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Question Video: Identifying the Molecule That Is Not a Polymer Biology

Which of the following is not a polymer? [A] Nucleic acids [B] Cellulose [C] Proteins [D] Fatty acids

03:15

Video Transcript

Which of the following is not a polymer? (A) Nucleic acids, (B) cellulose, (C) proteins, or (D) fatty acids.

This question asks us to identify which of the biological molecules listed is not considered a polymer. To answer this question, we need to review what polymers are and look at some examples of them.

The word “polymer” itself gives us some clues as to what this means, since the prefix poly- means many and “mer” means unit. This describes the structure of the polymers nicely, as they consist of repeating subunits called monomers. The prefix mono-, meaning one, is used, since monomers are single building blocks.

Nucleic acids are examples of polymers. These biological macromolecules are composed of repeating nucleotides. The nucleotide monomers are joined together by phosphodiester bonds. A well-known nucleic acid is DNA, which is found in the nuclei of our cells. DNA stores the information that contains instructions for making all of the proteins in the body. Answer (A) is obviously not the correct answer, so we can cross it out.

Let’s now review our other answer choices in order to determine which one is not a polymer.

Cellulose is a component of the cell wall in plants, and it is composed of repeating monosaccharide units. We’ve already come across the prefix mono-, meaning one. Well, saccharide means sugar, so these are single sugar units, the basic building blocks of all carbohydrates. The monosaccharides that make up cellulose are glucose. They link together via glycosidic bonds to form long chains, ideal for structural support. Cellulose is a type of polymer called a polysaccharide, since it is made up of many sugar monomers. We can therefore discount option (B).

Now let’s look at proteins. Proteins are made up of monomers called amino acids. Amino acids get their name from the amino group and carboxylic acid group in their structure. Many amino acids are linked together by peptide bonds to form polypeptides. A protein is made up of one or more polypeptide chains. There are 20 different main amino acids. There are another two more recently discovered ones, but they are not used much.

When these different amino acids are linked together in different formats, we can make over 100,000 unique proteins. Each protein has its own unique shape due to interactions between the different amino acids. Because proteins are made up of long chains of amino acids, they are also polymers. So we can rule out option (C).

This leaves us with fatty acids, which are made up of a hydrocarbon chain attached to a carboxylic acid group. They are a component of many complex lipid molecules. An example is a triglyceride, which is made up of three fatty acids attached to a glycerol molecule by ester bonds. Since fatty acids are not made up of repeating subunits, they are not polymers.

Therefore, the correct answer to our question is option (D). The molecule that is not a polymer is fatty acids.

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