Globular vs Fibrous Proteins and Biuret Test
How protein shape decides its job, comparing globular and fibrous proteins (collagen), and how the biuret test shows that protein is present. Includes the 4-mark exam question, a worked model answer and the examiner trap on what biuret detects.
Chapters (9)
Exam question [4 marks]
Collagen is a fibrous protein found in tendons.
(a) Explain how the structure of collagen suits its function in a tendon.
(b) Describe how you would show that a sample of tendon extract contains protein.
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 | Three polypeptide chains wound together (like a rope / triple helix) with many hydrogen bonds between them | "three chains twisted", "rope", "many hydrogen bonds cross-link the chains" | "one long chain"; "covalent bonds hold the chains"; "peptide bonds between the chains" |
| 2 | So it is strong, able to withstand a pulling force / not stretch ("and insoluble" is optional; mark 2 can be gained if the link to the structure in mark point 1 is made in either order) | "strong", "high tensile strength", "resists pulling", "does not dissolve" | "collagen is strong" with no link to the structure at all |
| 3 | Add sodium hydroxide, then (dilute) copper sulfate solution | "biuret reagent", either order if both named | "add copper sulfate only"; "Benedict's reagent"; "heat" |
| 4 | Blue to purple / lilac (so protein is present); reference to peptide bonds is not required | "blue turns violet / mauve" | "red", "pink", "brick red"; "detects amino acids" |
Part (a) needs the structure and then the property it gives; a function with no structure scores nothing. In (b) the reagents and the colour change are separate marks; the colour mark needs an unambiguous change to purple. Globular proteins (compact, soluble, specific shape) are taught in the video but not tested in this question.
Model answer (4/4)
(a) Collagen is made of three polypeptide chains wound together like a rope, with many hydrogen bonds between them. So it is strong and insoluble, which suits a tendon that must withstand a pulling force. (b) Add sodium hydroxide solution to the sample, then add dilute copper sulfate solution and mix. If protein is present, the solution changes from blue to purple, which shows peptide bonds, so protein is present.
Examiner trap
- Saying the biuret test detects amino acids (it detects peptide bonds)
- Saying a positive result is 'red' or 'pink' instead of purple/lilac
- Describing structure without linking it to the property asked
Key terms
- protein function
- fibrous
- globular
- biuret reagent
- sodium hydroxide
- copper sulfate
- blue to purple
- peptide bonds
Lesson notes
A written version of the video, chapter by chapter, for revision. Times link to the video.
0:32Shape decides job
Proteins do a huge range of jobs in every living organism. Enzymes catalyse reactions, antibodies defend the body, haemoglobin carries oxygen, and collagen holds tissue together. The rule for every exam answer is the same. Name the structure, give the property it creates, then say what that lets the protein do.
0:54Globular proteins
First group, the globular proteins. Enzymes and haemoglobin are examples. The chain is folded into a compact, rounded shape. The hydrophilic side groups sit on the outside, facing the water, so the protein is soluble. That suits transport in blood and reactions in the cell. And the exact fold gives a precise shape, such as an active site that fits only one substrate.
1:20Fibrous proteins
Second group, the fibrous proteins, such as collagen. Collagen is three polypeptide chains wound together like a rope. Hydrogen bonds hold the chains tightly together. Each bond is weak, but there are many, so the rope is very strong. It is long and insoluble, so it is perfect for a tendon, which must take a strong pull without stretching.
Globular, round and soluble. Fibrous, long and strong.
1:50The biuret test: what it detects
Now the test for protein. It is called the biuret test, and it detects peptide bonds. Every protein is a chain of amino acids joined by peptide bonds, so any protein will give a positive result.
2:05Biuret method step by step
Step one. Add a few drops of sodium hydroxide solution to your sample. Step two. Add a few drops of dilute copper sulfate solution, and mix. The reagent starts out blue.
2:18Reading the result
No protein, and it stays blue. Protein present, and it turns purple, sometimes called lilac. Not red, and not pink. That is a different test. The colour change happens because the copper ions react with the peptide bonds.
3:09Examiner trap and recap
The trap. Biuret does not detect amino acids. It detects peptide bonds. And a positive result is purple, not red or pink.
Recap. Globular means round, soluble, specific. Fibrous means long, insoluble, with high tensile strength. Biuret goes from blue to purple.
Next, we look at the most important group of globular proteins in detail. Enzymes, and how the active site really works.
Watch first
This lesson builds on:
- Protein Structure: Primary to Quaternary · AQA 7402 · 3.1.4.1
Keep going
More from 3.1 Biological molecules
- Amino Acids and Peptide Bonds Explained · AQA 7402 · 3.1.4.1
- Protein Structure: Primary to Quaternary · AQA 7402 · 3.1.4.1
- How Enzymes Work: Active Site and Induced Fit · AQA 7402 · 3.1.4.2
- Enzymes: Temperature and pH Effects · AQA 7402 · 3.1.4.2
All 21 3.1 Biological molecules lessons Whole playlist on YouTube
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-009.