The cell must be able to control biochemical reactions. If there is an adequate amount of an end product then there is no reason to continue making it. The cell must have a means of shutting the pathway down. Reactions are regulated by controlling the enzymes associated with them. If just one enzyme in a pathway is inhibited in its function then the entire pathway stalls. Conversely, if an end product is needed, enzymes can be activated to turn on a metabolic pathway.