This article deals with an example of nonlinear control systems and the interlacing between a biochemical system, the mathematical model and the constrains derived from the discrete implementation of a continuous control policy. The theory is developed on a simplified model of a bioreactor to be regulated, and the sliding mode control is presented as a robust control technique. The biological interpretation of the results derived from the mathematical model is pointed out, especially of those more closely involved with the implementation, as is the case of sample period, which seems to be very enough with respect to the minimum time needed for sample analysis.