Testing for biological molecules
Cambridge A-Level Biology (9700) · Unit 2: Biological molecules · 7 flashcards
Testing for biological molecules is topic 2.1 in the Cambridge A-Level Biology (9700) syllabus , positioned in Unit 2 — Biological molecules , alongside Carbohydrates and lipids, Proteins and Water. In one line: The iodine test detects the presence of starch. Iodine solution (potassium iodide) is added to the sample. A positive result is a color change from yellow/brown to blue/black, indicating starch presence.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 2 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
The purpose of the iodine test
The iodine test detects the presence of starch. Iodine solution (potassium iodide) is added to the sample. A positive result is a color change from yellow/brown to blue/black, indicating starch presence.
What the Cambridge 9700 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- describe and carry out the Benedict’s test for reducing sugars, the iodine test for starch, the emulsion test for lipids and the biuret test for proteins
- describe and carry out a semi-quantitative Benedict’s test on a reducing sugar solution by standardising the test and using the results (time to first colour change or comparison to colour standards) to estimate the concentration
- describe and carry out a test to identify the presence of non-reducing sugars, using acid hydrolysis and Benedict’s solution
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9700 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Testing for biological molecules
- › Remember: three polypeptide chains form one collagen molecule (triple helix). Molecules then arrange into fibrils, and fibrils into fibres.
- › Distinguish between the direct Benedict’s test and the non-reducing sugar test which requires prior acid hydrolysis and neutralization.
- › Amylose is unbranched with 1,4-glycosidic bonds only; amylopectin is branched and contains both 1,4 and 1,6-glycosidic bonds.
- › Recall that hydrolysis reactions use water to split molecules, whereas condensation reactions release water.
- › Understand that the non-reducing sugar test hydrolyzes disaccharides into monosaccharides, increasing the total concentration of reducing sugars detected.
Describe the Benedict's test and its positive result.
The Benedict's test identifies reducing sugars. Add Benedict's reagent to a sample and heat in a water bath. A positive result is a color change from blue to green/yellow/orange/red, indicating the presence of reducing sugars.
What is the purpose of the iodine test?
The iodine test detects the presence of starch. Iodine solution (potassium iodide) is added to the sample. A positive result is a color change from yellow/brown to blue/black, indicating starch presence.
Outline the emulsion test for lipids.
The emulsion test identifies lipids. Mix the sample with ethanol, then add water. A positive result is a milky white emulsion forming, indicating the presence of lipids.
Describe the Biuret test and its positive result.
The Biuret test detects proteins. Add Biuret reagent (copper(II) sulfate and sodium hydroxide) to the sample. A positive result is a color change from blue to purple/violet, indicating protein presence.
How can a semi-quantitative Benedict's test be carried out?
A semi-quantitative Benedict's test involves standardising the procedure (
Explain the purpose of acid hydrolysis when testing for non-reducing sugars.
Acid hydrolysis breaks down non-reducing sugars into their constituent reducing sugars. This allows them to be detected by the Benedict's test, which cannot directly react with non-reducing sugars like sucrose.
Outline the procedure for testing for non-reducing sugars after acid hydrolysis.
Boil the sample with hydrochloric acid, then neutralise with sodium hydrogencarbonate. Perform the Benedict's test. A positive result (color change) indicates the presence of non-reducing sugars in the original sample.
More topics in Unit 2 — Biological molecules
Testing for biological molecules sits alongside these A-Level Biology decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Testing for biological molecules deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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