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D3. The Light Reactions of Photosynthesis - Biology

D3.  The Light Reactions of Photosynthesis - Biology


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Photoexcitation of the non-reaction center chlorophyll turns that molecule into a good reducing agent, which transfers its electron to the nearest excited state level of the reaction center chlorophyll. If you count both step together, the non-reaction center chlorophyll gets "photooxidized", in the process producing the "strong" oxidizing agent which is the positively charged chlorophyll derivative. The extra electron passed onto the second molecule will eventually be passed on to NADP+ to produce NADPH. Electrons from water are moved through PSII to a mobile, hydrophobic molecule, plastaquinone (PQ) to form its reduced form, PQH2. Another photosystem, PS1, is also found further "downstream" in the electron transport pathway. It takes electrons from another reduced mobile carrier of electrons, plastocyanin (PCred) to ferredoxin, which becomes a strong reducing agent. It ultimately passes its electrons along to NADP+ to form NADPH. Protons then can move down a concentration gradient through the C0C1ATPase to produce ATP required for reductive biosythesis of glucose.

Figure: THE LIGHT REACTIONS OF PHOTOSYNTHESIS

Figure: Detailed View of Light Reaction of Photosynthesis (reprinted with permission from Kanehisa Laboratories and the KEGG project: www.kegg.org )

Boxed number represent Enzyme Commission Number. Original KEGG Map with imbedded links.

Contributors

  • Prof. Henry Jakubowski (College of St. Benedict/St. John's University)

D1. Introduction

"Of all the biochemical inventions in the history of life, the machinery to oxidize water — photosystem II — using sunlight is surely one of the grandest." (Sessions, A. et al, 2009)

  • A strong oxidizing agent must be formed which can take water and oxidize it to dioxygen. We know that redox reactions occur in the direction of stronger to weaker oxidizing agent (just as acid base reactions are thermodynamically favored in the direction of strong to weak acid). Somehow we must generate a stronger oxidizing agent than dioxygen, which often has the most positive standard reduction potential in tables.
  • Plants must have high concentrations of a reducing agent for the reductive biosynthesis of glucose from CO2. The reducing agent used for most biosynthetic reactions in nature is NADPH, which differs from NADH only by the addition of a phosphate to the ribose ring. This phosphate differentiates the pool of nucleotides in the cells used for reductive biosynthesis (NADPH/NADP + ) from those used for oxidative catabolism (NADH/NAD + )
  • Finally, plants need an abundant source of ATP which will be required for reductive biosynthesis.

We will discuss only the light reaction of photosynthesis which produces these three types of molecules. The dark reaction , which as the name implies can occur in the dark, involves that actual fixation of carbon dioxide into carbohydrate using the ATP and NADPH produced in the light reaction.


D3. The Light Reactions of Photosynthesis

  • Contributed by Henry Jakubowski
  • Professor (Chemistry) at College of St. Benedict/St. John's University

Photoexcitation of the non-reaction center chlorophyll turns that molecule into a good reducing agent, which transfers its electron to the nearest excited state level of the reaction center chlorophyll. If you count both step together, the non-reaction center chlorophyll gets "photooxidized", in the process producing the "strong" oxidizing agent which is the positively charged chlorophyll derivative. The extra electron passed onto the second molecule will eventually be passed on to NADP + to produce NADPH. The light reaction of photosynthesis in green plants is shown below. In this process, in a scheme that is reminiscent of electron transport in mitochondria, water is oxidized by photosystem II. Electrons from water are moved through PSII to a mobile, hydrophobic molecule, plastaquinone (PQ) to form its reduced form, PQH 2 . PSII is a complicated structure with many polypeptide chains, lots of chlorophylls, and Mn, Ca, and Fe ions. A Mn cluster, called the oxygen evolving complex, OEC, is directly involved in the oxidation of wate. Two key homologous 32 KD protein subunits, D1 and D2, in PSII are transmembrane proteins and are at the heart of the PSII complex. Another photosystem, PS1, is also found further "downstream" in the electron transport pathway. It takes electrons from another reduced mobile carrier of electrons, plastocyanin (PC red ) to ferredoxin, which becomes a strong reducing agent. Ferrodoxin is a protein with an Fe-S cluster (Fe-S-Fe-S in a 4 membered ring, with 2 additions Cys residues coordinating each Fe). It ultimately passes its electrons along to NADP + to form NADPH. A summary of the light reaction in plants and standard reduction potentials of the participants, are shown below. Note that many of the complexes produce a transmembrane proton gradient. In contrast to mitochondria, the lumen (as compared to the mitochondrial matrix) becomes more acidic that the other stroma. Protons then can move down a concentration gradient through the C 0 C 1 ATPase to produce ATP required for reductive biosythesis of glucose.

Figure: THE LIGHT REACTIONS OF PHOTOSYNTHESIS

Figure: Detailed View of Light Reaction of Photosynthesis (reprinted with permission from Kanehisa Laboratories and the KEGG project: www.kegg.org )

Boxed number represent Enzyme Commission Number. Original KEGG Map with imbedded links.


Free Response

Describe the pathway of energy in light-dependent reactions.

The energy is present initially as light. A photon of light hits chlorophyll, causing an electron to be energized. The free electron travels through the electron transport chain, and the energy of the electron is used to pump hydrogen ions into the thylakoid space, transferring the energy into the electrochemical gradient. The energy of the electrochemical gradient is used to power ATP synthase, and the energy is transferred into a bond in the ATP molecule. In addition, energy from another photon can be used to create a high-energy bond in the molecule NADPH.


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