Questions: Question 18 0.5 pts Before entering the metabolic pathways, amino acids first face - removal of their nitrogen-containing amino group - removal of their nitrogen - conversion to protein - condensation of their bonds - removal of their acetyl group

The answer is the first one: removal of their nitrogen-containing amino group.

Explanation for each option:

1. Removal of their nitrogen-containing amino group: This is correct. Before amino acids can enter metabolic pathways, they typically undergo deamination, which involves the removal of the nitrogen-containing amino group. This process is essential for the conversion of amino acids into intermediates that can enter pathways like the citric acid cycle.

2. Removal of their nitrogen: This option is not specific enough. The process specifically involves the removal of the nitrogen-containing amino group, not just nitrogen in general.

3. Conversion to protein: This is incorrect. Amino acids are the building blocks of proteins, not the other way around. The question is about amino acids entering metabolic pathways, not forming proteins.

4. Condensation of their bonds: This is incorrect. Condensation reactions typically refer to the formation of bonds, such as peptide bonds in protein synthesis, not a step in preparing amino acids for metabolic pathways.

5. Removal of their acetyl group: This is incorrect. Amino acids do not inherently have acetyl groups that need to be removed before entering metabolic pathways.

In summary, the correct process before amino acids enter metabolic pathways is the removal of their nitrogen-containing amino group.