What you'll learn
Amino acids, proteins and DNA as naturally occurring polymers is a topic in AQA GCSE Chemistry that connects organic chemistry to biology, and it is examined only on the separate Chemistry course rather than on Combined Science. The unifying idea is that nature builds enormous molecules by the same method chemists use: joining many small repeating units into long chains. Understanding this requires you to distinguish two ways of joining monomers — addition and condensation — and to recognise that all the natural polymers in this topic are condensation polymers. By the end of this guide you should be able to describe the general structure of an amino acid, explain how amino acids join to form proteins, identify the repeating structure of DNA and its four bases, describe starch and cellulose as polymers of glucose, distinguish addition from condensation polymerisation, and explain why a small number of monomers can produce an immense variety of polymers.
Key terms and definitions
Polymer — a very large molecule formed from many small repeating units joined by covalent bonds
Monomer — the small molecule from which a polymer is built
Addition polymerisation — polymerisation in which monomers containing a carbon–carbon double bond join with no other product formed
Condensation polymerisation — polymerisation in which monomers join and a small molecule, usually water, is released each time a bond forms
Amino acid — a molecule containing both an amino group and a carboxyl group, the monomer of proteins
Amino group — the group containing nitrogen and hydrogen present in every amino acid
Carboxyl group — the acidic group containing carbon, oxygen and hydrogen present in every amino acid
Peptide bond — the covalent link formed between two amino acids when water is released
Polypeptide — a chain of amino acids joined by peptide bonds
Protein — a polymer of amino acids folded into a specific shape that determines its function
DNA — deoxyribonucleic acid, the polymer that encodes genetic instructions
Nucleotide — the monomer of DNA, consisting of a sugar, a phosphate group and a base
Complementary base pairing — the specific pairing of bases that holds the two DNA strands together
Core concepts
Two kinds of polymerisation
Distinguishing these two processes is the foundation of the topic, and questions test the distinction directly.
In addition polymerisation, the monomers each contain a carbon–carbon double bond. The double bond opens and the monomers join end to end. Nothing else is produced: the polymer is the only product, and it contains exactly the same atoms as the monomers that formed it. Poly(ethene) from ethene is the standard example, and all addition polymers are synthetic.
In condensation polymerisation, each monomer has two functional groups, one at each end. When two monomers join, a small molecule — usually water — is released. So there are always two products: the polymer and the small molecule.
Every natural polymer in this topic is a condensation polymer. This is a reliable classification point: if water is released, the process is condensation.
Amino acids
An amino acid contains two functional groups on the same molecule: an amino group, which contains nitrogen, at one end, and a carboxyl group, which is acidic, at the other. Between them sits a carbon atom carrying a side group that differs from one amino acid to another.
The simplest amino acid is glycine. There are twenty amino acids used to build proteins in living organisms, and they differ only in that side group.
Because an amino acid has a reactive group at each end, it can join to another amino acid at either end, which is exactly what makes polymerisation possible. A molecule with only one functional group could join once and then stop.
Forming proteins
When two amino acids react, the amino group of one reacts with the carboxyl group of the other. A molecule of water is released and a new covalent bond, called a peptide bond, forms between them.
Because the resulting molecule still has a free amino group at one end and a free carboxyl group at the other, the process can repeat indefinitely, building a long chain called a polypeptide. A protein consists of one or more polypeptide chains.
Since a molecule of water is released each time a bond forms, this is condensation polymerisation. A chain of one hundred amino acids therefore releases ninety-nine molecules of water during its formation — a calculation that appears in examinations and follows the simple rule that the number of water molecules released is one fewer than the number of monomers joined.
The polypeptide chain then folds into a specific three-dimensional shape, and that shape determines the protein's function. This is why enzymes, which are proteins, have active sites of a particular shape, and why changing the shape by heating destroys their function.
Why twenty monomers produce such variety
A common and important examination question asks how so few amino acids can produce the enormous range of proteins found in living organisms.
The answer lies in sequence and length. A chain of just ten amino acids drawn from twenty possibilities can be arranged in an extremely large number of different orders, and real proteins contain hundreds or thousands of units. Different sequences fold into different shapes, and different shapes give different functions.
The principle is the same as that by which twenty-six letters produce every word in a language: it is the order, not the number of available units, that generates the variety.
DNA
DNA, or deoxyribonucleic acid, encodes the genetic instructions for the development and functioning of living organisms and viruses.
Its monomers are called nucleotides. Each nucleotide consists of three parts: a sugar, a phosphate group, and one of four bases.
The four bases are adenine, cytosine, guanine and thymine, usually written A, C, G and T. The sequence of these bases along the chain carries the genetic information, in the same way that the sequence of amino acids determines a protein.
Most DNA molecules consist of two polymer chains wound together to form a double helix. The two strands are held together by complementary base pairing: A always pairs with T, and C always pairs with G. This specific pairing is what allows DNA to be copied accurately, because each strand carries the information needed to rebuild the other.
DNA is a condensation polymer, formed by nucleotides joining with the release of water.
Starch and cellulose
Both starch and cellulose are natural polymers made from sugars, and in both cases the monomer is glucose. Both are condensation polymers, formed with the release of water as the glucose units join.
