Let us take an opportunity to review an important bit of nomenclature, which you might run into on an MCAT passage, describing the 'phosphate series'. What you have likely become accustomed to denoting simply phosphate, or a phosphate group, may be sometimes referred to as 'orthophosphate'. Don't let it be a set-back to comprehension if you see PO4 referred to as 'orthophosphate'. Here is an example in usage suitable to the present context: 'Just as hydrolysis involves breaking a bond using water, phosphorolysis involves breaking a bond using orthophosphate, as in glycogen breakdown.'

Two orthophosphate units (two phosphate groups) combined through dehydration, in other words, orthophosphate anhydride, is sometimes referred to as 'pyrophosphate'. Orthophosphate and pyrophosphate are how one refers to units of one and two phosphates respectively.