growth hormone and insulin are protein hormones that regulate carbohydrate metabolism by hepatocytes through the activation of intracellular signaling pathways. researchers investigative whether the two hormones trigger the same or different intracellular signaling pathways in hepatocytes. the researchers added either growth hormone or insulin to parallel cultures of hepatocytes. at several times points after the hormone addition they purified total intracellular protein from the cells and quantified the level of the phosphorylated forms of two proteins involved in two different signaling pathways (figure 1, A and b) .identify a dependent variable in the experimental design identify negative control for the effects of hormone addition identify a likely reason why the researchers continue their experiments for 240 minutes