Amino acids (HT only)

What you'll learn

What makes a molecule an amino acid.
How amino acids join by condensation polymerisation.
How to write the repeat unit for a polypeptide made from glycine.
Why different amino acids can make many different proteins.

Starting point: functional groups

Organic molecules are based on carbon atoms bonded to other atoms. A functional group is a small group of atoms in a molecule that gives it particular chemical properties.

Definition

Functional group

A functional group is a specific atom or group of atoms in an organic molecule that controls many of the molecule’s reactions.

For this topic, you need two functional groups:

the amino group, $-NH2\text{-NH}_2$
the carboxylic acid group, $-COOH\text{-COOH}$

An amino acid contains both of these functional groups in the same molecule.

Key Idea

Amino acids have two functional groups

Every amino acid has an amino group, $-NH2\text{-NH}_2$ , and a carboxylic acid group, $-COOH\text{-COOH}$ . This is what allows amino acids to join together into chains.

General structure of an amino acid

A simple way to picture an amino acid is:

a carbon atom in the middle
an amino group, $-NH2\text{-NH}_2$
a carboxylic acid group, $-COOH\text{-COOH}$
a hydrogen atom
an R group, which is the part that changes between different amino acids

Definition

R group

The R group is the variable part of an amino acid. Different R groups give different amino acids.

For the amino acid glycine, the R group is just hydrogen. Its formula is:

$H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$

Labelled amino acid and glycine condensation to form a peptide link

Example

Recognising an amino acid

A molecule has the formula $CH3CH(NH2)COOH\text{CH}_3\text{CH(NH}_2\text{)COOH}$ . Explain why it is an amino acid.

Identify the amino group: the formula contains $-NH2\text{-NH}_2$ , so it has an amino group.
Identify the carboxylic acid group: the formula contains $-COOH\text{-COOH}$ , so it has a carboxylic acid group.
Apply the definition: because both functional groups are present in the same molecule, the molecule is an amino acid.

Amino acids as monomers

A monomer is a small molecule that can join with many similar molecules. A polymer is a large molecule made from many monomers joined together.

Definition

Monomer and polymer

A monomer is a small molecule that can join to others. A polymer is a large molecule made from many repeating monomer units.

Amino acids are monomers. When many amino acids join together, they form a polymer called a polypeptide.

A protein is made from one or more polypeptide chains. In GCSE Chemistry, the key idea is that proteins are naturally occurring polymers made from amino acid monomers.

Condensation reactions

Amino acids join by a type of reaction called condensation.

Definition

Condensation reaction

A condensation reaction joins molecules together and produces a small molecule as a by-product, usually water.

When two amino acids join:

an $-OH\text{-OH}$ from the carboxylic acid group of one amino acid is removed
an $-H\text{-H}$ from the amino group of another amino acid is removed
the removed $H\text{H}$ and $OH\text{OH}$ combine to form water, $H2O\text{H}_2\text{O}$
the two amino acids join by a peptide link, $-CO-NH-\text{-CO-NH-}$

Definition

Peptide link

A peptide link is the $-CO-NH-\text{-CO-NH-}$ link formed when two amino acids join by condensation.

Example

Forming a peptide link

Two glycine molecules join together. Describe what happens during the condensation reaction.

Choose the reacting groups: the carboxylic acid group, $-COOH\text{-COOH}$ , on one glycine reacts with the amino group, $-NH2\text{-NH}_2$ , on another glycine.
Remove water: the $-OH\text{-OH}$ from $-COOH\text{-COOH}$ and one $-H\text{-H}$ from $-NH2\text{-NH}_2$ combine to make $H2O\text{H}_2\text{O}$ .
Form the new link: the carbon from the $-COOH\text{-COOH}$ group bonds to the nitrogen from the $-NH2\text{-NH}_2$ group, producing a peptide link, $-CO-NH-\text{-CO-NH-}$ .

Common Mistake

Forgetting the water molecule

In condensation polymerisation, water is not just “lost” vaguely. It is made from $OH\text{OH}$ from the carboxylic acid group and $H\text{H}$ from the amino group.

