Which of the following biological macromolecules is a polymer formed by linking many smaller subunits together?

Introduction / Context:
In biochemistry, many important molecules are polymers, meaning they are made of many repeating smaller units called monomers. Recognising which biomolecules are polymers and which are monomers is essential for understanding the structure and function of proteins, nucleic acids and polysaccharides. This question asks you to identify the polymer among several biological molecules, some of which are small building blocks and others which are large chains made from these building blocks.

Given Data / Assumptions:

• Glycerol is a three carbon alcohol used in forming triglycerides.
• An amino acid is a monomer used to build proteins.
• Nucleic acids, such as DNA and RNA, are long chains of nucleotides.
• Fatty acids are long chain carboxylic acids used to build lipids.
• Monosaccharide sugars are small carbohydrate units.

Concept / Approach:
A polymer is a large molecule made by joining many similar or identical monomer units through covalent bonds. Nucleic acids are classic examples: DNA and RNA consist of long chains of nucleotide monomers linked by phosphodiester bonds. In contrast, glycerol, amino acids, fatty acids and monosaccharides listed here are monomers or small molecules that can be combined to form larger structures, but are not themselves polymers. The approach is to identify which option describes a macromolecule built from many smaller repeating units rather than a single small unit.

Step-by-Step Solution:
Step 1: Consider glycerol, which has three carbon atoms with hydroxyl groups and acts as a backbone in triglycerides, but a single glycerol molecule is not a polymer. Step 2: An amino acid is a small molecule consisting of an amino group, carboxyl group and side chain; proteins are polymers made from many amino acids, but one amino acid alone is not a polymer. Step 3: Nucleic acids, including DNA and RNA, are built from many nucleotide monomers linked together, forming long chains with sequence information. Step 4: A single fatty acid molecule is a monomeric unit that can form part of lipids but is not itself a polymer. Step 5: A monosaccharide is a single sugar unit; polysaccharides such as starch or cellulose are polymers, but one monosaccharide molecule is not. Step 6: Therefore, the polymer in the list is a nucleic acid, which is made of many nucleotide subunits.

Verification / Alternative check:
Biology textbooks classify nucleic acids, proteins and polysaccharides as major classes of biological polymers, with lipids often treated separately. DNA is described as a long polymer composed of nucleotide monomers arranged in a specific sequence, responsible for genetic information. In contrast, amino acids are described as monomers of proteins, and monosaccharides as monomers of polysaccharides. Glycerol and fatty acids are building blocks of triglycerides and phospholipids, which are not classic polymers in the same sense. This classification confirms nucleic acids as the correct answer.

Why Other Options Are Wrong:
Option A, glycerol, is a small three carbon molecule and not a chain of repeating units. Option B, an amino acid, is a monomer for proteins, not a polymer itself. Option D, a fatty acid, is a single long chain carboxylic acid, again a monomeric unit. Option E, a simple monosaccharide sugar, is a single unit of carbohydrate, whereas polymers such as starch and cellulose are made of many monosaccharides. Only option C, a nucleic acid such as DNA or RNA, fits the definition of a polymer formed by linking many smaller units together.

Common Pitfalls:
Students sometimes confuse the building blocks with the larger structures they form, assuming that any large molecule is automatically a polymer. Another pitfall is to overlook nucleic acids and focus only on proteins and carbohydrates when thinking about polymers. To avoid confusion, remember that polymer always implies a repeated chain of similar units. Nucleic acids are polymers of nucleotides, proteins are polymers of amino acids and polysaccharides are polymers of monosaccharides. Lipids are built from smaller components but are not usually considered true polymers in the same way.

Final Answer:
The biological macromolecule that is a polymer in this list is A nucleic acid such as DNA or RNA, which is made of many nucleotide monomers linked in a chain.