reaction; thylakoid membrane; photosystems; photosynthetic electron transfer; photophosphorylation

Figure 1.

The ‘Z scheme’. This shows how the two primary photoacts of oxygenic photosynthesis are linked by thermochemical electron transfer through the cytochrome b6f complex coupled to ATP synthesis. The energetics of electron transfer are illustrated by reference to the scale of midpoint redox potentials (Em) drawn underneath the ‘Z’. P680, the special pair of chlorophyll a molecules acting as the primary electron donor of photosystem II; hv, a quantum of light absorbed by antenna chlorophylls; QA/B, the primary and secondary plastoquinone acceptor molecules associated with photosystem II; PQ, the pool of plastoquinone molecules in the thylakoid membrane; cyt, cytochrome; FeS, the ‘Rieske’ iron–sulfur protein; PC, plastocyanin; P700, the primary donor chlorophylls a of photosystem I; FB, one of two iron–sulfur centres associated with subunit PsaC of photosystem I; Fd, ferredoxin.

Figure 2.

The vectorial ‘Z scheme’. This represents the Z scheme in terms of the arrangement of the components within the thylakoid membrane according to the chemiosmotic theory, and illustrates the accumulation of H+ in the thylakoid lumen (‘in’) coupled to electron transfer. hem bL, hem bH, low‐ and high‐potential haems of cytochrome b6; other abbreviations as in Figure . The stoichiometry of H+ translocation is not fully represented.

Figure 3.

Diagrammatic arrangement of protein complexes and diffusible components in relation to a thylakoid. PSII, photosystem II; D1/D2, the two main subunits of the reaction centre of photosystem II; O, subunit PsbO, which protects the manganese cluster of the OEC; b, f, cytochromes b6 and f, respectively, of the cytochrome b6f complex; IV, subunit IV of the cytochrome b6f complex; PC, plastocyanin; c6, cytochrome c6; PSI, photosystem I; A/B, the two main subunits of photosystem I (PsaA, PsaB); C, D, E, F, subunits of photosysem I; Fd, ferredoxin; FNR, ferredoxin–NADP oxidoreductase; F0, F1, intrinsic and extrinsic components of ATP synthase; III, multiple copies of subunit III involved in proton translocation; β, γ, subunits of F1.

Figure 4.

Cofactors of the reaction centres of photosystems II and I, drawn from the X‐ray crystal structures. The plane of the thylakoid membrane is horizontal and at right angles to the plane of view. Light‐induced electron transfer takes place from bottom to top of the figure. (a) Photosystem II from Thermosynechococcus elongatus, pdb entry 1S5L. Oxygen evolving centre (OEC); TyrZ, side‐chain of Tyr‐161 of D1 which acts as an intermediate in electron transfer between OEC and P680; TyrD, side‐chain of Tyr‐160 of D2, which can donate electrons to P680, but is inactive in water oxidation; P680, the special pair of chlorophyll a molecules acting as the primary electron donor of photosystem II; ChlD1, ChlD2 chlorophyll a molecules bound to D1 and D2, respectively, only the former normally being active in electron transfer; PheoD1, PheoD2, phaeophytin molecules associated with D1 and D2, respectively; QA, tightly bound molecule of plastoquinone acting as primary acceptor; Fe, ferrous ion important for electron transfer from QA to QB; QB, loosely bound molecule of plastoquinone, the secondary acceptor. (b) Photosystem I from Synechococcus elongatus, pdb entry 1JB0; P700(eC1), pair of chlorophyll a molecules associated with PsaA and PsaB respectively; QK, a pair of vitamin K (naphthoquinone) molecules; Fx, iron–sulfur centre bound to interface between PsaA and PsaB; FA, FB, iron–sulfur centres bound to the subunit PsaC.

Figure 5.

Cofactors of the cytochrome b6f complex of Chlamydomonas reinhardtii, drawn from the X‐ray crystal structure, pdb entry 1Q90. The orientation is as in Figure . Hem bL, Hem bH, low‐potential and high‐potential haems, respectively, of cytochrome b6; Hem f, a c‐type haem bound to cytochrome f; Fes, iron–sulfur centre of the Rieske iron–sulfur protein subunit; Hem ci, a c‐type haem associated with the cytochrome b polypeptide and of uncertain function; Chl, a molecule of chlorophyll a associated with the cytochrome b polypeptide and of unknown function.

Figure 6.

Model of thylakoid membrane structure in higher‐plant chloroplasts, illustrating the distribution of the major protein complexes between grana and stroma lamellae, and between appressed membranes, grana margins and end membranes within the grana stacks.

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Further Reading

Albertsson PA (2001) A quantitative model of the domain structure of the photosynthetic membrane. Trends in Plant Science 6: 349–354.

Allen JF (2003) Cyclic, pseudocyclic and noncyclic photophosphorylation: new links in the chain. Trends in Plant Science 8: 15–19.

Allen JF (2004) Cytochrome b6f: structure for signalling and vectorial metabolism. Trends in Plant Science 9: 130–137.

Allen JF and Forsberg J (2001) Molecular recognition in thylakoid structure and function. Trends in Plant Science 6: 317–326.

Blankenship RE (2002) Molecular Mechanisms of Photosynthesis. Oxford: Blackwell Science.

Ferreira KN, Iverson TM, Maghlaoui K, Barber J and Iwata S (2004) Architecture of the photosynthetic oxygen‐evolving center. Science 303: 1831–1838.

Joliot P and Joliot A (2002) Cyclic electron transfer in plant leaf. Proceedings of the National Academy of Sciences of the USA 99: 10209–10214.

Leslie AGW and Walker JE (2000) Structural model of F1‐ATPase and the implications for rotary catalysis. Philosophical Transactions of the Royal Society of London B. 355: 465–472.

Mullineaux CW (1999) The thylakoid membranes of cyanobacteria: structure, dynamics and function. Australian Journal of Plant Physiology. 26: 671–677.

Stock D, Leslie AGW and Walker JE (1999) Molecular architecture of the rotary motor in ATP synthase. Science. 286: 1700–1705.

Walker DA (2002) The Z‐scheme – down hill all the way. Trends in Plant Science 7: 183–185.

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Bendall, Derek S(Jan 2006) Photosynthesis: Light Reactions. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001311]