Condensation polymerisation

Polymerisation means joining many monomers together to form a polymer.

So condensation polymerisation means many monomers join together, with a small molecule produced each time a link forms.

Definition

Condensation polymerisation

Condensation polymerisation is polymerisation where monomers join together and a small molecule, usually water, is formed as a by-product.

Amino acids can react by condensation polymerisation because each molecule has two useful functional groups:

the $-NH2\text{-NH}_2$ group can react at one end
the $-COOH\text{-COOH}$ group can react at the other end

That means amino acids can keep joining together in a chain.

Key Idea

Why amino acids form chains

Amino acids can form long chains because each amino acid has two different functional groups, so it can join to another amino acid at both ends.

Glycine forming a polypeptide

Glycine has the formula:

$H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$

When many glycine molecules polymerise, the repeat unit in the polypeptide is:

$(HNCH2CO)n(\text{HNCH}_2\text{CO})_n$

The simplified GCSE equation is:

H2O(l)n\,\text{H}_2\text{NCH}_2\text{COOH(s)} \to (\text{HNCH}_2\text{CO})_n\text{(s)} + n\,\text{H}_2\text{O(l)}

The $n$ means “many”. It tells you that many glycine molecules are reacting, and a long chain is formed.

Example

Writing the repeat unit for glycine

Glycine is $H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$ . Work out the repeat unit in the polypeptide.

Start with the full glycine molecule: $H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$ .
Remove the atoms that form water during condensation: one $H\text{H}$ is removed from $-NH2\text{-NH}_2$ , and one $OH\text{OH}$ is removed from $-COOH\text{-COOH}$ .
Write what remains in the chain: $HNCH2CO\text{HNCH}_2\text{CO}$ , so the repeat unit is $(HNCH2CO)n(\text{HNCH}_2\text{CO})_n$ .

Tip

Spot the repeat unit

In condensation polymers, the repeat unit is not usually the whole monomer unchanged. Some atoms have been removed to form the small molecule by-product, usually water.

Common Mistake

Short chains versus polymer equations

For a short straight chain of 5 amino acids, there are 4 peptide links and 4 water molecules formed. GCSE polymer equations often use a simplified repeat-unit equation with $H2On\,\text{H}_2\text{O}$ because the chain ends are not shown.

Polypeptides and proteins

A polypeptide is a polymer chain made from amino acids joined by peptide links.

A protein is made from one or more polypeptide chains. Proteins are examples of naturally occurring polymers.

Different amino acids can be combined in the same chain. This matters because the order of amino acids changes the structure and properties of the protein.

For example, a chain containing glycine, alanine, and serine in one order is not the same as a chain containing the same amino acids in a different order. The monomers may be the same, but the sequence is different.

Key Idea

Proteins are sequence polymers

Proteins are polymers made from amino acids. Different amino acids can be joined in different orders, producing different proteins.

What you need to remember

For this Higher Tier Chemistry topic, focus on the chemistry of joining:

Amino acids contain $-NH2\text{-NH}_2$ and $-COOH\text{-COOH}$ functional groups.
Amino acids join by condensation reactions.
Each new peptide link, $-CO-NH-\text{-CO-NH-}$ , forms with water as a by-product.
Many amino acids joined together form a polypeptide.
Different amino acids in the same chain can produce proteins.

Exam technique

In the exam

If you are asked why amino acids form polymers, mention both functional groups: $-NH2\text{-NH}_2$ and $-COOH\text{-COOH}$ .
If you are asked about condensation polymerisation, state that water is formed as a by-product.
If drawing or identifying the link between amino acids, look for the peptide link, $-CO-NH-\text{-CO-NH-}$ .

Self review

Check yourself

What two functional groups are found in every amino acid?
What atoms are removed to form water when two amino acids join?
What is the repeat unit formed when glycine, $H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$ , polymerises?

Amino acids (HT only)

What you'll learn

What makes a molecule an amino acid.
How amino acids join by condensation polymerisation.
How to write the repeat unit for a polypeptide made from glycine.
Why different amino acids can make many different proteins.

