A kinetic model is presented that describes functional biomass, starch and storage lipid (TAG) synthesis in the microalga Neochloris oleoabundans as a function of nitrogen and light supply rates to a nitrogen-limited turbidostat cultivation system. The model is based on the measured electron distribution in N. oleoabundans, which showed that starch is the primary storage component, whereas TAG was only produced after an excess of electrons was generated, when growth was limited by nitrogen supply. A fixed 8.6% of the excess electrons ended up in TAG, suggesting close metabolic interactions between nitrogen assimilation and TAG accumulation, such as a shared electron pool. The proposed model shows that by manipulating the cultivation conditions in a light or nitrogen limited turbidostat, algal biomass composition can be customised and the volumetric productivities and yields of the major biomass constituents can be changed on demand.