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In this work, we present Monte Carlo simulations for the transport cycle of Escherichia coli lactose permease, using as starting point the model proposed by Kaback et al., which is based on functional properties of mutants and X-ray structures. Kaback's model suggests the existence of six states for the whole cycle of lactose/proton symport. However, the free energy differences between these states have not yet been reported in the literature. Here, we analyzed the biochemical structure of each state and determined a range of possible values for each one of the five free energy variations. Then, using the Metropolis algorithm in a non-homogeneous random walk model, we tested all the possible combinations with these values to find the free energy curve that best reproduces the dynamics of LacY. The agreement between our model predictions and the experimental data suggests that our free energy curve is appropriate for describing the lactose/proton symport. We found not only this curve, but also the time of occupancy of the lactose permease at each conformation. In addition, we paved the way in this work to solve an open question related to this transport mechanism, which is the importance of protonation for lactose binding to LacY.