Starting point: functional groups

Organic molecules are based on carbon atoms bonded to other atoms. A functional group is a small group of atoms in a molecule that gives it particular chemical properties.

Definition

Functional group

A functional group is a specific atom or group of atoms in an organic molecule that controls many of the molecule’s reactions.

For this topic, you need two functional groups:

the amino group, $-NH2\text{-NH}_2$
the carboxylic acid group, $-COOH\text{-COOH}$

An amino acid contains both of these functional groups in the same molecule.

Key Idea

Amino acids have two functional groups

Every amino acid has an amino group, $-NH2\text{-NH}_2$ , and a carboxylic acid group, $-COOH\text{-COOH}$ . This is what allows amino acids to join together into chains.

General structure of an amino acid

A simple way to picture an amino acid is:

a carbon atom in the middle
an amino group, $-NH2\text{-NH}_2$
a carboxylic acid group, $-COOH\text{-COOH}$
a hydrogen atom
an R group, which is the part that changes between different amino acids

Definition

R group

The R group is the variable part of an amino acid. Different R groups give different amino acids.

For the amino acid glycine, the R group is just hydrogen. Its formula is:

$H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$

Labelled amino acid and glycine condensation to form a peptide link

Example

Recognising an amino acid

A molecule has the formula $CH3CH(NH2)COOH\text{CH}_3\text{CH(NH}_2\text{)COOH}$ . Explain why it is an amino acid.

Identify the amino group: the formula contains $-NH2\text{-NH}_2$ , so it has an amino group.
Identify the carboxylic acid group: the formula contains $-COOH\text{-COOH}$ , so it has a carboxylic acid group.
Apply the definition: because both functional groups are present in the same molecule, the molecule is an amino acid.

Amino acids as monomers

A monomer is a small molecule that can join with many similar molecules. A polymer is a large molecule made from many monomers joined together.

Definition

Monomer and polymer

A monomer is a small molecule that can join to others. A polymer is a large molecule made from many repeating monomer units.

Amino acids are monomers. When many amino acids join together, they form a polymer called a polypeptide.

A protein is made from one or more polypeptide chains. In GCSE Chemistry, the key idea is that proteins are naturally occurring polymers made from amino acid monomers.

Condensation reactions

Amino acids join by a type of reaction called condensation.

Definition

Condensation reaction

A condensation reaction joins molecules together and produces a small molecule as a by-product, usually water.

When two amino acids join:

an $-OH\text{-OH}$ from the carboxylic acid group of one amino acid is removed
an $-H\text{-H}$ from the amino group of another amino acid is removed
the removed $H\text{H}$ and $OH\text{OH}$ combine to form water, $H2O\text{H}_2\text{O}$
the two amino acids join by a peptide link, $-CO-NH-\text{-CO-NH-}$

Definition

Peptide link

A peptide link is the $-CO-NH-\text{-CO-NH-}$ link formed when two amino acids join by condensation.

Example

Forming a peptide link

Two glycine molecules join together. Describe what happens during the condensation reaction.

Choose the reacting groups: the carboxylic acid group, $-COOH\text{-COOH}$ , on one glycine reacts with the amino group, $-NH2\text{-NH}_2$ , on another glycine.
Remove water: the $-OH\text{-OH}$ from $-COOH\text{-COOH}$ and one $-H\text{-H}$ from $-NH2\text{-NH}_2$ combine to make $H2O\text{H}_2\text{O}$ .
Form the new link: the carbon from the $-COOH\text{-COOH}$ group bonds to the nitrogen from the $-NH2\text{-NH}_2$ group, producing a peptide link, $-CO-NH-\text{-CO-NH-}$ .

Common Mistake

Forgetting the water molecule

In condensation polymerisation, water is not just “lost” vaguely. It is made from $OH\text{OH}$ from the carboxylic acid group and $H\text{H}$ from the amino group.

Condensation polymerisation

Polymerisation means joining many monomers together to form a polymer.

