The Z-scheme owes its origin to several investigators. First it was Robert Emerson and his co-workers, including the authors (1956-1964) at the University of Illinois (at Urbana-Champaign) who discovered and embellished the “Enhancement effect” in oxygen evolution, which occurred when light absorbed in one photosystem (currently called PSI) was added to light absorbed in another photosystem (currently called PSII).

Historical Origin of the Z-Scheme

The Z-scheme owes its origin to several investigators. First it was Robert Emerson and his co-workers, including the authors (1956-1964) at the University of Illinois (at Urbana-Champaign) who discovered and embellished the “Enhancement effect” in oxygen evolution, which occurred when light absorbed in one photosystem (currently called PSI) was added to light absorbed in another photosystem (currently called PSII). Experiments using chloroplasts, and those using a mass spectrometer, absorption spectrometer, a fluorometer and electron spin resonance spectrometer were crucial to the establishment of the concepts. These suggested the existence of “two light reactions and two photosystems”. It was Bessel Kok and co-workers (1959-1960) at Baltimore, Maryland, and Lou NM Duysens, J. Amesz and co-workers (1959-1961) in Leiden, The Netherlands, who discovered the crucial antagonistic effect of light absorbed in PSI and PSII on the oxidation-reduction state of P700 and of cytochrome f (Cyt f, the electron carrier in the middle of the intersystem chain of intermediates), respectively. Duysens’s experiments established the “series” nature of the present scheme. Light captured by PSI, in the “Z”-scheme, leads to oxidation of Cyt f (i.e., takes an electron away from it and places it on, say, “NADP+), whereas when light is captured by PSII, oxidized Cyt f is reduced by an electron coming from PSII. The theoretical concepts of Robin Hill and Fay Bendall (1960) (who essentially gave us the Z-scheme), the earlier schemes in books by Eugene Rabinowitch (1945-1956), and the detailed work of Horst Witt and co-workers (1961) in Berlin, Germany, played important and crucial roles in the origin of the “Z-scheme”. The final evidence of its validity came from state-of-the-art detailed biophysics, biochemical, molecular biology, and genetic research in about 20 laboratories around the world. This is not to say that mutants and organisms may not be found in the future, that use alternate means to transfer electron from water to NADPH.

Z-scheme

Whenever molecules gain or lose electrons energy is involved. The Z-scheme is an energy diagram for electron transfer in the “light reactions” of plant photosynthesis. It applies equally well to photosynthesis by algae and cyanobacteria. The vertical energy scale shows each molecule’s ability to transfer an electron to (i.e., to reduce) the next one from left to right. The ones at the top transfer electrons easily to the ones below them as it is a “downhill” reaction, energy-wise. However, for electron transfer from those at the bottom to those above them it is an “uphill” reaction and requires input of outside energy. The Z scheme shows the pathway of electron transfer from water to NADP+. Using this pathway, plants transform light energy into “electrical” energy (electron flow) and hence into chemical energy as reduced NADPH and ATP. Later in the “dark reactions” of photosynthesis, that chemical energy is further transformed into the chemical bonds in sugar molecules. In the complete process, carbon dioxide is joined into the sugar molecules and oxygen is released. Although it is not a structural diagram, the Z-scheme does give the sequence of electron flow (oxidation and reduction) with an energy perspective. The source of electrons is water (H2O). The large vertical red arrows represent excitation of reaction center chlorophyll molecules (by light energy) and the black arrows represent electron flow, which is downhill energy-wise. 

