It is commonly known that photons necessarily participate in atomic and molecular interactions. On the other hand, plant, animal and human cells spontaneously emit electromagnetic radiation. The universality of ultraweak cell luminescence is inherently associated with fundamental processes in living organisms. The majority of researchers think that this very weak radiation (10-105 photons/(cm2-s)) results from such radical reactions as, for instance, lipid peroxidation. Having at our disposal the spectra of ultraweak luminescence (UL) from intact Characeae cells and their particular cellular structures and fractions, we could confirm that the electron carriers of the mitochondrial respiration chain were active in cytoplasm and in the intact cells but not in the cell walls. We obtained a band of chlorophyll emission in UL spectra. The dark chlorophyll should not emit fluorescence, which we observed under illumination. Nevertheless in our experiments, in which plants were kept in darkness for ca. 12 hours prior to measurement, this emission was observed. The only way to account for this phenomenon is that the energy of excitation is transferred to chlorophyll by other molecules. In this ultraweak sensibilized luminescence, the excited carbonyl molecules 1,3(=C=O*) transfer their energy to chlorophyll molecules with a high quantum efficiency. As we found in the spectral analysis of UL, the intensity at the wavelength of 634 nm - corresponding to dimoles of singlet oxygen - markedly decreased when the cells were exposed to the action of ascorbic acid (AsA) and lignocaine, which are singlet oxygen quenchers.