Question

Which of the following is an example of dehydration synthesis?
A. Starch $\to$ Glucose
B. Cellulose $\to$ Carbohydrate
C. Amino acid $\to$ Polypeptide
D. DNA $\to$ Nucleotide
E. Glycerol $\to$ Fatty acids

Answer 1

Hint: Dehydration synthesis is a process in which a larger macromolecule is formed by union of smaller molecules accompanied by the loss of a molecule of water. In dehydration synthesis, smaller molecules join together to form larger molecules.

Complete answer:
> Amino acids are organic compounds comprising amine and carboxyl functional groups. A side chain is present, which is specific to each amino acid. There are 20 known natural amino acids. Some of these 20 amino acids are alanine, arginine, tryptophan, glycine. The amino acids are linked by peptide bonds to form peptide chains. If a peptide chain comprises more than fifty amino acids, it is known as polypeptide chains. The linking of two or more amino acids is accompanied by the release of a water molecule, hence the reaction is a dehydration synthesis.
> Carbohydrate is a molecule containing carbon and hydrogen. The simplest form of carbohydrate is a glucose molecule. Glucose is a monosaccharide comprising six carbon atoms and five hydroxyl (OH) groups. It cannot be further broken down into simpler forms.
> Starch is a carbohydrate that is formed when several glucose units are joined together by glycosidic bonds. It is synthesized by plants during photosynthesis. Animals consume starch while having plant derived products like maize, potato, etc. The breakdown of starch into glucose does not involve release of a water molecule. Thus, it is not a dehydration synthesis.
> Cellulose is a polysaccharide comprising many glucose molecules. Since, conversion of cellulose to glucose involves breakdown of the former into smaller units, this reaction is not a dehydration synthesis. > Cellulose is abundant in nature. Animals like cows, goats can digest cellulose. Humans, take in cellulose while consuming leafy vegetables. It acts as roughage in human nutrition.
> Nucleotides are structural units of DNA and RNA. They comprise a five- carbon sugar, a phosphate group and a nitrogenous base. Several nucleotides are linked by phosphodiester bonds to form nucleic acids like DNA and RNA. Hence, conversion of DNA into nucleotides does not involve synthesis, but breakdown. Hence, it is not a dehydration synthesis.
> Glycerol is a simple compound. It contains three hydroxyl groups and can form glycerides by esterification reactions. The hydroxyl groups are esterified with a maximum of three fatty acids forming monoglyceride, diglyceride and triglycerides.
Therefore, we can conclude that amino acids are linked by peptide bonds to form polypeptides. The reaction involves the release of water molecules. The reaction is a dehydration synthesis.

Hence, option (C) is correct.

Note: The process of dehydration synthesis involves formation of polymers from monomers. Monomers are the smaller molecules that unite to form larger molecules or polymers. The process always involves formation of bigger molecules by linking of smaller units. The linking of amino acids to form polypeptide chains is a classic example of dehydration synthesis.