Metabolism in the Solar Photon Field A physical framework for photon-assisted modulation of mitochondrial electron-transfer kinetics

<jats:p>Within a single human lifetime, the spectral environment has been fundamentally reshaped.Broadband daylight, rich in infrared (IR) photons arising from solar and atmospheric physics,has been replaced in the built environment by narrow, engineered spectra that largely exclude long wavelengths. While modern lighting is optimised for vision, the non-visual photobiology of metabolism may depend on spectral components that are now absent. When expressed in photon-energy units, the solar spectrum exhibits a broad maximum near 0.75 eV. This range overlaps with the activation and reorganisation energies governing mitochondrial electron-transfer kinetics. Within a Marcus-type framework, IR photons are therefore positioned to modulate rate-limiting metabolic steps by biasing barrier-crossing probabilities rather than supplying chemical energy. These wavelengths also penetrate deeply into tissue in a scattering-dominated regime, forming a diffuse internal photon field capable of interacting with distributed mitochondrial networks. We propose the term photometabolism: a solar-driven, non-photosynthetic modulation of core metabolic processes. A scaling analysis shows that photon interception in this band varies with body mass in parallel with basal metabolic rate, suggesting that ambient sunlight provides sufficient flux to influence metabolic kinetics across the biosphere. These findings have implications for physiology, ecology and the design of indoor environments whose lighting spectra increasingly diverge from their evolutionary context.</jats:p>