Basic Concepts in Photobiology

Abstract

Photobiology encompasses many diverse subjects: Photomedicine, vision, photosynthesis, bioluminescence, and others. The effect of nonionizing radiation on biologic systems, as well as the use of these wavelengths to study biologic phenomena, constitutes photobiology. Photomedicine, which includes photoimmunology, is concerned with the effect of nonionizing radiation on the human organism. The areas of photomedicine share a common pathway from the absorption of nonionizing radiation by molecules in the system to the observable biologic effects. This pathway is depicted in Fig. 1.1. In the first step, radiation is absorbed by a molecule in the biologic system, such as DNA, protein, or porphyrin. The light-absorbing molecule is referred to as a chromophore. Specific chromophores are present for each photobiologic response. After absorbing the energy of the radiation (electromagnetic energy), the molecule is in an excited state. This form exists for only a fraction of a second. In the second step, the excited-state molecule undergoes a chemical change to form a photo-product. The photoproduct may result from rearrangements within the molecular framework of the chromophore or from reaction of the chromophore with another molecule.