Question 1 · 4.1 Cell Communication
Why is it useful for bacteria to release a toxin only when quorum sensing signals a large population?
- A Cells at low density lack the gene for the toxin
- B Toxin can be made only after food runs out
- C Quorum signals supply the energy to make toxin
- D Toxin from many cells at once overwhelms a host
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D — Toxin released by a handful of cells would be too little to harm a host, so waiting until the signal shows many cells are present lets the whole population act together. Every cell carries the toxin gene; quorum sensing controls when that gene is switched on.
Question 2 · 4.2 Introduction to Signal Transduction
A hormone carried in the blood reaches nearly every cell in the body, but only a few cell types respond. Why?
- A Only those cells let the hormone cross into the cytoplasm
- B Only those cells lie close to the gland that made it
- C Only those cells have the gene that codes for the hormone
- D Only those cells make a receptor that fits the hormone
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D — A cell responds to a signal only if it makes a receptor whose binding site fits that signal, so the hormone's target cells are the ones that express the matching receptor. Distance from the gland does not matter for a hormone in the blood, which reaches all tissues.
Question 3 · 4.3 Signal Transduction Pathways
In a green alga, kinase M relays a nutrient signal to target enzyme E. Researchers replaced the one serine on M that normally receives a phosphate with alanine, which accepts no phosphate, or with glutamate, whose negative charge resembles that of a phosphate group. Which conclusion about the phosphate on kinase M is best supported by the data?
- A It switches M on, and the signal acts on M chiefly by adding it
- B It switches M off, and M is active only while the site is bare
- C It supplies the energy that M spends to phosphorylate enzyme E
- D It plays a minor role, and the signal activates M by another route
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A — The two substitutions test the site from both sides. With alanine, the site can never be phosphorylated and the signal barely works (6 units against 100), so phosphorylation of that site is how the signal reaches M. With glutamate, whose negative charge imitates a phosphate, M is active even without the signal (88 units). Glutamate carries no phosphate and no high-energy bond, so what activates M is the added negative charge, which changes the protein's shape. Phosphorylation is a shape switch, not an energy supply; the energy for M's own phosphate transfers comes from the ATP it uses as a substrate.