Measurement of Quantum Yield, Quantum Requirement, and Energetic Efficiency of the O2-Evolving System of Photosynthesis by a Simple Dye Reaction

Photosynthesis is the conversion of absorbed radiant energy from sunlight into various forms of chemical energy by the chloroplasts of higher green plants. The overall process of photosynthesis consists of the oxidation of water (with the release of O₂ as a product) and the reduction of CO₂ to form carbohydrates. In the test tube electrons produced by the photolytic cleavage of H₂) may be deviated from their true acceptor by inserting a suitable dye in the electron chain; i.e.; 2,6-dichlorophenol indophenol (DCPIP) (E'_o = + 0.217 V), which is blue in the oxidized quinone form and which becomes colorless when reduced to the phenolic form. This dye-electrom acceptor also has the advantage that it accepts electroms directly from the quinone (Q_a) electron-acceptor of the photosystem II< the reaction center associated with the O₂-evolving (or water-slplitting) system. Based in the bleaching of DCPIP by illuminated spinach leaf chloroplasts, a classroom laboratory protocol has been developed to determine the quantum yield (QY = micromol O₂ s^-1 / micromol photons s^-1, the quantum requirement (1/QY) and the energetic efficiency (f = chemical energy stored / light energy supplied) of the O₂-evolving system of photosynthesis. Although values for the quantum yield, the quantum requirement and the energetic efficiency calculated in the classroom laboratory differ widely from those expected theoretically, these calculations are useful for illustrating the transformation of light energy into chemical energy by the chloroplasts of green plants.