Ch 1 · Biological Molecules · O Level Biology 5090
Topic 1 of 1 in Biological Molecules — O Level Biology 5090 — 48 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 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 simple car, you might only need red and blue bricks. To build a complex spaceship, you might need red, blue, and yellow bricks. 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 work properly.
📖 Definitions
🔍 Worked example — Easy
A student is testing an unknown food sample. The test shows the presence of Carbon, Hydrogen, and Oxygen. The sample is not detected by the Biuret test. Identify the most likely class of biological molecule.
🔍 Worked example — Standard
Complete the following table regarding the composition of biological molecules:
🔍 Worked example — Exam-style
A sample contains 1.2 g of glucose. If the glucose is broken down into its constituent elements, calculate the mass of Carbon present, assuming the mass percentage of Carbon in glucose is approximately 40\%.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) Identify the type of molecule present. [1] (b) Explain why the Biuret test was negative. [1] [2 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 Lego bricks. A single Lego brick is a small molecule (a monomer). When you snap hundreds of these identical bricks together, you get a long, complex chain (a polymer). In biology, the 'bricks' are small molecules like glucose or amino acids, and the 'structures' are large molecules like starch or proteins.
📖 Definitions
🔍 Worked example — Easy
Identify the monomer for each of the following polymers: (i) Starch, (ii) Protein, (iii) Fats.
🔍 Worked example — Standard
A student performs a test on a food sample. The sample turns from blue to brick-red after being heated with Benedict's solution. Identify the type of molecule present and name the specific monomer.
🔍 Worked example — Exam-style
Compare the roles of starch and glycogen in living organisms.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Identify the type of biological molecule present. [1] (b) Explain why the Biuret test was positive. [2] [3 marks]
Key ideas — Food tests
Learn the reagent, the method and both the starting and finishing colours.
🧑🏫 Teacher's explanation
Think of food tests like a chemical 'identity check' for your food. Just as you might use a specific tool to check if a metal is gold or silver, biologists use specific chemical reagents that change colour when they encounter a specific molecule.
📖 Definitions
🔍 Worked example — Easy
A student performs a Benedict's test on a solution. The initial colour is blue. After heating in a water bath, the solution turns green. Interpret this result.
🔍 Worked example — Standard
A food sample is shaken with ethanol and then added to water. The resulting mixture is clear and transparent. What is the conclusion regarding the lipid content?
🔍 Worked example — Exam-style
In a Vitamin C titration, 10 ext ml of blue DCPIP solution is used. Sample A requires 2 ext ml of orange juice to decolourise the DCPIP. Sample B requires 5 ext ml of orange juice to decolourise the DCPIP. Which sample has a higher concentration of Vitamin C? Justify your answer.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe how the student would test for the presence of reducing sugars in the juice. [3] (b) The student performs the test and the solution turns brick-red. Explain what this result indicates about the concentration of reducing sugar. [1] [4 marks]
Key ideas — Structure of DNA
DNA structure. A DNA molecule consists of two strands coiled around each other to form a double helix. Each strand is a chain of nucleotides, and each nucleotide carries one of four bases: A (adenine), T (thymine), C (cytosine) and G (guanine). Pairs of bases hold the two strands together like the rungs of a twisted ladder, and pairing is always complementary: A pairs with T and C pairs with G. So if one strand reads A T T G C, the opposite strand reads T A A C G. This is what allows DNA to be copied accurately before a cell divides.
🧑🏫 Teacher's explanation
Think of DNA as a long, twisted ladder. The sides of the ladder are made of a repeating sugar-phosphate backbone, and the rungs are made of four specific chemical bases. To keep the ladder stable, the rungs must fit together perfectly; only certain pieces can click into others.
📖 Definitions
🔍 Worked example — Easy
Identify the four nitrogenous bases found in a DNA molecule.
🔍 Worked example — Standard
A single strand of DNA has the following sequence of bases: G - A - C - T - T - A. Determine the sequence of the complementary strand.
🔍 Worked example — Exam-style
A DNA molecule is composed of nucleotides. List the three chemical components that make up a single nucleotide.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 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 solution starts as a pale blue color. After adding the reagent, the solution turns purple. State what this indicates about the contents of the solution.
🔍 Worked example — Standard
A sample of food is tested with Benedict's solution. The initial color is blue. After heating in a water bath, the color changes to brick-red. Identify the type of molecule present and describe the necessary conditions for this test.
🔍 Worked example — Exam-style
A student is given a mystery liquid. They perform three tests: 1) Iodine solution turns blue-black. 2) Biuret test turns purple. 3) Ethanol emulsion test forms a white emulsion. Identify the three types of biological molecules present.
⚠️ Common errors
Common mistakes
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].