As discussed in the blue/green levels of this chapter microbial cells use nutrients for growth, energy production and product formation as indicated in the following expression; Nutrients + microbial cells > cell growth + energy + reaction products Consider the operation of the "Batch" system shown in  Figure 1. This container initially contains a known growth substrate concentration S. The container is well mixed and therefore the dissolved oxygen concentration O2 does not become a limiting factor for microbial growth. Initially a known concentration X of viable microbial cells (i.e. inoculum) is added to the container and, with time, growth substrate S is utilized for cell growth. We therefore over time will observe a decrease in S (negative dS/dt) and a corresponding  increase in X