Paper 2 · November 2006 · Proteins and Amino Acids
Fig. 1.1 shows two amino acids, one with a hydrogen atom as its side chain and one with a benzene ring carrying a hydroxyl group. When these two molecules are joined together,
Athe two side chains react with one another and a molecule of carbon dioxide is released from the pair
Bthe two carboxyl groups react with one another and a molecule of hydrogen is released from the pair
Cthe amino group of one reacts with the carboxyl group of the other and a molecule of water is released
Dthe amino group of one reacts with the amino group of the other and a molecule of water is taken in
Explanation
Every amino acid carries an amino group at one end and a carboxyl group at the other, and it is these two groups, not the side chains, that react. The hydroxyl of the carboxyl group and a hydrogen of the amino group leave together as water, which makes this a condensation reaction. The side chains stay free and go on to decide how the finished chain folds.
Derived from ZIMSEC Biology Paper 2, Nov 2006, Q1