Monomers and polymers

In a nutshell

Almost every large molecule in a living thing is built the same way: many small units are joined into a long chain.

This subtopic gives you the vocabulary for that pattern. It defines a monomer and a polymer, and the two opposite reactions that build and break them: condensation (which joins molecules and releases water) and hydrolysis (which splits them using water).

Get these four terms exactly right here, because carbohydrates, proteins and nucleic acids all reuse them.

Core content

Monomers and polymers

A monomer is a smaller unit from which larger molecules are made.

A polymer is a molecule made from a large number of monomers joined together.

The word to hold on to is many: a polymer is not two or three units, it is a large number of similar units repeated along a chain.

AQA gives you three monomers to know, each of which builds its own kind of polymer:

• Monosaccharides (for example glucose) are the monomers of carbohydrates.
• Amino acids are the monomers of proteins.
• Nucleotides are the monomers of nucleic acids (DNA and RNA).

You meet each of these bonds in its own subtopic. For now, the point is that the same idea (monomers joined into a polymer) is reused three times, with a different named bond each time.

Not every large biological molecule is a polymer. A lipid such as a triglyceride is a large molecule (a macromolecule), but it is not a polymer, because it is not built from many repeating monomer units. Do not describe lipids as polymers.

Condensation: building a polymer

Monomers are joined by a condensation reaction: a reaction that joins two molecules together with the formation of a chemical bond and involves the elimination of a molecule of water.

So a condensation reaction does two things at once:

• it forms a chemical bond between the two molecules, and
• it releases a molecule of water (H₂O).

Building a polymer means many condensation reactions in a row, one for each new monomer added. This is how cells make polymers, for example joining amino acids during protein synthesis.

Hydrolysis: breaking a polymer

The reverse reaction is hydrolysis: a reaction that breaks a chemical bond between two molecules and involves the use of a water molecule.

• it breaks a chemical bond, and
• it uses a molecule of water (H₂O) to do so.

Hydrolysis is how polymers are broken back down into monomers, for example when large food molecules are digested, or when a stored polymer is broken down for use.

Still don't get it? · condensation vs hydrolysis

Think of joining two links of a paper chain. To lock the two links together you fold a tab over and a tiny scrap of paper is snipped off and drops to the floor. To take the chain apart again, you have to add something back to un-stick the join.

Now the biology. When two monomers join, a new bond forms and a small piece is thrown out: that scrap is a molecule of water. That building reaction is a condensation reaction: a bond forms and a molecule of water is eliminated.

To undo it, a water molecule is put back in to break the bond apart. That splitting reaction is hydrolysis: a bond is broken and a molecule of water is used. The clue is in the name: "hydro" (water) + "lysis" (splitting).

So the single fact that keeps the two straight: condensation gives water out, hydrolysis takes water in. In the exam, name the reaction and say what happens to the water and the bond, because both carry marks.

The two reactions side by side

Worked examples

Model answer, "Explain what is meant by a polymer." (2 marks)

1. A polymer is a (large) molecule made from many monomers (smaller/similar molecules).
2. The monomers are joined together (in a chain).

The examiner point: writing "two or more" units, or just "small molecules joined", does not score. The mark needs many and, if you avoid the word monomer, you must still show the units are similar/repeating.

Model answer, "Describe how monomers are joined to form a polymer, and how a polymer is broken down into its monomers." (4 marks)

1. Monomers are joined by condensation reactions.
2. Each condensation forms a (chemical) bond and eliminates a molecule of water.
3. A polymer is broken down by hydrolysis reactions.
4. Each hydrolysis breaks a bond using a molecule of water.

Notice the structure: four distinct, creditable points, each naming the reaction and saying what happens to the bond and to the water. That is what a "describe how" answer at this level is counting.

Common exam mistakes

• Naming the wrong reaction. Confusing condensation and hydrolysis is the single most common error here. If a bond is being broken to release monomers, the answer is hydrolysis, not condensation (and not "digestion" or "breaking down").
• "Two or more" monomers. A polymer is made of many monomers. Answers that say a polymer is "two or more" units, or that call glucose or maltose a polymer, are rejected: those are a monomer and a disaccharide.
• A half-defined monomer. Writing only "a small unit" (or "a building block") loses the mark. You must add "from which larger molecules are made."
• Forgetting the water. In condensation, a molecule of water is released; in hydrolysis, a molecule of water is added/used. Leaving water out of the definition drops a marking point.
• Not showing the monomers are similar. If you describe a polymer without using the word "monomer", you must still make clear the repeating units are identical or similar, or the answer is incomplete.
• Mismatching monomer to polymer. Nucleic acids are polymers of nucleotides (not amino acids, and a whole DNA strand is not "a monomer"); proteins are polymers of amino acids; polysaccharides are polymers of monosaccharides.
• Calling a lipid a polymer. A triglyceride is a macromolecule but not a polymer, because it is not built from many repeating monomers.

Key definitions

• Monomer: a smaller unit from which larger molecules are made. (Monosaccharides, amino acids and nucleotides are examples.)
• Polymer: a molecule made from a large number of monomers joined together.
• Condensation reaction: a reaction that joins two molecules together with the formation of a chemical bond and involves the elimination of a molecule of water.
• Hydrolysis reaction: a reaction that breaks a chemical bond between two molecules and involves the use of a water molecule.

Specification

• I can state that a monomer is a smaller unit from which larger molecules are made, and give monosaccharides, amino acids and nucleotides as examples.
• I can state that a polymer is a molecule made from a large number of monomers joined together.
• I can define a condensation reaction: it joins two molecules, forms a chemical bond, and eliminates a molecule of water.
• I can define a hydrolysis reaction: it breaks a chemical bond between two molecules using a molecule of water.

Related notes

• Carbohydrates
• Proteins
• Nucleic acids