The big idea: Amino acids are the small subunits (monomers) that join together to build proteins. Every amino acid is built around one central carbon atom.
Four things bonded to the central carbon
- An amino group (—NH₂)
- A carboxyl group (—COOH)
- A single hydrogen (—H)
- A variable R group (a side chain)
Only one part changes: Three of these four are identical in every amino acid — only the R group changes.
The generalized amino acid: one central carbon bonded to an amino group (—NH₂), a carboxyl group (—COOH), a hydrogen (—H) and a variable R group (the side chain).
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- Amino acid
- The monomer (subunit) that proteins are built from. It has a central carbon bonded to an amino group, a carboxyl group, a hydrogen and a variable R group.
- Central carbon
- The single carbon atom at the centre of every amino acid; the four groups are all attached to it.
- Amino group (—NH₂)
- A nitrogen-containing group found in every amino acid. Its name gives the 'amino' in 'amino acid'.
- Carboxyl group (—COOH)
- An acidic group found in every amino acid. It gives the 'acid' in 'amino acid'.
- R group (side chain)
- The part of an amino acid that varies from one amino acid to the next. There are 20 different R groups, so there are 20 different amino acids.
Where the name comes from: The name amino acid is a clue to its two key groups:
amino = the amino group (—NH₂), and acid = the carboxyl group (—COOH).
Both of these groups appear in every amino acid.
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If three of the four groups are the same in every amino acid, then the R group must be the part that makes them different.
There are 20 different amino acids in living things, and they differ only in their R group — everything else about the core structure is identical.
The variable R group: The R group is the side chain: a different chemical group sticking off the central carbon in each amino acid.
Because there are 20 different R groups, there are 20 different amino acids.
How R groups differ
- The R group can be small (just a hydrogen in glycine) or large.
- It can be charged, water-loving or water-hating.
- These differences in the R groups are what give finished proteins their wide range of properties.
The variable R group (highlighted) is the only part that differs between the 20 amino acids; the rest of the structure is identical in all of them.
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Element composition — look for nitrogen: Amino acids (and the proteins built from them) always contain the elements carbon (C), hydrogen (H), oxygen (O) and nitrogen (N).
Why nitrogen is the clue: The nitrogen comes from the amino group (—NH₂), and it is the element that carbohydrates and lipids do NOT have.
So if a molecule's elements include nitrogen, it could be a protein / amino acid — but never a carbohydrate or a fat.
| Molecule | Elements it contains | Key point |
|---|---|---|
| Carbohydrate | C, H, O | No nitrogen |
| Lipid (fat) | C, H, O | No nitrogen |
| Amino acid / protein | C, H, O AND N (some also S) | Nitrogen is present — this is the giveaway |
Same in every amino acid
- The central carbon
- The amino group (—NH₂)
- The carboxyl group (—COOH)
- The single hydrogen (—H)
Different in every amino acid
- The R group (side chain)
- 20 different R groups = 20 amino acids
- R groups vary in size, charge and water-loving / water-hating nature
- These differences give proteins their variety
An easy way to remember: R = 'Rest of it' = the part that's different. The amino group, carboxyl group and hydrogen are the fixed core; the R group is the changeable bit.
And: nitrogen = protein. Carbohydrates and lipids have C, H and O only — only proteins add N.
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The big idea: Draw an amino acid as one alpha carbon with four groups around it, each group drawn and labelled.
The points to remember
- Start with the alpha carbon: the central carbon, with four single bonds.
- Amine group: an N with two H atoms (NH₂), bonded to the alpha carbon.
- Carboxyl group: a C with a double bond to O and a single bond to OH (COOH).
- A single hydrogen atom on the alpha carbon.
- The R-group on the alpha carbon: write R, or the real group (CH₃ in alanine).
Remember it as: Amine left, carboxyl right, H down, R up.
Real example: glycine and valine: In glycine the R-group is just H; in valine it is CH(CH₃)₂. Everything else in the drawing is the same.
How this is tested: Paper 1A tests the R group and element composition. Paper 2 tests the whole structure.
What you may be asked
- Paper 1A: a multiple-choice question often shows the structure of a named amino acid (such as leucine). You identify the part that is the variable R group — the side chain that makes one amino acid differ from another.
- Paper 1A: you are given the element composition of two molecules and asked which one is the protein / amino acid. The answer is the one that contains nitrogen.
- Paper 2: you may be asked to draw or label the generalized structure of an amino acid, showing all four groups on the central carbon.
Exam-style question — draw a labelled amino acid
Draw the generalized structure of an amino acid and label its four groups. [3]
How to score all three marks
- Start with the central carbon. Draw one central carbon atom in the middle, with four bonds coming off it (up, down, left and right).
- Add the three fixed groups. Label an amino group (—NH₂), a carboxyl group (—COOH) and a single hydrogen (—H) on three of the four bonds. (These are the same in every amino acid.)
- Add the variable group. Label the fourth bond as the variable R group (side chain). (Award 1 mark for the central carbon with four bonds, 1 mark for correctly labelling the amino + carboxyl groups, 1 mark for the H and the R group.)
Final answer
A central carbon bonded to: an amino group (—NH₂), a carboxyl group (—COOH), a hydrogen (—H) and a variable R group.
The generalized amino acid: one central carbon bonded to an amino group (—NH₂), a carboxyl group (—COOH), a hydrogen (—H) and a variable R group (the side chain).
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What the diagram must show: All four groups must be on the one central carbon: amino group, carboxyl group, hydrogen and the R group.
Marking down usually comes from leaving out the R group or forgetting that the amino and carboxyl groups are both present.
| Group on the central carbon | Chemical formula | Same or different between amino acids? |
|---|---|---|
| Amino group | —NH₂ (contains nitrogen) | Same in all amino acids |
| Carboxyl group | —COOH (an acid group) | Same in all amino acids |
| Hydrogen | —H | Same in all amino acids |
| R group (side chain) | varies (—R) | DIFFERENT — this is what makes the 20 amino acids distinct |
A three-mark drawing in the Paper 2 style:
Draw the generalized structure of an amino acid.
Model answer plan
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