Photosynthesis: Light Reactions


Photosynthesis is the process by which light energy is captured to produce glucose from water and carbon dioxide. This process is divided into two parts: the light reactions where light is captured to generate reducing power in the form of nicotinamide–adenine dinucleotide phosphate (NADPH) and metabolic energy as adenosine triphosphate (ATP), and the dark reactions which subsequently use NADPH and ATP to synthesise carbohydrates. The light reactions take place inside the chloroplast on the thylakoid membranes and utilise four major membrane‐bound protein complexes: photosystems I and II (PSI and PSII), the cytochrome b6f complex and an ATPase. The complex series of processes include not only reactions in which light participates directly, but also closely associated reactions that indirectly depend on light.

Key Concepts

  • Photosynthesis is divided into two sets of reactions: the light reaction and the dark reaction (the Calvin cycle).
  • In the light reactions, light energy is captured and stored as reducing power in NADPH and metabolic energy in ATP.
  • The light reactions take place on an extensive system of membranes inside the chloroplast called the thylakoids.
  • Large protein–pigment complexes called light‐harvesting complexes (LHC) are arranged on the thylakoid membranes and act as antennas to capture photons of light.
  • Four major protein complexes embedded in the thylakoid membranes perform the light reactions: photosystems I and II (PSI and PSII), cytochrome b6f and ATP synthase.
  • The Z‐scheme describes how PSI and PSII act in series to remove an electron from water, shuttle the electron through the transport chain and ultimately use the electron to reduce NADP+ to NADPH.
  • As electrons move through the transport chain, protons are translocated to the thylakoid lumen, creating a proton motive force (pmf), which is subsequently used to power the ATP synthase.
  • The ATP and NADPH generated by the light reaction are consumed by the dark reaction to produce carbohydrates.

Keywords: light reaction; thylakoid membrane; photosystems; photosynthetic electron transfer; photophosphorylation; reactions centre; Z‐scheme

Figure 1. The ‘Z scheme’ shows how the two photosystems of oxygenic photosynthesis are linked by thermochemical electron transfer through the cytochrome b6f complex. The accumulation of H+ ‘In’ the lumen couples this process to ATP synthesis via the ATP synthase (not shown). 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; Light, a quantum of light absorbed by antenna chlorophylls; PQ/PQH2, the pool of plastoquinone molecules in the thylakoid membrane; cyt b6 and cyt f, cytochromes b6 and f, respectively; FeS, the ‘Rieske’ iron–sulfur protein; PC, plastocyanin; P700, the special pair of chlorophyll a molecules acting as the primary electron donor of photosystem I; Fd, ferredoxin; haemH/L, low‐ and high‐potential haems of cytochrome b6; NADPH, nicotinamide adenine dinucleotide phosphate; H2O, O2, H+ and e, water, oxygen, protons and electrons respectively. The stoichiometry of H+ translocation is not fully represented. This figure was created using Inkscape,
Figure 2. Diagrammatic representation of protein complexes and their cofactors as well as diffusible components that link them along the thylakoid membrane. Proteins are labelled with white text, and other components are labelled in black text. Black dashed lines represent the path of electrons and protons through the photosynthetic apparatus. All protein complexes span the thylakoid membrane, the stroma (outside) and lumen (inside) are labelled in grey text. (a) Photosystem II including: P680, the special pair of chlorophyll a molecules acting as primary electron donors; D1/D2, the two main subunits of the reaction centre of photosystem II; OEC, oxygen‐evolving complex/center; ChlD1, ChlD2, PheoD1 and PheoD2, paired chlorophyll and pheophytin cofactors of PSII reaction centre; QA, a plastoquinone tightly bound to subunit D1; QB, a mobile plastoquinone that temporarily binds subunit D2 and acts as the primary electron acceptor. PQH2, the fully reduced form of plastoquinone; CP43 and CP47, two antenna proteins containing chlorophyll a. (b) Photosystem I including: P700, the special pair of chlorophyll a molecules acting as primary electron donors; PsaA and PsaB, the two main subunits of the reaction centre of photosystem I; PsaC, minor protein subunit; A0A, A0B, PyQA and PyQB, paired chlorophyll and phylloquinone cofactors of PSI reaction centre; FeSX, iron–sulfur cluster bound to PsaA/B; FeSA and FeSB iron–sulfur clusters bound to PsaC; Fd, ferredoxin, the final electron acceptor, a soluble electron carrier that binds reversible to PsaC to received electrons from FB; PC, plastocyanin; FNR, ferredoxin–NADP+‐reductase. (c) The plastoquinone pool and cytochrome b6f including: PQ, plastoquinone; PSQ, partially reduced plastoquinone: PQH2, fully reduced plastoquinone; Cyt b and Cyt f, cytochromes b6 and f, respectively; Rieske, the Rieske iron–sulfur protein complex containing an iron–sulfur molecule labelled FeS; Haem bL, Haem bH, low‐ and high‐potential haems, respectively, of cytochrome b6; haem f, the haem molecule of cytochrome f; PC, plastocyanin. (d) ATP synthase including: CF0, CF1, intrinsic and extrinsic components of ATP synthase. CF0 consists of multiple c‐subunits, shown in blue; CF1 consists of multiple alpha and beta subunits and one gamma subunit and the stator which consists of subunits, a, b and b′. This figure was created using Inkscape,
Figure 3. 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 and Forsberg J (2001) Molecular recognition in thylakoid structure and function. Trends in Plant Science 6: 317–326.

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

Antal TK, Kovalenko IB, Rubin AB and Tyystjärvi E (2013) Photosynthesis‐related quantities for education and modeling. Photosynthesis Research 117: 1–30.

Blankenship RE (2014) Molecular mechanisms of Photosynthesis, 2nd edn. Oxford, UK: Wiley‐Blackwell.

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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.

Lincoln T, Zeiger E, Møller IM and Murphy A (2015) Plant Physiology and Development, 6th edn. Sunderland, MA: Sinauer Associates.

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

Wobb L, Bassi R and Kruse O (2015) Multi‐level light capture control in plants and green algae. Trends in Plant Science 21: 55–56.

Zhang R, Roose J and Williams ME (2015) Light‐Dependent Reactions of Photosynthesis. Teaching Tools in Plant Biology: Lecture Notes. The Plant Cell (online), doi: 10.1105/tpc.115.tt0515.

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St. Onge, Kate R(May 2018) Photosynthesis: Light Reactions. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001311.pub2]