Ch 1 · Biological Molecules · IGCSE Biology 0610
Topic 1 of 1 in Biological Molecules — IGCSE Biology 0610 — 47 lessons.
Introduction
Every living cell is built from, fuelled by and controlled by a small set of biological molecules: carbohydrates, fats and oils, proteins and nucleic acids. This topic asks which elements they contain, which small units they are built from, and how to test a food sample for each one. Food tests appear regularly on Paper 6, and the Supplement DNA structure underpins the inheritance topic later in the course.
Key ideas — The elements in biological molecules
Nitrogen is the discriminating element. If a question asks which element is found in proteins but not in carbohydrates or fats, the answer is nitrogen.
🧑🏫 Teacher's explanation
Think of biological molecules like LEGO sets. To build a basic car, you might only need red and blue bricks. To build a complex spaceship, you might need to add yellow and green bricks to your collection. In biology, the 'bricks' are chemical elements like Carbon, Hydrogen, and Oxygen. Most biological molecules use these three, but some complex structures require 'specialty bricks' like Nitrogen to function.
📖 Definitions
🔍 Worked example — Easy
Identify the elements present in a carbohydrate molecule and compare the proportion of oxygen relative to a fat molecule.
🔍 Worked example — Standard
A student is testing an unknown food sample. The sample turns blue-black when iodine is added. Identify the molecule present and state its monomer unit.
🔍 Worked example — Exam-style
A biochemical analysis shows a molecule contains Carbon, Hydrogen, Oxygen, and Nitrogen. Determine the most likely class of biological molecule and identify the 'discriminating element' that distinguishes it from a lipid.
⚠️ Common errors
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🎯 Exam question
(a) Identify the class of biological molecule. [1] (b) Name the monomer that makes up this molecule. [1] (c) State the element that is present in this molecule but absent in fats and oils. [1] [3 marks]
Key ideas — Large molecules are made from smaller molecules
Large biological molecules are long chains built by joining many small molecules together.
🧑🏫 Teacher's explanation
Think of biological molecules like complex structures built from simple building blocks. Imagine a long necklace: the individual beads are the 'small molecules' (monomers), and the entire necklace is the 'large molecule' (polymer). If you change the color or shape of the beads, you change the properties of the necklace, just as changing the sequence of amino acids changes the function of a protein.
📖 Definitions
🔍 Worked example — Easy
Identify the monomer unit for the following polymers: (i) Starch, (ii) Protein, (iii) Fats.
🔍 Worked example — Standard
A student is testing a food sample. The sample turns blue-black when iodine solution is added. State the biological molecule present and the monomer it is composed of.
🔍 Worked example — Exam-style
Compare the roles of starch and glycogen in living organisms.
⚠️ Common errors
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🎯 Exam question
(i) Iodine solution: turned blue-black. (ii) Benedict's solution (heated): remained blue. (iii) Biuret reagent: remained blue.
Key ideas — Food tests
Learn the reagent, the method and both the starting and finishing colours.
🧑🏫 Teacher's explanation
Think of food tests like chemical 'detective work'. When you eat, your body breaks down large, complex molecules into smaller building blocks. To identify what was in that food, we use specific chemical reagents that change colour when they encounter a specific molecule. It is like a lock and key system: the reagent is the key, and the nutrient is the lock. If the key fits, you see a colour change.
📖 Definitions
🔍 Worked example — Easy
A student tests a food sample for starch. They add two drops of iodine solution to a clear liquid. The liquid turns blue-black. State the conclusion regarding the presence of starch.
🔍 Worked example — Standard
A student performs a Benedict's test on a glucose solution. The solution starts blue. After heating in a water bath, the solution turns brick-red. Interpret this result in terms of sugar concentration.
🔍 Worked example — Exam-style
A student is testing the Vitamin C content in two different fruit juices, A and B, using a fixed volume of blue DCPIP. Juice A takes 5 drops to decolourise the DCPIP. Juice B takes 12 drops to decolourise the DCPIP. Which juice has a higher concentration of Vitamin C? Explain your answer.
⚠️ Common errors
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🎯 Exam question
(a) Identify the nutrients present in the food substance. [2] (b) Explain why the Benedict's test requires heating while the Biuret test does not. [1] [3 marks]
Key ideas — Structure of DNA
🧑🏫 Teacher's explanation
Imagine a spiral staircase or a twisted ladder. The side rails of the ladder provide the structural backbone, while the steps (the rungs) carry the actual information. In DNA, these 'steps' are made of chemical bases that must fit together perfectly to keep the ladder stable.
📖 Definitions
🔍 Worked example — Easy
Identify the sequence of bases on the complementary strand if the original DNA strand has the following sequence: 5'- extA-G-T-C-C-A-3'
🔍 Worked example — Exam-style
Describe the structural relationship between the two strands of a DNA molecule and explain how this structure relates to its function in replication.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. A student is given a sample of milk. Describe how the student could show that the milk contains both protein and reducing sugar. [5]
🔍 Worked example — Easy
A student performs the Biuret test on a solution. The initial color of the Biuret reagent is blue. After adding it to the sample, the solution turns purple. What is the conclusion regarding the presence of protein?
🔍 Worked example — Standard
A food sample is tested with Benedict's solution. The solution turns from blue to brick-red after heating in a water bath. Explain why heating is necessary for this test.
🔍 Worked example — Exam-style
A student is given three test tubes: A, B, and C. Tube A: Iodine solution turns blue-black. Tube B: Benedict's solution + heat turns green. Tube C: Ethanol emulsion test shows a cloudy white emulsion. Identify the biological molecule present in each tube.
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Practice
Answers (a) Carbon, hydrogen and oxygen [B1]; (b) carbon, hydrogen, oxygen and nitrogen (sulfur may also be present) [B1]. (a) Glucose [B1]; (b) fatty acids and glycerol [B1]; (c) amino acids [B1]. Add a few drops of iodine solution [M1]; colour changes from orange-brown [A1] to blue-black, showing starch is present [A1]. Juice X [B1]; fewer drops are needed to decolourise the same volume of DCPIP, so its vitamin C is more concentrated [B1]. C T A A G T [B1]; A always pairs with T and C always pairs with G [B1]; so each base is replaced by its complementary partner in the same order [B1].