Mn is the manganese center, a complex containing 4 manganese atoms, which splits two water molecules at a time into 4 protons (4H+), 4 electrons (4e-), and 2 oxygen atoms, as an oxygen molecule (O2). Tyr is a special tyrosine molecule, also sometimes referred to as Yz or simply as Z, which shuttles electrons to the “reaction center” of photosystem II (PSII). Chl P680 is the reaction center pair of chlorophyll a molecules of PSII. Excited Chl P680* has reached this state by absorbing a photon of light energy. Pheo is a pheophytin molecule, which is a chlorophyll with its central magnesium ion (Mg++) having been replaced by two hydrogens. It is the primary electron acceptor of PSII, whereas P680 is the primary electron donor. QA is a plastoquinone molecule, which is tightly-bound and immovable. It is known in some circles as the primary stable electron acceptor of PSII, and it accepts and transfers one electron at a time. QB is a loosely bound plastoquinone molecule, which accepts two electrons and then takes on two protons, before it detaches and becomes mobile and called PQ. PQ is the detached plastoquinone molecule, which is mobile within the hydrophobic core of the thylakoid membrane. FeS is the Rieske iron-sulfur protein. Cyt f is cytochrome f. Cyt b6L and Cyt b6H are two cytochrome b6 molecules (of lower and higher energy). PC is plastocyanin, a highly mobile copper protein. Chl P700 and Excited Chl P700* are respectively the ground energy state and the excited energy state of the chlorophyll molecule of the “reaction center” of photosystem I. AO is a special chlorophyll a molecule that is the primary electron acceptor of PSI, whereas P700 is the primary electron donor of PSI. A1 is a phylloquinone (vitamin K) molecule. FX, FA, and FB are three separate immobile iron-sulfur protein centers. FD is ferredoxin, a somewhat mobile iron-sulfur protein. FNR is the enzyme ferredoxin-NADP oxidoreductase, which contains the active group, called FAD (flavin adenine dinucleotide). NADP+ is the oxidized form of nicotinamide adenine dinucleotide phosphate. NADPH is its reduced form. .

In plants (as well as algae and cyanobacteria), photosynthesis has two major phases:

The Light Reactions comprise the light-dependent phase, which produces the reducing power (“reducing”, as in “oxidation and reduction”), ATP (adenosine triphosphate – the energy currency of life), and oxygen (O2). This all takes place in and around the thylakoid membranes.

The Dark Reactions are not directly dependent on the presence of light. Here the reducing power of NADPH and the energy of ATP (both generated by the light reactions) are used to convert, or “fix” CO2 into sugars. These reactions occur in the stroma matrix and are called the Calvin-Benson cycle or C3 cycle. They are not shown in the Z-scheme diagram. The pathway for ATP production is also not shown in the diagram.

The Z-scheme represents the steps in the light reactions, showing the pathway of electron transport from water to NADP+ (nicotinamide adenine dinucleotide phosphate). This leads to the release of oxygen, the “reduction” of NADP+ to NADPH (by adding two electrons and one proton), and the building-up of a high concentration of hydrogen ions inside the thylakoid lumen (needed for ATP production).

Why is it called Z-Scheme?

It is simply because the diagram, when first drawn, was in the form of the letter “Z”. 

Operation of the Z-Scheme

Excitation of Reaction Centers Photosynthesis starts with the simultaneous excitation of pairs of special reaction center chlorophyll(a) molecules, labeled as P680 (in photosystem II, or PSII) and P700 (in photosystem I, or PSI). [See the two red vertical arrows in the diagram.] The excitation energy comes either from directly absorbed light or (most often) by energy transfer from adjacent pigment molecules in protein complexes called antennas. These “antenna” pigment molecules (chlorophylls and carotenoids) absorb light energy and then transmit it by inductive resonance from one molecule to the next, finally to the reaction center. Excitation is over within a few femtoseconds (10-15 s).

The First Chemical Step Normally, one describes the Z-diagram from left to right as if water delivers electrons first. We shall, however, describe the steps as they appear in an actual approximate time sequence. The first chemical step happens within only a few picoseconds (10-12 s) when excited P680* loses an electron to Pheo, producing oxidized P680 (P680+) and reduced Pheo (Pheo-) in PSII, and excited P700* loses an electron to Ao, producing oxidized P700 (P700+) and reduced Ao (Ao-). This is the only step where light energy is converted to chemical energy, precisely oxidation-reduction energy. The rest of the steps are downhill energy-wise.

The Electron Transfer Steps A molecule with a plus (+) charge has one less electron than its counterpart and is said to be the oxidized species, as it has lost one electron. The species that has an added electron is called the reduced species. Reduction means the addition of electrons or of hydrogen atoms {One hydrogen atom is a combination of one proton (H+) and one electron (e-)}, and oxidation means removal of either H or e-. This oxidation and reduction process is what drives the activity in the Z-scheme sequence. Molecules higher on the diagram are able to reduce (pass an electron to) the next molecule lower down on the energy scale.

