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A polypeptide is a polymer formed from many amino acids joined by peptide bonds through condensation polymerisation. Free notes and 11 questions answered.
Amino Acids and Polypeptide Formation is part of Synthetic and naturally occurring polymers (chemistry only) in GCSE AQA Higher Chemistry.
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A polypeptide is a polymer formed from many amino acids joined by peptide bonds through condensation polymerisation. Free notes and 11 questions answered.
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Use this lesson to stay inside Synthetic and naturally occurring polymers (chemistry only) while connecting the detail back to the wider Topic 7: Organic chemistry topic.
Read the core explanations from Amino Acids and Polypeptide Formation before testing yourself from memory.
Your body contains thousands of different proteins - from the haemoglobin carrying oxygen in your blood to the enzymes breaking down your food. All of these complex molecules start from the same basic building blocks: amino acids.
What makes amino acids unique is their dual personality. Every amino acid molecule contains two completely different functional groups that give it special properties.
The amino group (-NH₂) acts as a base - it can accept protons. The carboxyl group (-COOH) acts as an acid - it can donate protons. Having both groups in one molecule means amino acids can react with themselves to form long chains.
Let's look at the simplest amino acid to understand the basic structure. Glycine has the formula H₂NCH₂COOH - it's the building block we'll use to see how polymerisation works.
In glycine's structure, you can clearly see both functional groups. The amino group (-NH₂) is on the left, the carboxyl group (-COOH) is on the right, and there's a simple -CH₂- group connecting them.
This structure is crucial because it allows one glycine molecule to react with another. The amino group of one molecule can react with the carboxyl group of another - and this is where the key scientific idea happens.
When amino acids join together, they don't just stick - they undergo a specific type of reaction called condensation polymerisation. This process eliminates water molecules as the amino acids link up.
Worked example: How Two Glycine Molecules Join Together
Show how two glycine molecules (H₂NCH₂COOH) undergo condensation polymerisation.
Answer: Two glycine molecules form a dipeptide with peptide bond -CO-NH-, eliminating one water molecule
Notice that the dipeptide still has a free amino group at one end and a free carboxyl group at the other end. This means it can react with more amino acids to keep growing the chain.
The process doesn't stop at two amino acids. The dipeptide can react with a third amino acid, then a fourth, and so on. This creates long chains called polypeptides.
When glycine polymerises completely, you get a repeating pattern. The backbone of the polymer alternates between -CO-NH- links (the peptide bonds) and -CH₂- groups from the glycine.
The general structure of the polypeptide from glycine looks like: ...NH-CH₂-CO-NH-CH₂-CO-NH-CH₂-CO... This pattern continues for hundreds or thousands of units.
Here's where biology gets really clever. While we've focused on glycine, there are 20 different amino acids that your body uses. Each has the same basic structure (amino and carboxyl groups) but different side chains.
Different amino acids can be combined in the same polypeptide chain. This creates enormous variety - the sequence of amino acids determines the final shape and function of the protein.
Think of it like a sentence made from different letters. Just as changing the order of letters changes the meaning of words, changing the order of amino acids changes the properties of proteins.
Amino acid questions often trip students up because they confuse condensation polymerisation with addition polymerisation. Remember the key differences and watch out for these common exam traps.
Mark-winning phrases: Use 'condensation polymerisation', 'peptide bond formation', 'elimination of water', and 'amide link' to show you understand the reaction.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
A polypeptide is a polymer formed from many amino acids joined by peptide bonds through condensation polymerisation. It has repeating -CO-NH- links along its backbone, and long chains of different amino acids make proteins.
Glycine is the simplest amino acid, with the formula H₂NCH₂COOH. It has an amino group (-NH₂) on one end and a carboxyl group (-COOH) on the other, joined by a -CH₂- group.
A peptide bond, also called an amide link, is the -CO-NH- bond that links amino acids together in a polypeptide. It forms when the -OH from a carboxyl group and an -H from an amino group combine to make water.
An amino acid contains two functional groups: an amino group (-NH₂), which is basic, and a carboxyl group (-COOH), which is acidic. Having both groups in one molecule lets amino acids react with each other to form long chains.
A dipeptide is two amino acids joined by a peptide bond, -CO-NH-. Two glycine molecules form a dipeptide, H₂NCH₂CONHCH₂COOH, eliminating one water molecule, and the dipeptide can react further at both ends.
Amino acids form polypeptides by condensation polymerisation: the amino group of one amino acid reacts with the carboxyl group of another, forming a peptide bond (-CO-NH-) and eliminating a water molecule. The chain keeps growing because each end still has a free group.
Two glycine molecules (H₂NCH₂COOH) join to form a dipeptide and water: H₂NCH₂COOH + H₂NCH₂COOH → H₂NCH₂CONHCH₂COOH + H₂O. The dipeptide still has a free amino group and a free carboxyl group, so it can react with more amino acids.
Different amino acids can be combined in the same polypeptide chain, and the sequence of amino acids determines the final shape and function of the protein. There are 20 different amino acids, each with amino and carboxyl groups but different side chains.
The body uses 20 different amino acids. Each has the same basic structure, an amino group and a carboxyl group, but a different side chain, and the sequence of amino acids determines a protein's shape and function.
The amino group (-NH₂) is basic, because it can accept protons, while the carboxyl group (-COOH) is acidic, because it can donate protons. Having both in one molecule lets amino acids react with each other to form chains.
The polymer from glycine has a repeating pattern: ...NH-CH₂-CO-NH-CH₂-CO-NH-CH₂-CO..., alternating -CO-NH- peptide bonds with -CH₂- groups. This pattern continues for hundreds or thousands of units.