chemistry

Photosystem II

Photosystem II is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Photosystem II rather than just read about it. In short: Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria.

Key takeaways

Photosystem II belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Photosystem II to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Photosystem II from memory before moving on to harder problems.

Reference excerpt

Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. By replenishing lost electrons with electrons from the splitting of water, photosystem II provides the electrons for all oxygenic photosynthesis to occur. The hydrogen ions (protons) generated by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP. The energized electrons transferred to plastoquinone are ultimately used to reduce NADP+ to NADPH or are used in non-cyclic electron flow. DCMU is a chemical often used in laboratory settings to inhibit photosynthesis. When present, DCMU inhibits electron flow from photosystem II to plastoquinone.

Structure of complex

The core of PSII consists of a pseudo-symmetric heterodimer of two homologous proteins D1 and D2. Unlike the reaction centers of all other photosystems in which the positive charge sitting on the chlorophyll dimer that undergoes the initial photoinduced charge separation is equally shared by the two monomers, in intact PSII the charge is mostly localized on one chlorophyll center (70−80%). Because of this, P680+ is highly oxidizing and can take part in the splitting of water. Photosystem II (of cyanobacteria and green plants) is composed of around 20 subunits (depending on the organism) as well as other accessory, light-harvesting proteins. Each photosystem II contains at least 99 cofactors: 35 chlorophyll a, 12 beta-carotene, two pheophytin, two plastoquinone, two heme, one bicarbonate, 20 lipids, the Mn4CaO5 cluster (including two chloride ions), one non heme Fe2+ and two putative Ca2+ ions per monomer. There are several crystal structures of photosystem II. The PDB accession codes for this protein are PDB: 3WU2, PDB: 3BZ1, PDB: 3BZ2 (3BZ1 and 3BZ2 are monomeric structures of the Photosystem II dimer), PDB: 2AXT, PDB: 1S5L, PDB: 1W5C, PDB: 1ILX, PDB: 1FE1, PDB: 1IZL.

Oxygen-evolving complex (OEC)

The oxygen-evolving complex is the site of water oxidation. It is a metallo-oxo cluster comprising four manganese ions (in oxidation states ranging from +3 to +4) and one divalent calcium ion. When it oxidizes water, producing oxygen gas and protons, it sequentially delivers the four electrons from water to a tyrosine (D1-Y161) sidechain and then to P680 itself. It is composed of three protein subunits, OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ); a fourth PsbR peptide is associated nearby. The first structural model of the oxygen-evolving complex was solved using X-ray crystallography from frozen protein crystals with a resolution of 3.8Å in 2001. Over the next years the resolution of the model was gradually increased to 2.9Å. While obtaining these structures was in itself a great feat, they did not show the oxygen-evolving complex in full detail. In 2011 the OEC of PSII was resolved to a level of 1.9Å revealing five oxygen atoms serving as oxo bridges linking the five metal atoms and four water molecules bound to the Mn4CaO5 cluster; more than 1,300 water molecules were found in each photosystem II monomer, some forming extensive hydrogen-bonding networks that may serve as channels for protons, water or oxygen molecules. At this stage, it is suggested that the structures obtained by X-ray crystallography are biased, since there is evidence that the manganese atoms are reduced by the high-intensity X-rays used, altering the observed OEC structure. This incentivized researchers to take their crystals to X-ray free electron laser facilities, such as SLAC. In 2014 the structure observed in 2011 was confirmed. Knowing the structure of Photosystem II did not suffice to reveal how it works exactly. So now the race has started to solve the structure of Photosystem II at different stages in the mechanistic cycle (discussed below). Currently structures of the S1 state and the S3 state's have been published almost simultaneously from two different groups, showing the addition of an oxygen molecule designated O6 between Mn1 and Mn4, suggesting that this may be the site on the oxygen evolving complex, where oxygen is produced.

Water splitting

Photosynthetic water splitting (or oxygen evolution) is one of the most important reactions on the planet, since it is the source of nearly all the atmosphere's oxygen. Moreover, artificial photosynthetic water-splitting may contribute to the effective use of sunlight as an alternative energy source. The mechanism of water oxidation is understood in substantial detail. The oxidation of water to molecular oxygen requires extraction of four electrons and four protons from two molecules of water. The experimental evidence that oxygen is released through cyclic reaction of oxygen evolving complex (OEC) within one PSII was provided by Pierre Joliot et al. They have shown that, if dark-adapted photosynthetic material (higher plants, algae, and cyanobacteria) is exposed to a series of single turnover flashes, oxygen evolution is detected with typical period-four damped oscillation with maxima on the third and the seventh flash and with minima on the first and the fifth flash (for review, see). Based on this experiment, Bessel Kok and co-workers introduced a cycle of five flash-induced transitions of the so-called S-states, describing the four redox states of OEC: When four oxidizing equivalents have been stored (at the S4-state), OEC returns to its basic S0-state. In the absence of light, the OEC will "relax" to the S1 state; the S1 state is often described as being "dark-stable". The S1 state is largely considered to consist of manganese ions with oxidation states of Mn3+, Mn3+, Mn4+, Mn4+. Finally, the intermediate S-states were proposed by Jablonsky and Lazar as a regulatory mechanism and link between S-states and tyrosine Z. In 2012, Renger expressed the idea of internal changes of water molecules into typical oxides in different S-states during water splitting.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Photosystem II

Start with the simplest possible case. Write down what Photosystem II claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Photosystem II before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Photosystem II ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Photosystem II

In research: Photosystem II appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Photosystem II in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Photosystem II is common in secondary-school and first-year university syllabi. It links to neighbouring topics EC 1.10.3, Light reactions, Manganese enzymes, so understanding it makes those chapters shorter.
In everyday life: Look for Photosystem II outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Photosystem II in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Photosystem II means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Photosystem II out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Photosystem II in simple terms?

Why does Photosystem II matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Photosystem II?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Photosystem II.