The recovery (reduction) of P680+ to P680 and of P700+ to P700 happens almost simultaneously. P700+ recovers to P700 by receiving an electron that was passed down from reduced Pheo to QA (which is bound to the reaction center protein complex), then to QB (another bound plastoquinone molecule). When QB has accepted two electrons from QA, it also takes on two protons from the stroma. Then it detaches from its protein binding site and diffuses through the hydrophobic core of the thylakoid membrane to the Cyt bf complex (see below), where the electrons are passed on to an iron-sulfur protein (FeS, the Rieske protein) and then to a mobile copper-protein (PC, or plastocyanin) which finally carries a single electron to the oxidized P700+. Thus the electron is passed in “bucket brigade” manner through the “intersystem chain of electron (or H-atom) carriers”.

There is a protein complex called the Cyt bf complex which contains FeS, Cytochrome f, and two cytochrome b6 molecules. The “bottleneck”, or the slowest step of the entire sequence, is the passage of the reduced PQ molecule to the Cyt bf complex and PQ’s oxidation by FeS. This takes several milliseconds (10-3 s). Cytochrome b6 is active in the Q-cycle.

In PSI the electron on AO- is passed ultimately to NADP+ via several other intermediates: A1, a phylloquinone (vitamin K); FX, FA, and FB which are immobile (bound) iron-sulfur proteins; ferredoxin, which is a somewhat mobile iron-sulfur protein molecule; and the enzyme ferredoxin-NADP reductase (FNR) which is actually an oxido-reductase and whose active group is FAD (flavin adenine dinucleotide).

The missing electron on P680+ is recovered, ultimately, from water molecules on the left bottom of the diagram via an amino acid tyrosine and a tetra-nuclear manganese (Mn) cluster. These reactions also require a few milliseconds.

A total of 8 quanta (photons) of light (4 in PSII and 4 in PSI), are required to transfer 4 electrons from 2 molecules of water to 2 molecules of NADP+. This produces 2 molecules of NADPH and 1 molecule of O2. This is the oxygen that both plants and animals need for respiration and life.

Energy Scale

An energy scale in terms of oxidation-reduction potential (Em) at pH 7. {At pH 7 the standard hydrogen electrode has an Em of –0.4 volts.} Intermediates that are higher up in the diagram have a lower (more negative) Em and can easily reduce any intermediate below them in the diagram by a downhill energy-wise process. This occurs in electron transfer:

from Pheo- to P700+ (see middle of diagram)

from AO- to NADP+ (see top right end of diagram)

from H2O to P680+ (see lower left of diagram)

Energy is needed to transfer electrons from P680 to Pheo and from P700 to AO, and this is where “light” energy is consumed.

Proton Gradient and ATP Synthesis

The light reactions provide not only the reducing power in NADPH but also the energy of ATP, both essential for producing sugars from CO2. ATP is produced through an enzyme called ATP synthase, using ADP (adenosine diphosphate), inorganic phosphate (Pi) and energy from a proton motive force (pmf) across the thylakoid membrane. This pmf is composed of two components:an electrical potential across the thylakoid membrane and a proton gradient across the thylakoid membrane. The proton gradient comes from the storing up of protons (hydrogen ions) inside the lumen, giving a pH of 6 inside the lumen and pH of 8 outside, in the stroma. Then, basically, protons escaping out from the thylakoid lumen through a central core of the enzyme ATP synthase (embedded in the membrane) cause conformational (rotational) changes in the enzyme, which catalyzes the phosphorylation of ADP and the release of ATP on the stromal side.

Protons (hydrogen ions) are concentrated into the lumen in several ways:Oxidation of water not only releases O2 and “sends” electrons to P680, but it also releases protons (H+) into the lumen. When QB is reduced in PSII, it not only receives two electrons from QA but it also picks up two protons from the stroma matrix and becomes PQH2. It is able to “carry” both electrons and protons (hydrogens). It is a H-atom carrier. At the Cyt bf complex it is then oxidized, but FeS and Cyt b6 can only accept electrons (not protons). So the two protons are released into the lumen.

The Q-cycle of the Cyt bf complex is great because it provides extra protons into the lumen. Here two electrons travel through the two hemes of cytochrome b6 and then reduce PQ on the stroma side of the membrane. The reduced PQ takes on two protons from the stroma, becoming PQH2, which migrates to the lumen side of the Cyt bf complex where it is again oxidized, releasing two more protons into the lumen. Thus the Q-cycle allows formation of more ATP..

When NADP+ is reduced by two electrons, it also picks up one proton, in effect removing it from the stroma and further increasing the gradient across the membrane.

Shamaila Qasim
M.Sc Botany