Amino acids and proteins

What you'll learn

Amino acids and proteins form part of the organic chemistry content in Edexcel GCSE Chemistry, specifically within the topic of natural and synthetic polymers. You need to understand the structure of amino acids, how they link together through condensation polymerisation to form proteins, and the reverse process of hydrolysis. These concepts regularly appear in Paper 2 questions worth 2-6 marks.

Key terms and definitions

Amino acid — an organic molecule containing both an amine group (-NH₂) and a carboxylic acid group (-COOH) attached to the same carbon atom.

Peptide bond — the covalent bond (also called an amide link) formed between the carboxylic acid group of one amino acid and the amine group of another, with the formula -CONH-.

Condensation polymerisation — a polymerisation reaction in which monomers join together with the elimination of a small molecule (water in the case of amino acids forming proteins).

Protein — a biological polymer (polypeptide) formed when many amino acid monomers join together through peptide bonds.

Hydrolysis — the breaking down of a polymer into monomers by reaction with water, reversing the condensation process.

Dipeptide — a molecule formed when exactly two amino acids join together through one peptide bond.

Polypeptide — a chain of many amino acids linked by peptide bonds; proteins are polypeptides.

Monomer — a small molecule that can join with other identical or similar molecules to form a polymer.

Core concepts

Structure of amino acids

All amino acids share the same basic structure at GCSE level. Each amino acid molecule contains:

• A central carbon atom
• An amine group (-NH₂) attached to the carbon
• A carboxylic acid group (-COOH) attached to the same carbon
• A hydrogen atom attached to the carbon
• An 'R' group (side chain) that varies between different amino acids

The general structural formula for an amino acid is:

        H   O
        |   ||
    R — C — C — OH
        |
        NH₂

The R group determines which specific amino acid the molecule is. For example:

• When R = H, the amino acid is glycine (the simplest amino acid)
• When R = CH₃, the amino acid is alanine
• There are approximately 20 different amino acids commonly found in proteins, each with a different R group

At GCSE level, you must be able to recognise the functional groups (-NH₂ and -COOH) in an amino acid structure and understand that different amino acids have different R groups.

Formation of peptide bonds through condensation

When two amino acids join together, a condensation reaction occurs. This type of reaction involves:

1. The -OH from the carboxylic acid group of one amino acid
2. An H from the amine group of another amino acid
3. These combine to form H₂O (water), which is eliminated
4. A peptide bond (-CO-NH-) forms between the two amino acids

The equation for two amino acids forming a dipeptide:

amino acid 1 + amino acid 2 → dipeptide + water

Structurally, if we represent two simple amino acids joining:

    H₂N-CHR₁-COOH + H₂N-CHR₂-COOH → H₂N-CHR₁-CO-NH-CHR₂-COOH + H₂O

The key features of this reaction:

• It is a condensation polymerisation because a small molecule (water) is eliminated
• The new covalent bond formed is the peptide bond (or amide link)
• The reaction can continue with more amino acids joining to form longer chains
• When many amino acids join this way, they form a polypeptide or protein

Proteins are therefore addition polymers formed through condensation reactions. The monomer units are amino acids, and the polymer is the protein molecule.

Building proteins from amino acid sequences

A protein forms when many amino acids undergo repeated condensation reactions:

• Two amino acids form a dipeptide (one peptide bond)
• Three amino acids form a tripeptide (two peptide bonds)
• Many amino acids (potentially hundreds or thousands) form a polypeptide or protein

The sequence and type of amino acids determine the protein's properties. Different proteins have:

• Different numbers of amino acid units
• Different sequences of amino acids
• Different R groups in their amino acid monomers

Real protein examples you might encounter:

• Insulin — a hormone protein made of 51 amino acids
• Haemoglobin — an oxygen-transport protein in red blood cells
• Enzymes — biological catalysts that are all proteins

Hydrolysis of proteins

Hydrolysis is the reverse of condensation. When a protein undergoes hydrolysis:

1. Water molecules react with the protein
2. The peptide bonds (-CO-NH-) are broken
3. The amino acid monomers are reformed
4. Each broken peptide bond uses one water molecule

The general equation:

protein + water → amino acids

Or for a dipeptide:

dipeptide + H₂O → amino acid 1 + amino acid 2

Hydrolysis conditions at GCSE level:

• Requires heating with acid (acid hydrolysis)
• Or heating with alkali (alkaline hydrolysis)
• Or using enzymes (biological hydrolysis during digestion)

This process occurs naturally during digestion when proteins in food are broken down into amino acids in the stomach and small intestine. The enzymes protease and peptidase catalyse protein hydrolysis.

Practical detection and testing

While not always examined extensively, you should know that:

• The biuret test detects proteins (turns from blue to purple in the presence of peptide bonds)
• Amino acids can be identified using chromatography techniques
• Proteins denature (lose their structure) when heated or exposed to extreme pH values

Comparison with other polymers

In Edexcel GCSE Chemistry, amino acids and proteins are taught alongside other polymers:

Addition polymerisation (e.g., poly(ethene)):

• No small molecule eliminated
• Monomer must contain C=C double bond
• Forms only one product (the polymer)

Condensation polymerisation (e.g., proteins, polyesters):

• Small molecule eliminated (usually water)
• Two different functional groups react
• Forms polymer + water

Proteins are specifically natural condensation polymers, whereas polyesters and nylon are synthetic condensation polymers.

Worked examples

Example 1: Identifying amino acid structure

Question: The structure below shows an organic compound.

