Electron Flow

Chemical reactions involve atoms and molecules, but chemistry at its heart is really the study of the electron flow between those atoms and molecules when these react.

Similarly, biochemistry at its heart is the study of the electron flow within living organisms. A common term in biochemistry for this electron flow is the electron transport chain, of which there are two types: cyclical and non-cyclical.

Electron Flow Requires Energy

Electron transport chains need energy to begin. As energy can neither be created nor destroyed, it must be input from somewhere. In photosynthesis, this energy comes from photons in the visible light spectrum.

Electron Flow in Photosystem II (PSII)

Energy Input

Oxygenic photosynthesis begins in photosystem II (PSII), a protein complex embedded in, and spanning, the thylakoid membranes of chloroplasts and cyanobacteria. Here, photon energy is transferred to the two chlorophyll-protein complexes in PSII collectively known as P680.

The following diagram has been shown previously here and here. It is shown again below (Fig. 1) to make the rest of this chapter easier to follow. Please note that the focus here is only on PSII and the electron flow to cytochrome b₆f — everything from cytochrome b₆f through to ATP synthase will be covered in other chapters. Electron flow in the diagram below is shown by the blue-dotted lines and ‘e^-’ cyan-coloured circles.

One electron in P680 absorbs one photon’s energy (or the accumulated energy of the light-harvesting/antenna complex, enters a highly excited state, and leaves the molecule. P680, in losing this electron, has become oxidised. With the loss of an electron its charge becomes +1 and the P680 molecule is now P680⁺. This molecule is also now a radical, or a highly reactive compound very much receptive to a replacement electron. (P680⁺ is sometimes written as P680^•+ to make this radical state clearer.)

Radicals can be very important in driving electron flow, as we shall see.

Electron Flow

It is the photon energy entering photosystem II which initiates electron flow through the whole of photosynthesis. All electron flows in PSII are non-cyclical, in that electrons are not recycled and reused over and over, but must be continually replaced as they cascade through the transport chain. In the case of photosynthesis, every electron used in that system comes from the continuous splitting of water.

Manganese is crucial in this water-splitting reaction, but it is ‘merely’ a ‘helper’ in this reaction. Splitting water is energy-intensive, and manganese’s role here is to assist in lowering the energy required for the water-splitting to proceed. In other words, manganese doesn’t provide the energy, it helps lower the energy input required. It is not a catalyst (biological catalysts are special proteins called enzymes), but a helper, or cofactor (a non-protein compound or metallic ion required for a catalyst to function).

In PSII, four electrons are released in the oxygen-evolving complex when two water molecules are split into four hydrogen ions (4 H⁺, also called protons) and two oxygen atoms (2 O, which combine to form oxygen gas, O₂):
2H₂O → 4H⁺ + O₂ + 4e^-

This reaction involves manganese (Mn) in a still not-quite-understood way, but what is clear from this is that manganese is a vital nutrient for plants, for without manganese available to remove electrons from water, oxygenic photosynthesis (by far the most important type today) could never proceed whether photons are present or not.

[The oxygen gas from this reaction leaves as a waste product, but the hydrogen ions (protons) remain for an important role we’ll cover later.]

The Primary Electron Donor and Acceptor in Electron Flow

Driven by energy entering the system via photons, P680 is the first molecule to become excited (sometimes written as P680^* to denote this) and donate a high-energy electron in photosynthesis. It is the primary electron donor.

The first molecule accepting that highly-excited electron is pheophytin, also called the primary electron acceptor. It becomes reduced (receives electrons) and negatively-charged, as P680 becomes oxidised (loses electrons) and positively-charged.

(More on reduction-oxidation reactions, which always occur together, is here.)

P680⁺

P680 donates electrons, but P680⁺ is the strongest biological oxidising agent, or electron acceptor known, and it is this property that makes the oxidation (splitting of) water possible at all in any living organism.

Once it has been energised by photons to donate an electron to pheophytin, P680⁺ becomes a highly-reactive radical wanting that electron back. P680⁺ pulls that electron from a tyrosine residue in the surrounding D1 protein.

(Tyrosine is an amino acid. All amino acids share a common structure but each has an additional part unique to them, and which identifies them. That unique, identifiable part is the residue.)

This is where radicals can be important in electron flow, as P680⁺ by being highly reactive, readily reduces (receives an electron) back to P680. As P680, this molecule is again available for further photon stimulation and oxidation to release a new electron to initiate a new electron flow.

The tyrosine residue meanwhile has become oxidised (loses an electron). It now seeks a new electron, which it obtains from manganese, via manganese’s splitting of water.

This electron flow continues from water to P680 for as long as P680 is stimulated by photons, and is why photosystem II participates in light-dependent reactions of photosynthesis. Photosystem II cannot operate in darkness, and nor can photosynthesis, which begins with PSII.

Pheophytin

Pheophytin is a chlorophyll molecule lacking the central magnesium ion (Mg²⁺) that all chlorophylls otherwise have.

As P680 becomes a positively-charged radical, phaeophytin becomes a negatively-charged radical, and again this is important. The pheophytin radical is likewise highly reactive, and readily passes the additional electron obtained from P680 onto yet another compound: plastoquinone.

Pheophytin’s role is as an intermediary electron carrier, passing the electron from P680 to plastoquinone.

[Fun aside: plastoquinone is very similar in structure to ubiquinone, more commonly known as coenzyme Q10. It is ubiquitous in bacteria and eukaryotes (plants, algae, fungi and animals) — hence its name — and in eukaryotes is found predominantly in the mitochondria. Considering plastoquinone is additionally found in chloroplasts and cyanobacteria, this is still more evidence for the microbial origins of eukaryotes and the endosymbiotic theory.]

Plastoquinone

Plastoquinone is yet another electron carrier (electron transport chains are called electron transport chains for good reason!) and vital in moving electrons from PSII to the next protein complex in the thylakoid membrane: cytochrome b₆f. This will be covered in another chapter.

To Summarise

Oxygenic photosynthesis begins in photosystem II (PSII), which is embedded in, and spans, the thylakoid membranes found only in cyanobacteria, algae, and plants. PSII is the first of four protein complexes in the thylakoid membrane involved in photosynthesis.

A photon inputs energy into the system by stimulating P680, which releases a highly energetic electron and oxidises to the highly reactive P680^•+ radical.

This electron moves to pheophytin, which reduces to a highly-reactive pheophytin radical, and then on to plastoquinone.

Plastoquinone acts as an electron carrier through the thylakoid membrane, and the electron enters the second protein complex, cytochrome b₆f. We’ll pick up on that electron’s further travels in other chapters.

Meanwhile, the highly reactive P680^•+ radical obtains an electron from a tyrosine residue in the surrounding D1 protein and reduces back to P680. The tyrosine residue then obtains an electron from manganese, which removed electrons from water when oxidising water to hydrogen ions and oxygen.

Electrons flow from water to P680 for as long as photons stimulate P680 to drive that electron flow.

Photosystem II in a Nutshell

For each photon input, an electron is released from P680, which then enters an elaborate electron transport chain and drives additional photosynthetic reactions in and on either side of the thylakoid membrane.

For each photon input, an electron is drawn from water to replace that removed from P680.

Every electron used in photosynthetic reactions comes from the splitting of water in PSII.

Photosystem II operates for as long as there are photon and electron inputs into P680.

Photosynthesis as a whole operates for as long as there are photon and electron inputs into P680.