▶️ Answer/Explanation
The production of nitrate ions from ammonium ions is a biological process called nitrification, which is carried out by specialized soil bacteria. In the control sample (not heated), these bacteria converted the added ammonium into nitrates over the two-week period. However, heating soil to $100$ °C acts as a sterilization method that kills most living organisms, including these bacteria. Since the heated sample failed to produce any nitrate ions, it indicates that the biological agents responsible for the conversion were destroyed. Therefore, the lack of nitrate production is due to the heat killing the nitrifying bacteria.
✅ Answer: (C)
✅ Answer: (C)
▶️ Answer/Explanation
The graph illustrates the process of natural selection in a bacterial population exposed to an antibiotic. Initially, a random mutation may provide a very small number of bacteria with resistance to antibiotic $X$. When the antibiotic is applied, it acts as a selection pressure, killing the susceptible bacteria while the resistant individuals survive and reproduce. Over time, these resistant bacteria pass on their advantageous alleles to their offspring. As this process repeats over many generations, the proportion of the population that is resistant increases significantly, as shown by the upward curve in the graph.
✅ Answer: (B)
✅ Answer: (B)
▶️ Answer/Explanation
Antibiotics act as a selective pressure in a population of bacteria. When antibiotics are used, they kill susceptible bacteria, while those with rare mutations for resistance survive and reproduce. Overusing antibiotics provides more opportunities for this natural selection process to occur, leading to a rise in resistant strains. By using antibiotics only when essential, we limit the exposure of bacteria to the drug, thereby reducing the rate at which natural selection for resistance occurs. This helps preserve the effectiveness of treatments for future infections.
✅ Answer: (D)