Question 1 · 4.1 Cell Communication
A 51-amino-acid peptide hormone acts through a receptor embedded in the plasma membrane of its target cells. Which property of the hormone best explains why its receptor sits where it does rather than in the cytosol?
- A The peptide is large and water-soluble, so the hydrophobic core of the bilayer blocks it
- B The peptide is chopped up by cytosolic enzymes within seconds of getting inside
- C The peptide carries no charge, so it would drift past a receptor held in the cytosol
- D The peptide is too dilute in the blood for a receptor in the cytosol to detect it
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A — Where a receptor sits is determined by the chemistry of its ligand. The bilayer's interior is a sheet of hydrocarbon tails, and it admits small nonpolar molecules such as steroids while excluding anything large or polar. A 51-residue peptide is both, so it can never reach a cytosolic receptor and must be read at the outer surface, with the message relayed inward by transduction. Predicting receptor location from ligand solubility is one of the most reliable inferences in this unit.
Question 2 · 4.2 Introduction to Signal Transduction
In a cultured cell, a receptor coupled to a G protein binds its ligand. Which event occurs at the G protein immediately after the ligand-bound receptor contacts it?
- A GTP is hydrolyzed to GDP, freeing the alpha subunit
- B GDP is phosphorylated to GTP while it stays in place
- C GDP is released and replaced by GTP from the cytosol
- D GTP is released and replaced by GDP from the cytosol
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C — A G protein is a molecular switch whose setting is read from which guanine nucleotide it holds. The activated receptor acts as an exchange factor: it loosens the grip on GDP so that GDP diffuses away and the far more abundant GTP takes its place. That swap changes the shape of the alpha subunit, which then separates and acts on its target enzyme. Hydrolysis of the bound GTP comes later and is what turns the switch back off.
Question 3 · 4.3 Signal Transduction Pathways
A mutant version of a cascade kinase stays in its active shape at all times. Cells carrying this mutant kinase also carry a receptor that has lost its ligand-binding site. What behavior is expected of these cells?
- A They carry out the response only when ligand is present at high dose
- B They fail to carry out the response, since the receptor is broken
- C They carry out the response only after the receptor is repaired
- D They carry out the pathway's response in the absence of ligand
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D — Everything upstream of a relay protein exists only to change that protein's state. If the kinase is already in its active shape, the upstream steps have nothing left to accomplish, and a dead receptor costs the cell nothing. The response runs continuously with no signal at all. Reasoning about where a lesion sits relative to a constitutively active component is the standard way to order a pathway, and it is why constitutively active kinases downstream of a growth-factor receptor drive division in many cancers.