Practice Test 3

In the context of the passage, free radicals means...

A biology lecturer connects radiation physics to chemistry. When ionizing radiation passes through cells, it often interacts with water, the most abundant cellular molecule. The passage states that this interaction can split water into highly reactive fragments, including free radicals, which are species that possess an unpaired electron. Because electrons prefer to exist in pairs, an unpaired electron makes the molecule eager to steal an electron from nearby molecules, initiating chain reactions. The lecturer notes that free radicals can attack lipids in membranes and bases in DNA, creating lesions that cells must repair. Antioxidants are mentioned as molecules that can donate electrons to radicals without becoming dangerously reactive themselves, thereby “quenching” radical activity. The passage uses this context to explain why radiation damage is not only direct hits to DNA but also indirect chemical attack mediated by radicals.

The term annihilation in the passage can be replaced with...

A PET primer notes that emitted positrons do not persist indefinitely. The passage states that after a positron slows in tissue, it encounters an electron, and the two particles undergo annihilation. In this event, the particles disappear as particles and their mass is converted into energy, producing two gamma photons that fly away in opposite directions. The scanner detects these photons in coincidence, using their back-to-back geometry to localize the origin. The passage clarifies that annihilation is not “destruction” in the everyday sense of leaving nothing; rather, it is a conversion from matter into photon energy consistent with conservation laws.