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Photosynthesis

Introduction

Photosynthesis is a process performed by organisms to convert light energy into chemical energy. Around 40% of the plants dry mass as carbon is produced by photosynthesis.[1] The photosynthetic process is usually divided in two types: oxygenic photosynthesis and anoxygenic photosynthesis.[2] The anoxygenic photosynthesis is typically performed by bacteria without releasing oxygen but e.g. sulfur. Due to the complexity of the topic, this section will only give a short overview over the oxygenic photosynthesis.

Oxygenic photosynthesis is most common and performed by plants, algae and cyanobacteria. In the oxygenic photosynthesis, usually electrons are transferred by light energy from H2O to CO2, thus CO2 is reduced and H2O is oxidized by finally releasing O2 and producing carbohydrates.

Mechanism

It is possible to divide the (oxygenic) photosynthesis into three main processes:

  • absorption of photons by pigments,
  • transport of electrons from the splitting of water, production of nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP) (“light reactions”) and
  • the carbon-reduction cycle under consumption of ATP and NADPH (“dark reactions”). The absorption of photons by pigments, mainly by chlorophylls, is associated to two reaction centers, photosystem I (PS I) and photosystem II (PS II). The pigments are embedded in membrane structures and absorb a major part of the photosynthetically active radiation (400nm to 700nm).[1] This leads to an excitation of an electron.

The absorption of photons by pigments, mainly by chlorophylls, is associated to two reaction centers, photosystem I (PS I) and photosystem II (PS II). The pigments are embedded in membrane structures and absorb a major part of the photosynthetically active radiation (400nm to 700nm).[1] This leads to an excitation of an electron.

Light Reactions PS I is a chlorophyll dimer with an absorption maximum at 700 nm. The reaction center of PS II contains redox components including chlorophyll α with an absorption maximum of 680 nm.For the formation of one oxygen molecule out of two water molecules, 8 photons are needed – 4 photons have to be received by PS I and 4 photons by PS II.[3] PS II is responsible for water splitting. With 4 photons a 4-electron-oxidation of plastoquinone (PQ) to plastohydroquinone (PQH2) is initiated, that leads to of water and the production of elementary oxygen:

                    PS II :       hν + 2H2O + 2PQ  → O2 + 2PQH2                                                                               (1)     

The Cytb6I f-complex is used for the transport of the electrons to the reaction center of PS I. PQH2 and plastocyanin (PC) works as mobile electron transport molecule.

              Cytb6I f :          PQH2 + 2PCox  → PQ + 2PCred +2 H+                                                                       (2)     

The PS I reaction system is responsible for the regeneration of NADPH in a 2-electron reduction step:

                    PS I :           hν + H+ + 2PQred +NADP+ → 2PCox + NADPH                                       (3)     

At the same time, protons are transported to the cell membrane with embedded thylacoides, where ATP is regenerated:

                                          ADP3+ HPO42+ H+ → ATP4+ H2O                                                  (4)     

Dark Reactions The enzymatic reduction of CO2 in the so called Calvin-cyclus does not need further photons. The reaction can be described by the following equation:

                   2NADPH+3ADP4+ 3H2O+CO→ 2NADP++ ATP3+ 3HPO42+H+ + [CH2O] + H2O      (5)     

In sum, the dark and the light reactions can be expressed simplified as follows:

 

                                             hν + 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O

    (6) 

Figure 1 illustrates the interaction between the different reaction steps in a simplified scheme.

PhotoschemeFigure 1: Simplified Scheme of the interactions of the different reaction centers. The reactions at the different sites are listed in equation 1 - 5.

 

  1. LAMBERS, H.; RAVEN, J.; SHAVER, G.; SMITH, S.: Plant nutrient-acquisition strategies change with soil age, Trends in Ecology & Evolution, 2008-February, 23 (2), 95– 103, DOI: 1016/j.tree.2007.10.008.
  2. BRYANT, D. A.; FRIGAARD, N.-U.: Prokaryotic photosynthesis and phototrophy illuminated, Trends in Microbiology, 2006, 14 (11), 488–496, DOI: 1016/j.tim.2006. 09.001.
  3. WÖHRLE, D.; TAUSCH, M. W.; STOHRER, W.-D.: Photochemie, 1998, Wiley VCH Verlag GmbH, ISBN 978–3–527–29545–6.

 

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