You are testing for the presence of starch: Iodine dissolved in an aqueous solution of potassium iodide - reacts with starch producing a deep blue-black color. This reaction is the result of the formation of polyiodide chains from the reaction of starch and iodine.
The answer here is ATP.
ATP is used in the first step of glycolysis to convert glucose into glucose-6-phosphate (that's where the phosphate comes from).
