Describe amino acids and peptide bonds
Amino acids share a general structure; condensation forms peptide bonds and hydrolysis breaks them.
Section 2.3 · Proteins
AS Level · Cambridge International Biology 9700, 2028–2030. These lessons explain syllabus concepts; self-checked practice does not measure practical competence or qualify as teacher-reviewed assessment.
Before you begin
- Recognise that biological molecules have structures that relate to their roles in cells and organisms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What type of bond joins amino acids in a polypeptide?
What you will learn
- Draw and label the general structure of an amino acid
- Explain peptide-bond formation by condensation and breakage by hydrolysis
One central carbon and variable side chain
A general amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen atom and a variable side chain, often labelled R. The R group differs between amino acids and gives each amino acid its particular properties.
Amino acids are monomers of polypeptides. In a simple general formula, show all four groups attached to the central carbon; do not confuse the variable R group with a separate amino acid.
Peptide bonds join amino acids
A condensation reaction between the carboxyl group of one amino acid and the amino group of another forms a peptide bond and releases water. Repeating this reaction produces a polypeptide chain.
Hydrolysis adds water to break a peptide bond. Digestion uses hydrolysis to split proteins or polypeptides into smaller peptides and amino acids. The two reactions have opposite effects on the bond.
Pause and explain
What is released when a peptide bond forms by condensation?
Worked example
Two amino acids join to form a dipeptide. State the bond and the reaction, then describe the reverse reaction.
Show the worked solution
- Amino acids join by condensation.
- The link is a peptide bond and water is released.
- Hydrolysis adds water to break the link.
Answer A condensation reaction forms a peptide bond between the amino acids and releases water. Hydrolysis is the reverse: water is added to break the peptide bond.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe the general structure of an amino acid.
Give me a hint
Name the central carbon and the four attached groups.
Compare my reasoning
- A central carbon is present.
- It is bonded to an amino group, carboxyl group, hydrogen and variable R group.
A general amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen atom and a variable R group.
Look for these in your work
- Central carbon identified
- All four attached groups included
Explain how a peptide bond forms between two amino acids.
Give me a hint
Include the reaction and removed molecule.
Compare my reasoning
- The carboxyl group of one reacts with the amino group of another.
- Condensation forms a peptide bond and releases water.
A condensation reaction joins the carboxyl group of one amino acid to the amino group of another, forming a peptide bond and releasing a water molecule.
Look for these in your work
- Condensation named
- Bond and water stated
A protease digests a polypeptide. Explain the bond-level change.
Give me a hint
Use the reaction that breaks peptide bonds.
Compare my reasoning
- Hydrolysis adds water.
- Peptide bonds are broken to form smaller peptides or amino acids.
Hydrolysis adds water to peptide bonds and breaks them, producing smaller peptides and eventually amino acids.
Look for these in your work
- Hydrolysis identified
- Bond and products described
Common mistakes
- Calling the peptide bond a glycosidic bond
- Saying condensation adds water
- Drawing the R group as the same for every amino acid
What can you explain now?
What type of bond joins amino acids in a polypeptide?
Compare with the explanation
Peptide bond
Peptide bonds link amino acids.
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Make it stick
How do condensation and hydrolysis have opposite effects on peptide bonds?
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