Amino Acids
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of proteins and monomers and polymers. You can test your knowledge on these below.
What are the monomers that make up proteins?
Amino acids.
What type of reaction joins two amino acids together?
A condensation reaction.
What are the four levels of protein structure?
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Primary: Amino acid sequence held together by peptide bonds
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Secondary: Alpha-helix and beta-pleated sheets held together by hydrogen bonds.
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Tertiary: Specific, 3D folding due to hydrogen, ionic, and disulphide bonds
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Quaternary: A protein with multiple polypeptide chains.
Topic Explainer Videos
Check out this @LauraDoesBiology video that explains amino acids or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Amino Acids: The Building Blocks of Proteins
General Structure:
- Central carbon (C) atom bonded to:
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Amine group (-NH₂)
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Carboxyl group (-COOH)
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Hydrogen atom (H)
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Variable R group (different in each amino acid).
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You need to be able to draw the general structure of an amino acid and recognise the main groups.
- All amino acids contain the elements carbon, hydrogen, oxygen and nitrogen. Two amino acids contain sulphur: methionine and cysteine.
- There are 20 Common Amino Acids:
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Differ in their R group structure.
- Some are polar (hydrophilic), others are non-polar (hydrophobic).
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- The central carbon, amine group, carboxyl group and hydrogen are common to all amino acids. The R group varies.
Here are some examples of R-groups. You do not have to know any specific R groups for amino acids.
Peptide Bond Formation
- Amino acids join via condensation reactions forming a peptide bond and removing water.
- The peptide bond forms between the carboxyl group of one amino acid and the amine group of the next. This is a strong, covalent bond.
- Dipeptide: Two amino acids joined by a peptide bond:
- Polypeptide: Long chain of many amino acids.
Biochemical Test for Proteins
Biuret Test:
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Add Biuret reagent to the sample.
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Positive result: Light blue → Purple.
Key Terms
- Peptide Bond: The bond linking amino acids in a protein.
- Denaturation: Loss of a protein’s 3D shape due to increase in temperature or pH change.
- Globular Proteins: Soluble proteins with metabolic roles (e.g., enzymes).
- Fibrous Proteins: Structural proteins (e.g., collagen, keratin).
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Exam Tips
Always link protein structure to function.
For the Biuret test, mention both reagent and colour change.
Common mistake: Confusing peptide bonds with glycosidic or ester bonds.
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This is the amino acid alanine. Draw a diagram of the dipeptide formed when two molecules of alanine are joined together. Label the bond and show anything else that is produced in this reaction (3 marks)
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!