Section 1 of 4Structure of an Amino Acid
Amino acids are small organic molecules containing two different functional groups attached to the same carbon atom.
Amino group
- Amino group The -NH₂ group can accept a hydrogen ion, so it behaves as a base.
Carboxylic acid group
- Carboxylic acid group The -COOH group can release a hydrogen ion in solution, so it behaves as an acid.
R group
- R group The variable side chain attached to the central carbon, which differs between amino acids and gives each its properties.
Example
- Example Glycine, H₂NCH₂COOH, is the simplest amino acid, and its R group is a single hydrogen atom.
Section 2 of 4Acid and Base Behaviour
The two functional groups react differently depending on the acidity of the solution.
1
In an acidic solution, the amino group accepts a hydrogen ion and the molecule carries a positive charge.
2
In an alkaline solution, the carboxylic acid group loses a hydrogen ion and the molecule carries a negative charge.
3
Because both behaviours are possible, amino acids can react as either an acid or a base, depending on the conditions.
| Solution | Group that reacts | Charge on the ion formed |
|---|---|---|
| Acidic | Amino group accepts H⁺ | Positive |
| Alkaline | Carboxylic acid group loses H⁺ | Negative |
Section 3 of 4Joining Amino Acids into Proteins
Joining Amino Acids into Proteins
Peptide bond
- Peptide bond The carboxylic acid group of one amino acid reacts with the amino group of another, forming a -CO-NH- link.
Water loss
- Water loss A molecule of water is released each time two amino acids join, so the reaction is a condensation.
Polypeptide
- Polypeptide Repeated joining produces a long chain called a polypeptide, which folds into a specific shape to form a protein.
Sequence
- Sequence The order of amino acids in a chain determines the shape and function of the protein that forms.
Section 4 of 4Exam Checklist & Tips
7.3.3 – Key Points for the Exam
- Amino acid A molecule containing both an amino group and a carboxylic acid group attached to the same carbon atom.
- Condensation polymerisation Polymerisation in which small molecules join with the loss of a small molecule such as water.
- Peptide bond The -CO-NH- link formed between two amino acids in a polypeptide chain.
- Polypeptide A chain of many amino acids linked by peptide bonds.
- R group The variable side chain of an amino acid that distinguishes one amino acid from another.
Common Mistakes
- Confusing the amino group with the carboxylic acid group, and forgetting that both are attached to the same carbon atom.
- Stating that addition polymerisation forms proteins; proteins are formed by condensation polymerisation.
- Forgetting that water is released in each condensation step, so H₂O must appear as a product.
- Writing that all amino acids are identical; the R group varies and determines each amino acid's properties.