Drilling Down

There are two types of photosynthesis: anoxygenic and oxygenic.

Anoxygenic photosynthesis was the first to appear on Earth around 3.5 billion years ago, was and is performed solely by several groups of bacteria to this day, and doesn’t produce oxygen. (Sulfur is typically produced instead.)

Oxygenic photosynthesis came later, and first appeared around 2 billion years ago. This type does produce oxygen, and was first performed by bacteria known as cyanobacteria, which are also with us to this day.

Oxygenic photosynthesis is by far the most dominant type today, and still performed by cyanobacteria (the only bacteria which can), along with the relatively ‘new’ algae and plants. Oxygenic photosynthesis is  completely responsible for the oxygenation of Earth’s modern atmosphere.

All oxygenic photosynthesis occurs in structures known as thylakoids, which are found only in cyanobacteria and the chloroplasts of algae and plants. (There is in fact much evidence that chloroplasts were once cyanobacteria.)

In the thylakoid membranes are four major protein complexes essential for oxygenic photosynthesis (Fig. 1) — photosystem II (PSII), cytochrome b₆f, photosystem I (PSI), and ATP synthase:

Quick Summary of PSII

(For more detail on PSII’s role in initiating photosynthesis, please go here, and here for more detail on the electron flow within it.)

Photosynthesis begins in PSII, but only in the presence of light, when photon energy activates a complex flow of electrons derived from the splitting of water.

Photon energy begins the process by stimulating an electron in the two chlorophyll-protein complexes in PSII collectively known as P680. That electron becomes highly excited, leaves P680, and is replaced with an electron derived from the splitting of water elsewhere in PSII.

That first highly-excited electron moves to a molecule called pheophytin, and then onto another called plastoquinone.

Plastoquinone’s role is to shunt electrons from the first protein complex, PSII, to the second, the cytochrome b₆f complex.

Cytochrome b₆f

Cytochrome b₆f is the second of four protein complexes needed for oxygenic photosynthesis, and the one to receive the electrons sent out of PSII via plastoquinone.

Knowing its other name — plastoquinol-plastocyanin reductase — immediately reveals its role! Whenever you see the ending ‘-ase’ in any of the life sciences, think ‘enzyme’. And whenever you see ‘enzyme’, think also ‘a protein which acts as a catalyst’. And a reductase is a specific enzyme which catalyses a reduction reaction.

Here, cytochrome b₆f is catalysing the reduction of plastoquinol to plastocyanin, but let’s back-track a bit to work out what all this means.

Back in PSII, plastoquinone (PQ in Fig. 1 above) is really two molecules: plastoquinone Q_A and plastoquinone Q_B. Q_A transfers two electrons from pheophytin to Q_B. These two electrons enables plastoquinone Q_B to acquire two hydrogen ions (H⁺, also called a proton) from outside the thylakoid membrane (the stroma). Q_B leaves PSII and enters the thylakoid membrane. In acquiring two electrons, plastoquinone enters a reduced state and is called plastoquinol (PQH₂ in Fig. 1 above).

Cytochrome b₆f — also in the thylakoid membrane — now catalyses the transfer of those electrons to yet another molecule, plastocyanin (PC in Fig. 1 above). (Plastocyanin is a copper-containing protein, and one reason why copper is an essential plant nutrient.)

Plastocyanin, in receiving electrons, is now reduced in turn. And this is why cytochrome b₆f is also called plastoquinol-plastocyanin reductase, as it catalyses the reduction of plastoquinol to plastocyanin. (Enzyme names can be unambiguous and highly descriptive this way!)

In catalysing the movement of two electrons from plastoquinol to plastocyanin, cytochrome b₆f additionally pumps the two protons (H⁺ ions) brought in by plastoquinol from the stroma outside the thylakoid into the lumen inside the thylakoid. These protons have a further role which will be covered in a later chapter.

Further Electron Flow

Biochemistry is the flow of electrons, and biochemical pathways involve complex electron transport chains. Photosynthesis is no exception, and those electrons don’t end with plastocyanin!

Plastocyanin’s role is to move electrons to the third major protein complex in photosynthesis: photosystem I (PSI), and this is the topic of the next chapter.