Amino acids
Cambridge A-Level Chemistry (9701) · Unit 34: Organic nitrogen compounds · 8 flashcards
Amino acids is topic 34.4 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 34 — Organic nitrogen compounds , alongside Primary and secondary amines, Phenylamine and azo compounds and Amides. In one line: A zwitterion is an amino acid molecule with both a positive and negative electrical charge. This occurs when the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻).
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 8 flashcards — 3 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 3 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Zwitterion
A zwitterion is an amino acid molecule with both a positive and negative electrical charge. This occurs when the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻).
What the Cambridge 9701 syllabus says
2025-2027 syllabus, in our words · A2 LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Describe the acid and base behaviour of amino acids, how zwitterions form, and the isoelectric point.
- Describe how amide (peptide) bonds form between amino acids to make dipeptides and tripeptides.
- Interpret and predict electrophoresis results for mixtures containing amino acids and dipeptides at various pH values; setting up the apparatus is not tested.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Amino acids
- › At a pH higher than the isoelectric point, the zwitterion loses a proton to form a negatively charged ion.
What is a zwitterion?
A zwitterion is an amino acid molecule with both a positive and negative electrical charge. This occurs when the amino group is protonated (NH₃⁺) and the carboxyl group is deprotonated (COO⁻).
Define the isoelectric point (pI) of an amino acid.
The isoelectric point (pI) is the pH at which a particular amino acid exists predominantly as a zwitterion. At this pH, the amino acid has no net electrical charge and will not migrate in an electric field.
Describe how amino acids behave in acidic conditions (pH < pI).
In acidic conditions (pH < pI), the zwitterion acts as a base: its COO⁻ group accepts a proton to become COOH. The amino group stays as NH₃⁺. The species H₃N⁺-CHR-COOH carries a net positive charge (+1).
Describe how amino acids behave in alkaline conditions (pH > pI).
In alkaline conditions (pH > pI), the zwitterion acts as an acid: its NH₃⁺ group donates a proton to become NH₂. The carboxyl group stays as COO⁻. The species H₂N-CHR-COO⁻ carries a net negative charge (−1).
Describe the formation of a peptide bond.
A peptide bond (amide bond) is formed via a condensation reaction between the carboxyl group of one amino acid and the amino group of another. This reaction releases a water molecule (H₂O).
What is a dipeptide?
A dipeptide is a molecule consisting of two amino acids joined by a single peptide bond.
What is electrophoresis and how can it be used to separate amino acids?
Electrophoresis is a technique that separates charged molecules based on their size and charge by applying an electric field. Amino acids migrate towards the electrode with the opposite charge, with the separation dependent on their pI and the buffer pH.
Predict the direction of movement of an amino acid during electrophoresis if the pH of the buffer is less than the amino acid's pI.
If the pH < pI, the amino acid will be positively charged overall. Therefore, it will migrate towards the cathode (negative electrode).
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Amino acids sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Amino acids deck
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