9. Four tubes of P32-labelled ATP with a half-life of 30 days are dispensed and stored
at four different temperatures (25°C, 4°C, -20°C and -80°C). After 30 days, the
residual radioactivity will be in which one of the following orders?
a. 25°C > 4°C > -20°C > -80°C
b. 25°C = 4°C > -20°C = -80°C
c. 25°C = 4°C < -20°C = -80°C
d. 25°C = 4°C = -20°C = -80°C
P32-Labelled ATP Residual Radioactivity After 30 Days Storage
The correct answer is d. 25°C = 4°C = -20°C = -80°C. Radioactive decay of P32 in labelled ATP follows the same physical half-life regardless of storage temperature. After 30 days—one half-life—each tube retains exactly 50% of initial radioactivity.
Radioactive Decay Basics
Radioactive decay rates depend solely on nuclear properties, unaffected by temperature or pressure under normal lab conditions. P32 undergoes beta decay with a physical half-life around 14 days, though exam contexts may simplify to 30 days. After one half-life, activity halves uniformly across all tubes.
Option Analysis
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a. 25°C > 4°C > -20°C > -80°C: Incorrect; assumes higher temperatures accelerate decay, but nuclear decay ignores thermal effects.
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b. 25°C = 4°C > -20°C = -80°C: Wrong; implies warmer storage halves faster, confusing decay with chemical degradation.
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c. 25°C = 4°C < -20°C = -80°C: False; suggests cold boosts decay, opposite of chemical stability logic.
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d. 25°C = 4°C = -20°C = -80°C: Correct; all tubes lose exactly half radioactivity due to identical decay kinetics.
P32 labelled ATP with a half-life of 30 days serves as a key topic in CSIR NET Life Sciences, testing knowledge of radioactive decay principles. Four tubes stored at 25°C, 4°C, -20°C, and -80°C show identical residual radioactivity after 30 days, highlighting that temperature does not influence nuclear decay rates.
Decay Independence from Temperature
Radioactive decay follows first-order kinetics governed by nuclear forces, remaining constant across temperatures from -80°C to 25°C. After one half-life, all samples retain 50% activity: At=A0×(1/2)t/T, where t=30 days and T=30 days yields At=0.5A0. Lab storage varies for chemical stability of ATP, not P32 decay.
Why Options Fail
Higher temperatures might degrade ATP chemically, but measured residual radioactivity tracks P32 beta emissions, unaffected by molecular breakdown. Options a–c trap students mistaking decay for hydrolysis; d aligns with physics.
Exam Relevance
CSIR NET questions like this emphasize conceptual traps in molecular biology and radiation safety. P32-ATP labels nucleic acids or proteins, with storage optimizing usability despite uniform decay.
