Light harvesting complex
A light harvesting complex ( English light-harvesting complex , LHC ) or antenna complex (less often light collecting trap ) is a collection of membrane proteins in the photosynthetic membranes of organisms that photosynthesis operate.
function
Their job is to absorb light and direct the energy to the reaction center , where the light reaction of photosynthesis takes place.
The absorption of light causes electrons in the pigments to be put into an excited state. The absorbed energy is conducted to the so-called reaction center via a large number of pigments. This energy transfer process is very fast ( picosecond range) and extremely efficient.
Charge separation takes place in the reaction center, which is the first step in photosynthesis. It enables the conversion of ADP and NADP + to ATP and NADPH + H + , which ultimately leads to the build-up of glucose molecules in the light-independent reaction of photosynthesis.
Structure of various light harvesting complexes
The light-harvesting complexes of higher plants and algae are located in the inner membrane of the chloroplasts ( thylakoid membrane ). Plants have a number of different light harvesting complexes, all of which are transmembrane proteins . They are assigned to the various plant photosystems. The major complex associated with photosystem I is called LHC1 complex ( light harvesting complex 1 ) and the main complex associated with photosystem II is LHC2. The latter is the most common light-harvesting complex in the world in terms of number and is one of the most common proteins due to the spread of plants and algae. In addition to the main complexes , there are various other light harvesting complexes ( minor complexes ) that are also arranged around the photosystems. These are usually given with CP and their weight in kilodaltons , e.g. B. CP29. The figure shows the structure of the LHC2 complex when looking at the thylakoid membrane. The pigments chlorophyll a (green), chlorophyll b (cyan) and various carotenoids (yellow) are non- covalently bound by a protein structure (gray-transparent). The LHC2 complex has a diameter of 7.3 nm in this view. It consists of three identical monomers, 3.0 nm × 5.0 nm in size, each containing 8 chlorophylls a , 6 chlorophylls b , 2 lutein , 1 neoxanthine , 1 xanthophyll cycle pigment (violaxanthin, antheraxanthin or zeaxanthin) and approx. 232 amino acids.
In purple bacteria , the light-harvesting complexes are built into the plasma membrane. Like plant light harvesting complexes, they are transmembrane proteins. Most species have two types of light harvesting complexes called LH1 and LH2. Some species also have an LH3, others only have LH1. All complexes have a ring-shaped structure made up of identical subunits. For example, the figure shows the LH2 complex of Rhodopseudomonas acidophila , which consists of nine subunits. Each subunit consists of two closely spaced bacteriochlorophylls (red) and a wide bacteriochlorophyll (orange, not in LH1) and a carotenoid (yellow) tilted by 90 °. The pigments are bound non-covalently by two short proteins (gray-transparent). LH1 has a very similar structure, but consists of more units, e.g. B. 16. This results in a larger ring in the middle of which the reaction center is located.
Quantum physical phenomena
By means of femtosecond spectroscopy, it was possible to demonstrate that stable quantum entanglement of photons takes place over the entire complex , which makes the efficient use of light energy without heat loss possible. One of the remarkable things about this is the temperature stability of the phenomenon.
literature
General:
- Uwe Gerken: Spectroscopic investigations on individual light-collecting complexes of the purple bacterium R. rubrum . Stuttgart 2003, doi : 10.18419 / opus-4696 (dissertation, University of Stuttgart).
- Wendel Wohlleben: Femtosecond spectroscopy of biological systems using coherent control . Munich 2003, DNB 970629508 , urn : nbn: de: bvb: 19-18441 (dissertation, University of Munich).
- René Schödel: On the kinetics of singlet and triplet excitations in the light-collecting complex of photosystem II of higher plants (LHCII) . Berlin 1999, DNB 958166161 , urn : nbn: de: kobv: 11-1009399 (dissertation, Humboldt University Berlin).
- Frank Klimmek: The light collecting complex LHCI-730 of the photosystem I of higher plants: Investigations into the molecular assembly of the light collecting proteins Lhca1 and Lhca4 from barley (Hordeum vulgare, L.) and tomato (Lycopersicon esculentum) . Bremen 2001, DNB 964328089 , urn : nbn: de: gbv: 46-diss000002625 (dissertation, University of Bremen).
Diffraction experiments on LHC 2 for structure resolution:
- Werner Kühlbrandt , Da Neng Wang: Three-dimensional structure of plant light-harvesting complex determined by electron crystallography. In: Nature . Vol. 350, No. 6314, 1991, pp. 130-134, doi: 10.1038 / 350130a0 .
- Werner Kühlbrandt, Da Neng Wang & Yoshinori Fujiyoshi: Atomic model of plant light-harvesting complex by electron crystallography. In: Nature. Vol. 367, No. 6464, 1994, pp. 614-621, doi: 10.1038 / 367614a0 .
- Zhenfeng Liu, Hanchi Yan, Kebin Wang, Tingyun Kuang, Jiping Zhang, Lulu Gui, Xiaomin An, Wenrui Chang: Crystal structure of spinach major light-harvesting complex at 2.72 Å resolution. In: Nature. Vol. 428, No. 6980, 2004, pp. 287-292, doi: 10.1038 / nature02373 .
Individual evidence
- ↑ Lynn Yarris: Untangling the Quantum Entanglement Behind Photosynthesis: Berkeley scientists shine new light on green plant secrets. Berkeley Lab , May 10, 2010, accessed February 25, 2017 .
- ↑ Mohan Sarovar, Akihito Ishizaki, Graham R. Fleming, K. Birgitta Whaley: Quantum entanglement in photosynthetic light harvesting complexes . In: Nature Physics . tape 6 , no. 6 , 2010, p. 462-467 , doi : 10.1038 / nphys1652 , arxiv : 0905.3787v2 .