2. Nature, light and movement

Nature offers us prodigious examples of molecules and biological systems which, after absorbing a photon, change state by isomerization, inducing photomechanical actions of micro- or nanometric magnitude that are "invisible" to the naked eye, or of greater amplitude and this time perceptible.

One example is the mechanism of human vision, involving a chromophore, retinal, within the photoreceptors of the retina (rods and cones). This chromophore is a photoisomerizable molecule. When linked to a protein, opsin, the absorption of visible light causes the same change in configuration in a few hundred femtoseconds. The new, more elongated shape of the chromophore generates steric constraints in the surrounding protein pocket. These geometric modifications lead to a cascade of reactions, culminating in the propagation of a nerve impulse along the optic nerve to the central...