Tryptophan is a precursor for serotonin. Phenylalanine is a precursor for dopamine, norepinephrine, and epinephrine. Histamine acts both as a mediator of the inflammatory response and as a neurotransmitter in the central nervous system. Tyrosine is a precursor for dopamine. Serine is not a precursor for any neurotransmitter.

Insulin promotes the translocation of GLUT-4 receptors to the cell surface through cell signaling triggered by its binding to cell surface insulin receptors. GLUT-2 transporters are insulin-independent and are found in tissues like the pancreas and liver where immediate glucose sensing is important for whole body function (The pancreas needs to sense glucose so it can secrete insulin for the rest of the body. Imagine if the pancreas itself needed insulin.)

Glucagon receptors and beta-adrenoreceptors (for epinephrine) on cells trigger the release of cAMP, starting a phosphorylation cascade which ultimately activates glycogen phosphorylase and inhibits glycogen synthase. In liver cells, alpha-adrenoreceptors (also for epinephrine) releases calcium ions, which also begins a phosphorylation cascade ultimately leading to glycogen degradation. Glycogen is broken down into glucose which can undergo glycolysis for the production of ATP.

One must know the phosphorylation system in order to fully understand this conclusion, but logically, an increase of glucose in a cell (or insulin, which is released when blood glucose levels are high) shouldn't trigger a cell to make more glucose, as this implies there is an abundance of glucose in the cell.

Epinephrine, released by adrenal glands, is a neurotransmitter which is responsible for the "fight or flight" response, in which an organism needs energy fast. Therefore, an increase of glucose is needed for glycolysis.

Glucagon, released by the pancreas, is directly released when blood glucose levels are low, and therefore it is logical that it must signal for an increase of glucose production.

A hormone that has its receptor located on the surface of a cell will be a peptide hormone, not a steroid hormone. A steroid hormone can diffuse through the cell membrane and will find its receptor inside of the cell. The only peptide hormone listed as an answer choice is oxytocin.

Phospholipase C normally breaks down phosphatidylinositol (3,4,5)-trisphosphate (PIP3) into diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). This cascade eventually increases cytosolic calcium levels through its release from the endoplasmic reticulum and from the extracellular fluid. Malfunction in this enzyme results in PIP3 not being broken down.

A hormone is a signaling molecule that binds to a receptor and initiates a signaling cascade inside the cell. The receptor for a hormone can be found on the periphery of the cell (on plasma membrane) or inside the cell (cytoplasm or nucleoplasm). A steroid hormone is nonpolar and can traverse the hydrophobic interior of the plasma membrane whereas a peptide hormone is polar and cannot traverse the hydrophobic interior; therefore, a steroid hormone will have its receptor inside the cell whereas a peptide hormone will have its receptor on the plasma membrane. The question is asking us to find the polar, peptide hormone (because its receptor will be found on the plasma membrane, not in cytoplasm). To answer this question, we need to know which amino acids are polar. Recall that there are twelve polar amino acids. They are serine, threonine, cysteine, tyrosine, asparagine, glutamine, aspartic acid, glutamic acid, histidine, lysine, and arginine; therefore, the hormone containing phenylalanine, histidine, and methionine is most likely to be polar. The rest of the hormones have nonpolar amino acids only.

Steroid hormones are nonpolar molecules that can travel across the hydrophobic (or nonpolar) interior of the plasma membrane whereas peptide hormones are polar molecules that cannot travel across the hydrophobic interior. The question states that the hormone cannot enter the cell. This means that it cannot traverse the plasma membrane and, therefore, must be a peptide hormone. A peptide is made up of several amino acids. There are polar and nonpolar amino acids. Since they are polar, peptide hormones must have at least a few polar amino acids. These polar amino acids can be positively charged, negatively charged, or uncharged. There are twelve polar amino acids, five of which are charged (aspartic acid, glutamic acid, histidine, lysine, and arginine). Aspartic acid and glutamic acid are negatively charged at physiologic pH, whereas the other three are positively charged. A molecule that forms clumps in water is hydrophobic and nonpolar. Since we are dealing with a peptide hormone, the hormone will dissolve and not form clumps in water. A steroid hormone, on the other hand, is nonpolar and will form clumps in water.

Glucagon is a peptide hormone that is released in the fasted state. It stimulates macromolecule breakdown and the production and subsequent release of glucose into the blood stream. It is synthesized and released from the alpha-cells in the pancreatic islets.

Beta-cells are found in the majority of the inner area of pancreatic islets. They are the most common cell in the pancreatic islet. Beta-cells produce both insulin and C-peptide, and are primarily active in the fed state. The gamma cells of the pancreatic islets secrete somatostatin.

Insulin is a peptide hormone that is released in the fed state. Thus, it promotes glucose storage and DNA replication, but decreases glycogen breakdown and the release of glucose.