The Biology Z Scheme

The biology z scheme is the energy diagram that describes electron transfer in the “light reactions” of plant photosynthesis. It shows that the molecules at the top can easily transfer their electrons to those below them, which is an easy “downhill” reaction energy-wise.

H2- and O2-evolving photocatalyst powders with and without RGO were characterized in a media-free Z-schematic water splitting system. The system was active for both [Co(bpy)3]3+/2+ and Fe3+/2+ redox couples via interparticle electron transfer.

Light Reactions

The light reactions of photosynthesis transform solar energy into adenosine triphosphate (ATP). Two protein complexes, called PSI and PSII, use special pigment molecules to absorb and transfer electrons. When a molecule in the photosynthetic reaction center absorbs a photon, it is excited into an energy state called the charge separation state. This changes its redox potential, which is a measure of how easy it is for the molecule to pass an electron to another.

The biology z scheme is an energy diagram showing the sequence of electron transfer during the light reactions of photosynthesis. The arrows represent electron flow, and the vertical energy scale shows each molecular species’s reduction potential. The lower a molecule’s reduction potential, the easier it is for it to accept an electron from the higher molecular species below it. This is why the arrows in the diagram point downhill, energy-wise. The light reactions produce reducing power, ATP, and oxygen gas. The next step, the dark reactions, convert this reducing power into chemical energy to form sugar molecules.

Dark Reactions

The Z-scheme is an energy diagram for electron transfer during the light reactions of photosynthesis. It shows the reducing power of each of the carriers (reaction centers) on a vertical scale. The ones on the top have a more negative reduction potential and therefore can donate their electrons to those below them more easily.

When a photon is absorbed by PSI, it excites P680 to the more oxidized form, P680*. This forms an electron-carrier pair with cytochrome bf. Electrons can then pass from PSI to cytochrome bf in the ETC, or back to PSI in a cycle of electron flow.

The ETC is what produces ATP during photosynthesis. It is a non-cyclic version of the Light Reactions. This non-cyclic photophosphorylation was experimentally discovered by Robert Emerson and his coworkers in 1957. Robin Hill and Fay Bendall published the theoretical version of this scheme in 1960. This work inspired the development of ‘artificial photosynthesis’ which can be used to produce solar fuels such as hydrogen gas.

Electron Transfer

The electron transport chain (also known as the photosynthetic or respiratory redox loop) is a series of molecules that transfer electrons between different species to generate a transmembrane electrochemical gradient. During these reactions a number of oxidizing and reducing species are formed in succession.

The diagram below, called the Photosynthetic Z Scheme, illustrates the sequence of these steps in the light reactions of photosynthesis. Each step involves both an oxidation and a reduction, because electrons cannot be gained unless they are lost.

The arrows representing the absorption of energy by reaction center chlorophyll molecules at PS-I are drawn with their oxidized and reduced forms to demonstrate that the reduction is dependent on the oxidation. This “bucket brigade” mechanism is the key to establishing an continuous flow of electrons between water and NADP+ via PS-I and PS-II. This is the basis of cellular respiration and photosynthesis, and is one example of the many biological processes that are quantum mechanical in nature.


ATP (adenosine 5′-triphosphate) is the “energy currency” of all living cells. It acts like a tiny battery, storing energy when it is not needed and releasing it instantly when a cell needs it.

The biology z scheme shows the sequence of electron transfer steps that produce ATP in the “light reactions” of plant photosynthesis. Each molecule is marked on the energy scale according to its ability to transfer an electron to the one above it (i.e., to reduce it).

Molecules at the top of the scale easily pass an electron to molecules below them because they are at a lower energy level. The electrons then move down the chain and release energy each time a phosphate group is added to ADP. As the phosphate groups are attached to ADP they become ATP. The ATP molecule releases its energy to power all cellular processes, including muscle contraction, circulation of blood and locomotion. It is also used to make the multi-thousand types of proteins that are necessary for life.

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