**10. Cellular and tissue mechanisms of LLLT**

It appears that LLLT not only has a great range of effects at the molecular, cellular, and tissue levels and also its specific modes of action may vary among different applications. Within the cell, there is strong evidence to suggest that LLLT causes the mitochondria [46] to enhance ATP production [27, 31], modification of reactive oxygen species (ROS), and induce transcription factors [47].

LLLT also enhances the proliferation, maturation, and motility of fibroblasts, and increases the production of bFGF [48]. When a chromophore absorbs a photon of light (laser) in the treated cells, an electron in the chromophore has the potential to become excited and move from low‐ energy orbit to a higher energy orbit [49]. The system can then use this stored energy to achieve various cellular tasks. There are several pieces of data that suggest that a mitochondria chromophore is as the initial LLLT target. Radiation of tissue with light makes mitochondrial products such as ATP, nicotinamide adenine dinucleotide (NADH), proteins, and RNA, as well as a reciprocal augmentation in oxygen consumption to increase. Various in vitro experiments have showed that cellular respiration is upregulated when mitochondria are subjected to a He‐ Ne laser or other forms of illumination [50]. The relevant chromophore can be detected by matching the action spectra for the biological response to light in the NIR range to the absorption spectra of the four membrane‐bound complexes identified in mitochondria. This procedure demonstrates that complex IV or cytochrome *c* oxidase (CCO) is the essential chromophore in the cellular response to LLLT. CCO that consists of two copper and two heme‐ iron centers (components of the respiratory electron transport chain) is a large transmembrane protein complex [51]. The high‐energy electrons are passed from electron carriers through a series of transmembrane complexes (such as CCO) to the final electron acceptor which makes a proton gradient used to produce ATP. Therefore, the administration of light directly affects ATP production by influencing one of the transmembrane complexes in the chain. Especially, LLLT increases ATP production and electron transport [52].
