Excited states are a way of life on earth. The absorption of the free energy of photons from the sun produces excited states of the antenna pigments of photosynthesis and this "electronic energy" is converted to "chemical energy" for reduction and phosphorylation of pyridine nucleotides. This drives the plant-DNA-replicating engines and the animal (predator) DNA-replicating engines which we call living organisms. Photons captured by other pigment systems provide signals for pnototaxis, morphogenesis and vision, information for the engines to interact with the environment and with one another. Evolutionary pre-biotic photochemical synthesis selected for stable tetrapyrrole'and carotenoid structures whose electronic properties were coincidently ideal for excited state photosensitization, energy transfer and electron transfer. These basic structures are ubiquitous in all living organisms. In this paper we shall be concerned with biological chemiluminescence, the production of excited states of molecules in the cells and tissues of living organisms, from which photons are emitted as the consequence of chemical reactions with quantum efficiencies ranging from 10-15 to unity.
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