Growth hormone and insulin are protein hormones that regulate carbohydrate metabolism by hepatocytes (liver cells) through the activation of intracellular signaling pathways. Researchers investigated 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 time 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). 100 Level of Phosphorylated STAT5 (relative to maximum) 888888 50- 20 30 60 Time After Hormone Addition (min) 240 Level of Phosphorylated Protein Kinase B (relative to maximum) BIU E 100 90 80+ 70- .2 60+ 50 40- 30- 20 -Growth Hormone Insulin 30 60 Time After Hormone Addition (min) 240 Figure 1. The level of phosphorylated signaling proteins STAT5 (A) and protein kinase B (B) in cells cultured in the presence of growth hormone or insulin (a) Describe the first interaction that triggers signaling to begin in a target cell. (b) Identify a dependent variable in the experimental design. Identify a negative control for the effects of hormone addition. Identify a likely reason why the researchers continued their experiment for 240 minutes. B I U u | FNM ||| 0/10000 Word Limit (c) Describe the hormone and signaling protein combination that produced the greatest and most prolonged response. For the phosphorylated STAT5 response that reached 100 percent at 10 minutes in Figure 1A, calculate the rate of decrease (percent decrease per minute) in the detected level of phosphorylated STAT5 from 10 to 30 minutes. (d) The researchers claim that the similar regulation of carbohydrate metabolism by hepatocytes when the cells are treated with growth hormone or insulin results from the activation of different signaling pathways. Use the data from the researchers' experiment to support their claim. BIUE -NM ||| 0/10000 Word Limit