So condensation polymerisation means many monomers join together, with a small molecule produced each time a link forms.

Definition

Condensation polymerisation

Condensation polymerisation is polymerisation where monomers join together and a small molecule, usually water, is formed as a by-product.

Amino acids can react by condensation polymerisation because each molecule has two useful functional groups:

the $-NH2\text{-NH}_2$ group can react at one end
the $-COOH\text{-COOH}$ group can react at the other end

That means amino acids can keep joining together in a chain.

Key Idea

Why amino acids form chains

Amino acids can form long chains because each amino acid has two different functional groups, so it can join to another amino acid at both ends.

Glycine forming a polypeptide

Glycine has the formula:

$H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$

When many glycine molecules polymerise, the repeat unit in the polypeptide is:

$(HNCH2CO)n(\text{HNCH}_2\text{CO})_n$

The simplified GCSE equation is:

H2O(l)n\,\text{H}_2\text{NCH}_2\text{COOH(s)} \to (\text{HNCH}_2\text{CO})_n\text{(s)} + n\,\text{H}_2\text{O(l)}

The $n$ means “many”. It tells you that many glycine molecules are reacting, and a long chain is formed.

Example

Writing the repeat unit for glycine

Glycine is $H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$ . Work out the repeat unit in the polypeptide.

Start with the full glycine molecule: $H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$ .
Remove the atoms that form water during condensation: one $H\text{H}$ is removed from $-NH2\text{-NH}_2$ , and one $OH\text{OH}$ is removed from $-COOH\text{-COOH}$ .
Write what remains in the chain: $HNCH2CO\text{HNCH}_2\text{CO}$ , so the repeat unit is $(HNCH2CO)n(\text{HNCH}_2\text{CO})_n$ .

Tip

Spot the repeat unit

In condensation polymers, the repeat unit is not usually the whole monomer unchanged. Some atoms have been removed to form the small molecule by-product, usually water.

Common Mistake

Short chains versus polymer equations

Polypeptides and proteins

A polypeptide is a polymer chain made from amino acids joined by peptide links.

A protein is made from one or more polypeptide chains. Proteins are examples of naturally occurring polymers.

Different amino acids can be combined in the same chain. This matters because the order of amino acids changes the structure and properties of the protein.

Key Idea

Proteins are sequence polymers

Proteins are polymers made from amino acids. Different amino acids can be joined in different orders, producing different proteins.

What you need to remember

For this Higher Tier Chemistry topic, focus on the chemistry of joining:

Amino acids contain $-NH2\text{-NH}_2$ and $-COOH\text{-COOH}$ functional groups.
Amino acids join by condensation reactions.
Each new peptide link, $-CO-NH-\text{-CO-NH-}$ , forms with water as a by-product.
Many amino acids joined together form a polypeptide.
Different amino acids in the same chain can produce proteins.

Exam technique

In the exam

If you are asked why amino acids form polymers, mention both functional groups: $-NH2\text{-NH}_2$ and $-COOH\text{-COOH}$ .
If you are asked about condensation polymerisation, state that water is formed as a by-product.
If drawing or identifying the link between amino acids, look for the peptide link, $-CO-NH-\text{-CO-NH-}$ .

Self review

Check yourself

What two functional groups are found in every amino acid?
What atoms are removed to form water when two amino acids join?
What is the repeat unit formed when glycine, $H2NCH2COOH\text{H}_2\text{NCH}_2\text{COOH}$ , polymerises?

Amino acids (HT only)

What you'll learn

Starting point: functional groups

General structure of an amino acid

Amino acids as monomers

Condensation reactions

Condensation polymerisation

Glycine forming a polypeptide

Polypeptides and proteins

What you need to remember

Amino acids (HT only) Revision Guide

Amino acids (HT only)

What you'll learn

Starting point: functional groups

General structure of an amino acid

Amino acids as monomers

Condensation reactions

Condensation polymerisation

Glycine forming a polypeptide

Polypeptides and proteins

What you need to remember

Amino acids (HT only) Revision Guide