Cosmic rays are believed to play an essential role in determining the chemistry and the evolution of molecular clouds. This is because they are usually considered to be the main ionization agent of these star-forming regions. We have recently studied such a hypothesis from a theoretical point of view for the case of diffuse molecular clouds using the one-dimensional transport equation under the assumption that the cosmic-ray spectra measured locally are representative of the Galactic ones. Interestingly, it is found that cosmic ray density inside the cloud is significantly reduced and, thus, the predicted ionization rate is around 10 to 100 times smaller than the observational data. A brief discussion on the implication of this finding concerning fluctuations in the Galactic cosmic-ray spectra and additional sources of low-energy cosmic rays will be given at the end of this proceeding.