Photosystem

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A photosystem (also a photosystem ) is a collection of proteins and pigment molecules ( chlorophylls and carotenoids ) in the thylakoid membrane of cyanobacteria and chloroplasts , which convert light energy into chemical energy during the light reaction of oxygenic photosynthesis . They occur in phototrophic cyanobacteria and eukaryotic organisms ( plants and protists ).

Layout and function

A photosystem consists of a so-called antenna complex and a reaction center .

Depending on the type of photosystem, the antenna complex (also known as the light collecting complex ) consists of around 30 proteins that are connected to pigment molecules. They are lifted into an energetic, excited state by the light. This energy can be passed on to the reaction center through exciton transfer. The efficiency of the energy transfer in the light harvesting complex to a reaction center is more than 90% and takes place in 10 −13 seconds.

The reaction center of the photosystems contains two chlorophylls that act as the primary electron donor. The light energy sets an electron transport chain in motion.

In photosystem II, electrons are transferred from the water to a quinone by means of 4 light quanta in one cycle and at the same time protons are released from the water splitting. This creates oxygen as a by-product. The water-splitting complex contains a cluster of four manganese atoms, although the exact structure of this unit has not yet been clarified spectroscopically, since common X-ray structure analyzes reduce the manganese atoms and the spectrum obtained does not correspond to the native structure of the catalytic center. It is assumed that three manganese atoms are bridged by oxygen atoms and that one manganese atom “hangs” a little further away like an appendage.

In photosystem I, the light-driven electron transfer leads to the synthesis of NADPH + H + .

Types

  • The photosystem I contains approximately 200 molecules of chlorophyll a and b and 50 carotenes. The reaction center of photosystem I has an absorption maximum at a wavelength of 700 nm, which is why it is also referred to as “P700”.
  • The photosystem II contains 250 molecules of chlorophyll a and b as well as about 110 carotenoids. The reaction center of photosystem II has an absorption maximum at 680 nm (“P680”).

Anaerobic sulfur bacteria have a photosystem that is similar to PSI.

Stimulation by light

Simplified term scheme ( Jablonski diagram ) of chlorophyll a. The electron levels (terms) are composed of several vibration terms (spacing approx. 0.1 eV ), which in turn consist of rotation terms  at a spacing of 0.02 to 0.001 eV.

Chlorophylls act as light-absorbing components of the photosystems. Chlorophylls consist of a porphyrin ring, which complexes a magnesium ion (Mg 2+ ).

The system of delocalized π-electrons of chlorophyll is the place of light absorption: By supplying light energy, an electron can be raised from the ground state S 0 to higher energy levels. This energetic state of chlorophyll is called the excited state . However, only two wavelengths are suitable for excitation: lower-energy red light (with chlorophyll a a wavelength of 662 nm) raises the electron to a higher level (1st singlet, S 1 ), and higher-energy blue light (430 nm) to an even higher level Level (2nd singlet, S 2 ).

Transitions Half-life τ½ in seconds Form of the delivered energy proportion of Symbol in the picture
S 2 → S 1 10 −12 warmth   yellow arrow
S 1 → S 0 10 −9 Emission of light (fluorescence) 8th % F.
Emission of an electron (photochemical redox reaction)   R.
radiationless energy transfer to neighboring molecules   E.
T 1 → S 0 10 −2 Phosphorescence at 750 nm   P

Since the triplet state is very stable due to the long half-life of the transition from the triplet to the ground state, the slow photochemical processes in the isolated chlorophyll are started from this state, but not in the intact thylakoid membrane. There, based on the S 1 state, the energy of almost every light quantum is used for the light reaction. This means that an amount of energy of 174 kJ / mol is available from each absorbed quantum, regardless of whether from the blue or red area. However, the worse the light reactions, the higher the proportion of fluorescence and thus the loss of usable energy.

See also

literature

  • Gerhard Trageser: Nobel Prize for Chemistry: Light in the light reaction . In: Spectrum of Science. Born in 1988, No. 12, p. 14 ff.
  • Donat-Peter Häder (Ed.): Photosynthesis . Georg Thieme Verlag, Stuttgart, New York 1999, ISBN 3-13-115021-1

Web links

Individual evidence

  1. ↑ Bas Gobets, Rienk van Grondelle: Energy transfer and trapping in photosystem I. In: Biochim. Biophys. Acta . 1507, 2001, pp. 80-99.