Starch is the storage carbohydrate of plants. It is a compact, branched or coiled molecule, which suits it to storage, and it can be broken down by enzymes to release glucose when the plant needs energy.
Cellulose is the structural carbohydrate of plants. Its glucose units are arranged so that the chains lie straight and lie alongside one another, forming strong fibres. This is what gives plant cell walls their strength, and it is why cellulose is used to make paper and cotton.
The striking point, and one examiners like, is that starch and cellulose are made from the same monomer yet have completely different properties, because the glucose units are joined in different arrangements. This mirrors the diamond and graphite comparison in the bonding unit: identical building blocks, different arrangement, different properties.
Comparing natural and synthetic polymers
Natural polymers include proteins, DNA, starch, cellulose and natural rubber. They are made by living organisms and are generally biodegradable, because enzymes exist that can break them down.
Synthetic polymers include poly(ethene), poly(propene), nylon and polyesters. Most addition polymers are not biodegradable, because they are unreactive and no enzymes exist to break them down, which is why plastic waste persists in the environment.
Note that some synthetic polymers are also condensation polymers: nylon and polyesters are made by condensation and do contain the same kind of linkage found in proteins, which is why they can be broken down more readily than addition polymers.
Worked examples
Example 1: Classifying a polymerisation (3 marks)
A polymer is formed from monomers each containing an amino group and a carboxyl group, and water is produced during the reaction. Classify the polymerisation and justify your answer.
The polymerisation is condensation. The evidence is that a small molecule, water, is released as the monomers join, which does not happen in addition polymerisation. The presence of two different functional groups on each monomer, one at each end, is also characteristic of condensation polymerisation, since each monomer must be able to react at both ends for a long chain to form. In addition polymerisation the monomers would instead contain a carbon–carbon double bond and no other product would be formed.
Example 2: Calculating water released (3 marks)
A polypeptide is formed from 250 amino acids. Calculate the number of water molecules released, and explain your reasoning.
Each peptide bond formed releases one molecule of water. Joining 250 amino acids into a single chain requires 249 bonds, because the first amino acid needs no bond to join itself and every subsequent one adds a single link. The number of water molecules released is therefore 249. The general rule is that the number of small molecules released is one fewer than the number of monomers joined in a single chain.
Example 3: Explaining variety from few monomers (4 marks)
Only twenty different amino acids are used to build proteins in the human body, yet the body contains tens of thousands of different proteins. Explain how this is possible.
Proteins differ in the order in which the amino acids are joined and in the length of the chain. Because any of the twenty amino acids can occupy any position along a chain that may be hundreds of units long, the number of possible sequences is extremely large.
Each different sequence causes the polypeptide chain to fold into a different three-dimensional shape, and the shape of a protein determines its function. A small number of monomers arranged in different orders can therefore produce an enormous number of structurally and functionally distinct proteins, in the same way that a limited alphabet produces an unlimited vocabulary.
Common mistakes and how to avoid them
The most frequent error is classifying proteins or DNA as addition polymers. Every natural polymer in this topic is formed by condensation, with water released.
Students often state that amino acids contain one functional group. They contain two, an amino group and a carboxyl group, and having a reactive group at each end is precisely what allows a long chain to form.
In calculations of water released, many candidates give the same number as the number of monomers. It is always one fewer, because the number of bonds is one fewer than the number of units.
Another common slip is confusing the monomers of DNA with the bases. The monomer is the nucleotide, which consists of a sugar, a phosphate group and a base; the base is only one part of it.
Finally, candidates frequently say starch and cellulose are made from different monomers. Both are polymers of glucose, and their different properties arise from how the glucose units are arranged.
Exam technique for "Amino acids, proteins and DNA as naturally occurring polymers"
Whenever a question asks you to identify the type of polymerisation, look for a second product. Water released means condensation; no second product means addition.
State the monomer precisely. For proteins it is amino acids, for DNA it is nucleotides, and for both starch and cellulose it is glucose. Vague answers such as sugars or acids lose the mark.
For questions on variety, always include both the sequence and the resulting shape. Sequence alone is half the explanation; the link from shape to function completes it.
When describing DNA, name all four bases and state the complementary pairs. A and T, C and G are frequently worth a mark of their own.
Remember that this topic is assessed on the separate Chemistry course only, so it will not appear on a Combined Science paper.
Quick revision summary
Addition polymerisation joins monomers containing a carbon–carbon double bond with no other product; condensation polymerisation joins monomers with two functional groups and releases a small molecule, usually water, each time. All the natural polymers in this topic are condensation polymers. Amino acids contain an amino group at one end and a carboxyl group at the other, with a variable side group between, and there are twenty used in living organisms. They join by peptide bonds with the loss of water to form polypeptides and then proteins, and the number of water molecules released is one fewer than the number of amino acids joined. The sequence of amino acids determines the folded shape, and the shape determines the function, which is why twenty monomers can produce tens of thousands of proteins. DNA is built from nucleotides, each a sugar, a phosphate group and one of the four bases adenine, cytosine, guanine and thymine, and is usually a double helix held by A–T and C–G pairing. Starch and cellulose are both condensation polymers of glucose, differing in properties because the glucose units are arranged differently.