Proteins and Amino Acids: Structure, Function and Testing
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Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Key concept
Proteins are large molecules made of long chains of amino acids linked by peptide bonds. There are about 20 different amino acids. Proteins have many roles: enzymes, structural components (muscle, hair), antibodies, hormones and haemoglobin.
Proteins are the most versatile biological molecules: they catalyse reactions, provide structure, transport substances and defend against disease. This guide covers protein structure, the roles you need to know, and the biuret test for protein detection.
Protein Structure (Supplement)
Proteins are polymers: long chains of amino acid monomers.
There are about 20 different amino acids. The sequence and number of amino acids determines the protein's shape and function.
Amino acids are joined by peptide bonds in a condensation reaction (water is released).
The chain folds into a specific 3D shape held in place by bonds between different parts of the chain.
If the shape changes (e.g. By high temperature or extreme pH), the protein is denatured: it loses its function.
Amino acid structure: Each amino acid has an amino group (−NH₂), a carboxyl group (−COOH), and a variable R-group that makes each amino acid different.
Roles of Proteins in the Body
Role
Example protein
Function
Enzymes
Amylase, protease, lipase
Biological catalysts that speed up chemical reactions
Structural
Collagen, keratin
Collagen strengthens tendons and skin; keratin makes hair and nails
Transport
Haemoglobin
Carries oxygen in red blood cells
Defence
Antibodies
Bind to pathogens and mark them for destruction by white blood cells
Hormones
Insulin
Regulates blood glucose concentration
Contractile
Actin, myosin
Enable muscle contraction
The Biuret Test for Proteins
Add a few drops of dilute sodium hydroxide (NaOH) to the food solution to make it alkaline.
Add a few drops of dilute copper sulfate (CuSO₄) solution.
A colour change from blue to purple/violet indicates protein is present.
If no protein is present, the solution remains blue.
Note: Some exam boards use "biuret reagent" (a pre-mixed solution). The result is the same: blue to purple.
Common error: Do not heat the biuret test: unlike Benedict's test, no heating is required.
Try a focused question (3 marks)
Explain why heating a protein to a high temperature causes it to stop functioning. (3 marks)
Show the answer and marking guidance
High temperature breaks the bonds holding the protein in its 3D shape.
The protein's shape changes / the protein becomes denatured.
The active site (if an enzyme) or binding site no longer fits its substrate/molecule, so it cannot function.
Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.
Common mix-ups to avoid
Check these points against your own answer before moving on.
Saying proteins are "killed" by heat: proteins are not alive; they are denatured.
Heating the biuret test: no heat is required, unlike Benedict's test.
Confusing amino acids with fatty acids: amino acids make proteins; fatty acids make lipids.
Practical connection
Food Tests: Reagents, Methods and Colour Changes
Use Benedict’s plus heat for reducing sugar, iodine for starch, biuret for protein, ethanol then water for fat, and DCPIP decolourisation for vitamin C. State both the reagent and the positive colour change.