Heterocyst is a large-sized pale coloured thick-walled cell which occurs in terminal, intercalary or lateral position in filamentous cyanobacteria (BGA), e.g., Nostoc. The thick wall is impermeable to oxygen but permeable to nitrogen. Thylakoids lack phycobilisomes. In the absence of phycobilins (which are principle light absorbing pigments in oxygen evolving photosynthetic reactions) and chlorophyll a there is no CO2 fixation or O2 productions in heterocysts. Heterocyst is dependent for its nourishment on adjacent vegetative cells.