In diffusive molecular communication (DMC), the transmitter has to be able to control the release of signaling molecules for modulation of the information bits. In natural cells, pumping ions is an important control mechanism for releasing molecules which is carried out by ion pumps embedded in the membrane. The activity of the ion pumps is controlled by a driving parameter. In particular, light driven pumps are controlled by light intensity and enable a high degree of spatial and temporal control for modulation functionality. In this paper, a modulator based on ion pumps is proposed for DMC which controls the release rate of the molecules from the transmitter by modulating a light intensity signal. The pumping process of the ion pump is modeled by a Markov model based on which the stochastic nature of the modulated signal, i.e., the release rate of the ions from the transmitter is analyzed. A simple on-off keying modulation scheme is realized based on the proposed modulator. Our numerical results show that a realistic transmitter can not release ions instantaneously nor deterministically.