Final answer:
The proton's rapid movement in retinal is mainly due to its small size, enabling quick transfers between water molecules and subsequent formation of new H3O+ ions.
Step-by-step explanation:
The proton moves faster in retinal primarily due to its small size and mass. This characteristic allows the proton to interact with water molecules in such a way that it undergoes rapid proton hopping, which means it transfers between water molecules quickly, forming new H3O+ ions as it moves. The interaction with water molecules, rather than its interaction with the retinal molecule or its positive charge, is the primary reason for the increased mobility of protons in such environments. When discussing the photon-induced change in retinal, it's important to differentiate this photoisomerization process, where the molecule changes from 11-cis-retinal to all-trans-retinal, from the mechanism of proton movement in solution.