The photosynthetic pigments are organized into two discrete light harvesting complexes (LHCs) within the photosystem I and II. a. What is the advantage of having more than one pigment in the LHC? b. What forms the reaction center in PS I and PS II, respectively? c. Why are the photosystem named I and II? d. Write down the value indicated by the LHCs of photosystems.
Answers
Answer:
Water oxidation in photosynthesis takes place in photosystem II (PSII). This photosystem is built around a reaction center (RC) where sunlight-induced charge separation occurs. This RC consists of various polypeptides that bind only a few chromophores or pigments, next to several other cofactors. It can handle far more photons than the ones absorbed by its own pigments and therefore, additional excitations are provided by the surrounding light-harvesting complexes or antennae. The RC is located in the PSII core that also contains the inner light-harvesting complexes CP43 and CP47, harboring 13 and 16 chlorophyll pigments, respectively. The core is surrounded by outer light-harvesting complexes (Lhcs), together forming the so-called supercomplexes, at least in plants. These PSII supercomplexes are complemented by some “extra” Lhcs, but their exact location in the thylakoid membrane is unknown. The whole system consists of many subunits and appears to be modular, i.e., both its composition and organization depend on environmental conditions, especially on the quality and intensity of the light. In this review, we will provide a short overview of the relation between the structure and organization of pigment-protein complexes in PSII, ranging from individual complexes to entire membranes and experimental and theoretical results on excitation energy transfer and charge separation. It will become clear that time-resolved fluorescence data can provide invaluable information about the organization and functioning of thylakoid membranes. At the end, an overview will be given of unanswered questions that should be addressed in the near future.
Keywords: Excitation energy transfer, Picosecond fluorescence, Thylakoid membrane, Charge separation, State transitions
Introduction
The photosynthetic light reactions of green plants, algae, and cyanobacteria take place in photosystems I and II (PSI and PSII). Light-induced charge separation in the reaction center (RC) of PSII leads to the oxidation of water, the reduction of plastoquinone and the formation of a proton gradient across the thylakoid membrane in which PSI and PSII are embedded, which is crucial for the production of ATP. PSII and PSI work in series and together they also drive NADP+ to NADPH reduction with H2O as electron donor (Nelson and Yocum 2006). Light-induced charge separation in the RC of PSII starts from the primary donor P680 and an electron proceeds via a pheophytin onto plastoquinone QA and subsequently to plastoquinone QB. The primary cation radical P680+. has an Em value of +1.25 V (Rappaport et al. 2009), far higher than the value of +0.80 for Chl in solution (Kobayashi et al. 2007) and this high value is ultimately responsible for the oxidation of water.
The RC of PSII itself only contains six chlorophylls a (Chls a) and two pheophytins but it is always present in the so-called core complex that also contains the pigment-proteins CP43 and CP47, providing additional 13 and 16 Chls a, respectively, together with several β-carotene molecules (see (Umena et al. 2011) for the most recent PSII core structure). Both antenna complexes feed excitation energy into the RC. These antenna Chls are on the one hand at a “safe” distance from the RC pigments, which are highly oxidizing after charge separation ( 1), preventing direct pigment oxidation in the antenna, and on the other hand close enough to perform efficient excitation energy transfer (EET).
Answer:
The photosynthetic pigment is organized into two discrete light harvesting complexes within the photosystem I and II. a) What is the advantages of having more than one pigment in the LHC? b) What forms the reaction centre in PSI and PSII c) Why are the photosystems named I and II d) Write down the value indicated by LHC of photosynthesis