Amino Acids and Peptide Bonds Explained
The general structure of an amino acid, how the 20 amino acids differ in their R group, and how a condensation reaction forms a peptide bond in a dipeptide. Includes the 4-mark exam question, a worked model answer and the examiner trap on which groups react.
Chapters (8)
Exam question [4 marks]
Describe the general structure of an amino acid. State how the twenty amino acids found in organisms differ from each other. Describe how two amino acids join to form a dipeptide.
Try it on paper first, then check the mark scheme below. The question is original and was written for this lesson.
Mark scheme
| Mark | Mark point | Also accept | Do not accept |
|---|---|---|---|
| 1 | Central carbon bonded to an amine group, a carboxyl group, a hydrogen atom and an R group | "NH2 / COOH / H / R on one carbon"; "amino group"; "carboxylic (acid) group"; labelled diagram showing all four | Amine and carboxyl only, with no hydrogen or R group; "R is the acid group" |
| 2 | The twenty amino acids differ only in the R group (side chain / side group) | "only the side chain is different" | "differ in their amine / carboxyl group"; "differ in the number of amino groups" |
| 3 | A condensation reaction, releasing a molecule of water | "water is lost / produced / removed" (together with the word condensation) | "hydrolysis"; "water is added" |
| 4 | A peptide bond forms between the carboxyl group of one amino acid and the amine group of the other (giving a dipeptide) | "bond between C of COOH and N of NH2"; "C-N bond"; either order (amine of the first, carboxyl of the second) | "glycosidic bond"; "ester bond"; "peptide bond between the R groups" |
Mark 1 needs all four groups, so name each. Mark 2 needs the word "only". Mark 3 needs both the reaction name and the water; "condensation" alone scores nothing, and water loss alone scores only if the answer says this is how the two amino acids join (the joining reaction described as one that releases water). Mark 4 needs the correct groups involved, not just the words "peptide bond".
Model answer (4/4)
An amino acid has a central carbon atom bonded to an amine group, a carboxyl group, a hydrogen atom and an R group. The twenty amino acids differ only in their R group. Two amino acids join in a condensation reaction, which releases a molecule of water. A peptide bond forms between the carboxyl group of one amino acid and the amine group of the other, making a dipeptide.
Examiner trap
- Saying amino acids differ in amine or carboxyl groups (only R group differs)
- Confusing peptide bond with glycosidic or ester bond
- Drawing the peptide bond between wrong atoms
Key terms
- amino acid
- amine group
- carboxyl group
- R group
- side chain
- peptide bond
- dipeptide
- polypeptide
- condensation
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:26Amino acid structure
Amino acids are the monomers of proteins. Every one has the same basic layout around a central carbon atom. On one side, an amine group, written NH₂. On the other side, a carboxyl group, written COOH.
Then a single hydrogen atom, and finally a fourth arm, the R group, also called the side chain. So four things attach to the central carbon. Amine, carboxyl, hydrogen, and R. Write it out like that in the exam, and the first mark is yours.
0:59The R group: twenty amino acids
There are twenty amino acids found in all living organisms. And here is the key fact. They differ only in the R group. The amine group, the carboxyl group and the hydrogen are identical in every one. In the simplest amino acid, the R group is just a single hydrogen. In others it is a short carbon chain, or something bigger.
The R group is what makes each amino acid different, and later it decides how a protein folds.
1:30Joining two amino acids
Now, how do two amino acids join? Look at the carboxyl group of the first, and the amine group of the second. The OH from the carboxyl group, and one hydrogen from the amine group, are about to leave.
1:44Water leaves: the peptide bond
They leave together as a molecule of water. That is a condensation reaction, because water is released. The carbon of the first amino acid is now bonded directly to the nitrogen of the second. That new covalent bond is the peptide bond.
Two amino acids joined like this make a dipeptide.
2:04Dipeptides and polypeptides
Add a third amino acid, release another water, and the chain grows. Keep going, with many amino acids, and you have a polypeptide. The backbone repeats nitrogen, carbon, carbon, all the way along, with the R groups hanging off the side. Every join is the same reaction, a condensation, making a peptide bond.
Only the R groups are different, and the order of those R groups is what makes each polypeptide unique.
3:06Examiner trap and recap
The trap. Never say the amino acids differ in their amine or carboxyl groups. Only the R group differs. Recap. Amine, carboxyl, hydrogen, R. Condensation makes a peptide bond and releases water. Next time, we see how a long polypeptide folds into a working protein.
Watch first
This lesson builds on:
- Condensation and Hydrolysis Reactions · AQA 7402 · 3.1.1
Keep going
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- Globular vs Fibrous Proteins and Biuret Test · AQA 7402 · 3.1.4.1
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Independent revision resource, not affiliated with or endorsed by AQA. The exam question is original and was written for this lesson; it is not an AQA question. Spotted an error? Email info@medservatory.com with the lesson code AB-007.