11.3.4Organic Chemistry

Proteins — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 12 of 12 Unit quiz

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Part 1 of 5Amino Acids: The Monomers

Two functional groups

Two functional groups Every amino acid contains an amine group, –NH₂, and a carboxylic acid group, –COOH.

General formula

General formula An amino acid can be written H₂N–CH(R)–COOH, where R is a side chain that differs between amino acids.

Twenty common types

Twenty common types About twenty different amino acids occur in living things, each with its own R group.

Small molecules

Small molecules Amino acids are small, soluble molecules that act as the monomers from which proteins are built.

The –COOH group of one amino acid lines up next to the –NH₂ group of another amino acid.

A molecule of water is removed from the two groups, so this is a condensation reaction.

The carbon and nitrogen atoms join, forming the amide link –CONH–, which is called a peptide link in proteins.

The process repeats many times, so hundreds of amino acids can join into one long chain.

The overall change is n amino acids → protein + (n − 1) H₂O, so water is the small molecule lost.

Proteins as Polyamides

Natural polyamide

Natural polyamide A protein is a polyamide because its monomers are joined by amide links, and it is natural because it is made by living cells.

Condensation polymer

Condensation polymer A small molecule (water) is lost at each link, unlike addition polymers where only the polymer forms.

Different sequences

Different sequences The order of amino acids can vary, so a huge number of different proteins is possible.

Chain length

Chain length Proteins typically contain from about 60 up to several hundred amino acids joined in one chain.

Breakdown

Breakdown Digestive enzymes split the peptide links in proteins, releasing amino acids again.

Same link

Same link Both nylon and proteins contain the amide link, –CONH–, joining their repeat units.

Monomers

Monomers Nylon is made from two different monomers, whereas a protein is made from amino acids that each carry both functional groups.

Origin

Origin Nylon is a synthetic polymer made in industry, while proteins are made naturally in living organisms.

Small molecule lost

Small molecule lost Both are condensation polymers and both release water as the molecules join.

Protein A natural polymer made from many amino acids joined by peptide links.
Amino acid A small molecule containing both an amine group (–NH₂) and a carboxylic acid group (–COOH).
Peptide link The amide link –CONH– that joins amino acids in a protein.
Condensation polymerisation Joining of monomers with loss of a small molecule such as water at each link.
Polyamide A polymer whose repeat units are joined by amide links.

Saying that a protein is an addition polymer; it is a condensation polymer because water is lost.
Forgetting that amino acids have two different functional groups, –NH₂ and –COOH.
Calling the link in a protein an ester link instead of an amide (peptide) link.
Stating that all proteins contain the same amino acids in the same order.

Last updated: 26 October 2026