Photosynthetic unit = the minimum unit for carrying out photosynthetic O₂ production.

Using single-turnover flashes of light, Emerson & Arnold (1930) found that it took the cooperative action of around 2400 chlorophyll (chl) molecules to produce one molecule of O₂. This cooperation of chl molecules led to the concept that light-driven O₂ production involved a photosynthetic antenna complex. According to the model the antenna complex serves to "harvest" light energy and then channel the excitation energy to the "reaction center". The transfer of energy between the chl molecules is by excitation energy transfer (not electron transfer). Once the energy reaches the reaction center, a special pair of chla molecules carries out photochemistry and an excited electron is transferred to an electron acceptor molecule. The antenna/reaction center concept is shown in the following schematic diagram.

Measurements of the quantum efficiency of O₂ production showed that it takes a minimum of 8 photons to produce one molecule of O₂.

Since O₂ comes from water, it takes 4 electrons to produce one O₂.

2 H₂O ---> O₂ + 4H⁺ + 4 e^-

Since each photon can excite one electron, two photochemical events are needed for each electron involved. These and other observations lead researchers to propose a two photosystem (reaction center) model to explain the quantum requirement.

According to the two reaction center model a "photosynthetic unit" contains two reaction centers capable of carrying out photochemistry and these two reaction centers are associated with antenna comlexes of about 600 chl molecules each. Much research has confirmed the validity of the two photosystem model and its structure and mechanism of action have been extensively studied. Various diagrams of the current model for the photosynthetic electron transfer system are shown in Figures 7.12 and 7.22 in the text book. The various components of the system are also explained in detail in chapter 7.

According to the chemiosmotic theory for biological energy transduction, a transmembrane electrochemical gradient consisting of pH and membrane potential can be used to make ATP. Experiments done with chloroplasts provided some of the most convincing data in support of the theory.

The figure below represents results obtained when the pH is measured in a solution containing thylakoid membranes. In the light, the pH of the solution increases as protons move into the thylakoid lumen as a result of vectorial electron and proton transfer. After the light is truned off, the pH returns to the initial value as the protons slowly return to the external medium. In an intact chloroplast, the protons would move between the stroma and lumen. Compounds known as uncouplers cause the pH gradient to collapse and block ATP synthesis. Also, if ADP and Pi are added after the light is turned off and before the pH gradient has collapsed, ATP can be made in darkness.

Taking the experiment to another level, it was shown that artificially imposing a sufficiently large pH gradient to thylakoid membranes in darkness is sufficient to make ATP.

Taken together, the experiments outlined here provided compelling evidence that the basic tenents of the Chemiosmotic theory are valid. In addition, these experiments provide an explanation for the action of uncouplers and the essential role of light-driven electron tranfer reaction. Click here to see how the chloroplast ATPase motor works.