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Answers
Explanation:
Why is it called Z-Scheme?
It is simply because the diagram, when first drawn, was in the form of the letter "Z". Actually, you can see such a diagram in the older literature. (See e.g., Govindjee & R. Govindjee (1975) in: Bioenergetics of Photosynthesis, Academic Press, P. 27: and Demeter, S. and Govindjee (1989) Physiologia Plantarum, Volume 75, Pages 121-130. Currently, it is being drawn to emphasize the energy levels of the components. Thus, it has been turned 90 degrees counterclockwise.
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). [A second has as many femtoseconds in it as 31 million years has seconds.]
The First Chemical Step Normally, one describes the Z-diagram from left to right as if water delivers electrons first. This is not correct, but it is easy to describe the process that way.
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 (shown as a grey rectangle on this diagram) 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 (see below).
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 (short for 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 (a specific one in a protein of PSII, also referred to as Yz in the literature) 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.