        H   O
        |   ||
H₃C — C — C — OH
        |
        NH₂

(a) Explain how you can tell this molecule is an amino acid. [2 marks]

(b) Name the amino acid shown. [1 mark]

Mark scheme answer:

(a) The molecule contains both an amine/amino group (-NH₂) [1 mark] and a carboxylic acid group (-COOH) [1 mark] attached to the same carbon atom.

Alternative: It has both functional groups that define an amino acid / it has -NH₂ and -COOH groups [2 marks for both identified]

(b) Alanine [1 mark]

(Accept: the R group is CH₃ which makes it alanine)

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Example 2: Drawing a dipeptide

Question: Two glycine molecules can join together to form a dipeptide. Glycine has the structure:

        H   O
        |   ||
    H — C — C — OH
        |
        NH₂

(a) What type of reaction occurs when two glycine molecules join? [1 mark]

(b) Draw the structure of the dipeptide formed. [2 marks]

(c) Name the type of bond formed between the two amino acids. [1 mark]

Mark scheme answer:

(a) Condensation (polymerisation) [1 mark]

(Accept: condensation reaction)

(b)

        H   O       H   O
        |   ||      |   ||
    H — C — C — N — C — C — OH  [2 marks]
        |       |   |
        NH₂     H   H

Award 1 mark for showing the peptide bond -CO-NH- correctly formed Award 1 mark for correct placement of remaining groups

(c) Peptide bond [1 mark]

(Accept: amide link / amide bond)

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Example 3: Hydrolysis of proteins

Question: Proteins can be broken down into amino acids by hydrolysis.

(a) State what is meant by hydrolysis. [2 marks]

(b) Describe the conditions needed for the hydrolysis of proteins in a laboratory. [2 marks]

(c) Explain why proteins are hydrolysed during digestion. [2 marks]

Mark scheme answer:

(a) Breaking down (of a substance/polymer) [1 mark] by reaction with water [1 mark]

(Accept: addition of water breaks bonds)

(b) Heat/warm the protein [1 mark] with acid or alkali [1 mark]

(Accept: use of enzymes; specific acids like HCl or specific alkalis like NaOH)

(c) To break down large protein molecules [1 mark] into small amino acid molecules that can be absorbed (into the bloodstream) [1 mark]

(Accept: proteins are too large to be absorbed; amino acids are needed by the body for making new proteins)

Common mistakes and how to avoid them

• Mistake: Confusing addition polymerisation with condensation polymerisation. Correction: Amino acids join by condensation because water is eliminated when the peptide bond forms. Addition polymerisation (like forming poly(ethene)) produces no small molecule and requires C=C bonds.

• Mistake: Stating that only one water molecule is released when a protein forms. Correction: One water molecule is released for each peptide bond formed. A protein with 100 amino acids has 99 peptide bonds, so 99 water molecules are released during its formation.

• Mistake: Drawing the peptide bond incorrectly or in the wrong position. Correction: The peptide bond is -CO-NH- (not -CO-O- or -NH-NH-). It always forms between the C=O of one amino acid's carboxylic acid group and the N-H of another amino acid's amine group.

• Mistake: Writing that hydrolysis "adds hydrogen and oxygen" separately. Correction: Hydrolysis adds water (H₂O) across the broken bond. When a peptide bond breaks, the -OH rejoins one amino acid and the -H rejoins the other.

• Mistake: Forgetting that amino acids have both functional groups on the same carbon. Correction: Both -NH₂ and -COOH must be attached to the central carbon atom. If they're separated by several carbons, it's not an amino acid by the GCSE definition.

• Mistake: Stating that all amino acids are identical. Correction: While all amino acids share the same basic structure (-NH₂ and -COOH on the same carbon), they differ in their R groups. There are approximately 20 different amino acids in nature, each with a unique R group.

Exam technique for "Amino acids and proteins"

• Drawing structures: When asked to draw a dipeptide or show peptide bond formation, ensure you clearly show the -CO-NH- link. Examiners award marks for correct bond formation and for showing that water has been eliminated. Use displayed structural formulae showing all atoms and bonds unless the question specifies otherwise.

• Command word "explain": Questions asking you to explain peptide bond formation or hydrolysis typically require 3-4 marks. You must state what happens (e.g., "water is eliminated"), identify the groups involved (e.g., "-OH from -COOH and H from -NH₂"), and name the bond formed (peptide bond/amide link). One-word answers score zero marks.

• Comparing polymers: Six-mark questions sometimes ask you to compare protein formation with polyester or poly(ethene) formation. Structure your answer clearly: state the type of polymerisation for each, name the monomers, identify what (if anything) is eliminated, and describe the bonds formed. Use a comparison table format in your answer if appropriate.

• Practical context: Questions may present amino acids and proteins in contexts like food digestion, enzyme action, or industrial protein production. Read the context carefully but remember that the chemistry remains the same — identify functional groups, describe condensation/hydrolysis, and apply your knowledge to the specific scenario given.

Quick revision summary

Amino acids are monomers containing both -NH₂ and -COOH groups attached to the same carbon atom, plus a variable R group. They join through condensation polymerisation: the -COOH of one amino acid reacts with the -NH₂ of another, eliminating water and forming a peptide bond (-CO-NH-). Many amino acids form proteins (polypeptides). Hydrolysis reverses this process—water breaks peptide bonds to reform individual amino acids. This occurs during digestion using enzymes or in laboratories using acid/alkali and heat. Proteins are natural condensation polymers, distinct from addition polymers like poly(ethene).