AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The primary, secondary (α-helix and β–pleated sheets) and tertiary…
Full specification wording
Proteins (A-level only): The primary, secondary (α-helix and β–pleated sheets) and tertiary structure of proteins.
Primary structure is the sequence of amino acids in the chain, held by the covalent peptide links; it is the one level that hydrolysis has to break.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Primary structure is the sequence of amino acids in the chain, held by the covalent peptide links; it is the one level that hydrolysis has to break. Secondary structure is the regular local shape produced by hydrogen bonds between the C=O of one peptide link and the N-H of another along the backbone: in the α-helix the chain coils and the hydrogen bonds run parallel to the axis, and in the β-pleated sheet lengths of chain lie side by side with hydrogen bonds bridging between them. Tertiary structure is the way the whole chain, helices and sheets together, folds into one three-dimensional shape, held by hydrogen bonds, ionic attractions between charged R groups, van der Waals forces and disulfide bridges.
What earns the marks
- Primary structure defined as the sequence of amino acids joined by peptide links.
- Secondary structure held by hydrogen bonds between the C=O and N-H of the backbone.
- Naming the α-helix and the β-pleated sheet as the two secondary structures.
- Tertiary structure as the overall folding, held by interactions between R groups including S-S bridges.
Common mistakes and how to put them right
Saying the secondary structure is held together by peptide links.
Placing disulfide bridges in the secondary structure rather than the tertiary.
Describing tertiary structure as just the shape of the helix.
Confusing the primary structure with the molecular formula of the protein.
Exam tips
- Identify the level first from what is holding it: peptide links means primary, backbone hydrogen bonds means secondary, R group interactions means tertiary.
- In a diagram question, point at the feature and name both the level and the force that maintains it.
Try the lesson
The start of the Proteins (A-level only): The primary, secondary (α-helix and β–pleated sheets) and tertiary structure of proteins. lesson, made from checked teaching notes for this specification point.
Have a go first
What is meant by the primary structure of a protein?
Listen
Primary structure is the sequence of amino acids in the chain, held by the covalent peptide links; it is the one level that hydrolysis has to break. Secondary structure is the regular local shape produced by hydrogen bonds between the C=O of one peptide link and the N-H of another along the backbone: in the α-helix the chain coils and the hydrogen bonds run parallel to the axis, and in the β-pleated sheet lengths of chain lie side by side with hydrogen bonds bridging between them. Tertiary structure is the way the whole chain, helices and sheets together, folds into one three-dimensional shape, held by hydrogen bonds, ionic attractions between charged R groups, van der Waals forces and disulfide bridges. A strong answer earns marks for these points. primary structure defined as the sequence of amino acids joined by peptide links. secondary structure held by hydrogen bonds between the C=O and N-H of the backbone. naming the α-helix and the β-pleated sheet as the two secondary structures. tertiary structure as the overall folding, held by interactions between R groups including S-S bridges. Exam tip: Identify the level first from what is holding it: peptide links means primary, backbone hydrogen bonds means secondary, R group interactions means tertiary.
Key words
Say what each word means before you turn the card.
primary sequence
Try answering aloud or in your head before turning the card.
α-helix
Try answering aloud or in your head before turning the card.
β-pleated sheet
Try answering aloud or in your head before turning the card.
tertiary folding
Try answering aloud or in your head before turning the card.
See every definition
- primary sequence
- The sequence of amino acids in the chain, held by the covalent peptide links.
- α-helix
- A secondary structure in which the chain coils and the hydrogen bonds run parallel to the axis.
- β-pleated sheet
- A secondary structure in which lengths of chain lie side by side with hydrogen bonds bridging between them.
- tertiary folding
- The way the whole chain, helices and sheets together, folds into one three-dimensional shape, held by interactions between R groups.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Proteins (A-level only): The primary, secondary (α-helix and β–pleated sheets) and tertiary structure of proteins.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge