A one-dimensional isothermal model is used to calculate transmembrane water profiles and current-voltage curves under different operating conditions. The mechanistic model solves the mass balance over the system, basically by considering the main source-sink terms and transport contributions: fuel and air humidity, water generated by the electrochemical reaction; water concentration-diffusion and the electro-osmotic drag of water caused by the hydration shield of protons. The model is used to analyze the transient behavior of a fuel cell and the dynamics of water inside the membrane as a consequence of changes in the load, voltage, or power released by the cell. The current-voltage transient responses are analyzed and explained in terms of the hydration of the membrane and the heterogeneity of water distribution over the membrane. Temporary changes in the power released by the cell operated at constant load are associated with wetting-drying processes. Wetting curves show an induction stage because initially the membrane is critically dehydrated. The estimations from the model suggest that local water contents lower than 2.4 mol of water per 1 mol of the sulfonic group act as a bottleneck for proton transport.