One rate-limiting step of glycolysis is the conversion of Fructose-Phosphate (Fruc-P) to Fructose-Bisphosphate (Fruc-BP), catalysed by Phosphofructokinase (PFK). The reaction involves hydrolysing one ATP to ADP.
The reverse reaction of gluconeogenesis is catalysed by Fructose-Bisphosphatase (FBP). This reaction uses 1 H2O for hydrolysis and yields 1 phosphate (Pi).
One would expect the cell to be utilising only one of the two reactions at any one time, either to break down glucose or to generate it. However, the case is commonly that both reactions are taking place simultaneously in an equilibrium [ref tbc; I think this was in Voet & Voet Biochemistry], cycling Fruc-BP to no apparent benefit, at the expense of valuable ATP.
What is the purpose of such futile cycles?