3. Photosynthesis in oxygen-evolving organisms is said to involve two distinct photosystems. Describe the two photosystems and provide two lines of experimental evidence that led to their discovery.
Photosystem 1 (P700) and photosystem II(P680) , the two work in a series connected by an electron transport chain. Primary charge-seperation events in the PSI reaction center cause the reaction center chlorophyll a molecule to donate electrons to NADP+ via an electron transport chain, thus making NADPH recquired by the Calvin cycle. The oxidized chlorophyll a molecule in PSI reaction center is then re-reduced by electrons generated by events in PSII and transferred to PSI by an ETC. The energy generated by the ETC drives the transfer of protons from the stroma into the thylakoid lumen building up an electrochemical gradient. Protons move back down the gradient via the ATP synthase complex in the membrane è drives synthesis of the ATP recquired for Calvin cycle. (When photosystem II absorbs light, electrons in the reaction-center chlorophyll are excited to a higher energy level and are trapped by the primary electron acceptors. To replenish the deficit of electrons, electrons are extracted from water by a cluster of four Manganese ions in photosystem II and supplied to the chlorophyll via a redox-active tyrosine. Photoexcited electrons travel through the cytochrome b6f complex to photosystem I via an electron transport chain set in the thylakoid membrane. This energy fall is harnessed, (the whole process termed chemiosmosis), to transport hydrogen (H+) through the membrane, to the lumen, to provide a proton-motive force to generate ATP. The protons are transported by the plastoquinone. If electrons only pass through once, the process is termed noncyclic photophosphorylation. When the electron reaches photosystem I, it fills the electron deficit of the reaction-center chlorophyll of photosystem I. The deficit is due to photo-excitation of electrons that are again trapped in an electron acceptor molecule, this time that of photosystem I. ATP is generated when the ATP synthetase transports the protons present in the lumen to the stroma, through the membrane. The electrons may either continue to go through cyclic electron transport around PS I or pass, via ferredoxin, to the enzyme NADP+ reductase. Electrons and hydrogen ions are added to NADP+ to form NADPH. This reducing agent is transported to the Calvin cycle to react with glycerate 3-phosphate, along with ATP to form glyceraldehyde 3-phosphate, the basic building-block from which plants can make a variety of substances.)
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