# Biological Molecules, Food Tests and DNA Structure

> Biology · CIE IGCSE 0610
> Source: https://www.owlsprep.com/study/cie-0610-u3-biological-molecules-food-tests-and/

This guide covers core biological molecules, required food tests, and DNA structure for CIE IGCSE Biology 0610, with clearly labelled extended content for higher tier candidates. You will learn to apply these concepts to structured exam questions.

**Prerequisites:** [Basic cell structure and function](https://www.owlsprep.com/study/cie-0610-u2-cell-structure/); [Basic scientific practical skills](https://www.owlsprep.com/study/cie-0610-u1-practical-skills/)

## Learning objectives

- Identify the chemical elements present in carbohydrates, proteins and lipids
- Carry out and interpret results for all required IGCSE food tests
- Describe the double helix structure of DNA and complementary base pairing rules
- Extended: Recall the monomer subunits of each biological molecule class

## Core: Key Biological Molecules

All living organisms are made of four main classes of biological molecules: carbohydrates, proteins, lipids, and nucleic acids. Each class has a unique set of chemical elements and core functions in cells.

**Biological Molecules** — Large complex molecules synthesized by living cells, that carry out all essential functions for life

- Carbohydrates: made of carbon, hydrogen, oxygen (C, H, O); function as short-term energy stores
- Proteins: made of carbon, hydrogen, oxygen, nitrogen (C, H, O, N); some contain sulfur; function as structural components, enzymes, and antibodies
- Lipids: made of carbon, hydrogen, oxygen (C, H, O); function as long-term energy stores, insulation, and cell membrane components

**Worked example:** An unknown biological molecule contains nitrogen but no sulfur. Name the class of molecule it belongs to, and state one function of this class.

1. Step 1: Recall the element composition of each molecule class. Carbohydrates and lipids only contain C, H, O, so they are eliminated.
2. Step 2: Proteins contain C, H, O, N, and sometimes sulfur. The unknown molecule has nitrogen but no sulfur, so it is a protein.
3. Step 3: One valid function of proteins is acting as structural components of muscle tissue, or acting as biological catalysts (enzymes).

> **Exam tip:** If a question mentions nitrogen in a biological molecule, it is almost always a protein or DNA for IGCSE level; lipids and carbohydrates never contain nitrogen.

## Core: Required Food Tests

You are required to know the reagents, conditions and positive results for five core food tests for the 0610 exam. These are frequently tested in both practical and theory papers, for 2-4 marks per question.

| Molecule Tested | Reagent Used | Conditions | Positive Result |
| --- | --- | --- | --- |
| Reducing sugar (e.g. glucose) | Benedict's solution | Heat to 80°C in water bath | Blue → brick red precipitate |
| Starch | Iodine solution | Room temperature | Brown → blue-black |
| Protein | Biuret reagent | Room temperature | Blue → lilac/purple |
| Lipid (fat) | Ethanol + distilled water | Room temperature, shake well | Colourless → milky white emulsion |
| Vitamin C (ascorbic acid) | DCPIP solution (blue) | Add sample dropwise at room temperature | Blue → decolourised (goes colourless) |

> **tip**
>
> Always state that you cannot confirm the presence of untested molecules in food test questions; examiners frequently award 1 mark for this point.

**Worked example:** A student tests apple juice with Benedict's solution (heated) and observes an orange colour change. They test a separate sample with iodine solution and see no colour change. State which molecules are present and absent, explaining your answer.

1. Step 1: Interpret the Benedict's test result: an orange colour change is a positive result for reducing sugar, so reducing sugar is present.
2. Step 2: Interpret the iodine test result: no colour change (remains brown) is a negative result for starch, so starch is absent.
3. Step 3: No tests were carried out for protein or lipid, so you cannot conclude whether they are present or absent in the sample.

> **Exam tip:** Never refer to a positive lipid test result as a precipitate; you must use the term 'milky white emulsion' to get full marks.

## Core: DNA Structure

DNA (deoxyribonucleic acid) is the genetic material found in the nucleus of all eukaryotic cells. It carries the genetic code that determines all inherited characteristics of an organism.

**DNA Double Helix** — The twisted ladder shape of DNA, made of two anti-parallel strands linked by pairs of nitrogenous bases

- The two strands are held together by complementary base pairs: Adenine (A) always pairs with Thymine (T), Cytosine (C) always pairs with Guanine (G)
- Each strand has a sugar-phosphate backbone attached to the nitrogenous bases

**Worked example:** One strand of DNA has the base sequence A T C G G A. Write the complementary base sequence for the opposite strand.

1. Step 1: Recall complementary base pairing rules: A pairs with T, C pairs with G.
2. Step 2: Match each base on the given strand to its complement: A→T, T→A, C→G, G→C, G→C, A→T
3. Step 3: The complementary sequence is T A G C C T.

> **Exam tip:** Base pairing questions are almost always included in exam papers for 1-2 easy marks; write down A-T and C-G at the start of the question to avoid mixing up pairs.

## Extended Only: Biological Molecule Subunits

Extended candidates need to know the subunit structure of each biological molecule class, and that most large biological molecules are polymers made of smaller repeating monomers.

**Monomer** — Small, repeating subunit that bonds together with other identical subunits to form a larger polymer molecule

- Carbohydrate polymers (e.g. starch, glycogen) are made of glucose monomers
- Protein polymers are made of amino acid monomers (20 different amino acids exist in living organisms)
- Lipids are made of 1 glycerol molecule bonded to 3 fatty acid molecules
- DNA is a polymer made of nucleotide monomers, each containing a sugar, phosphate group and nitrogenous base

**Worked example:** Name the monomers that make up proteins, and state how many different types of this monomer exist naturally in living cells.

1. Step 1: Recall that proteins are polymers made of amino acid monomers.
2. Step 2: There are 20 different naturally occurring amino acids used to synthesize proteins in living organisms.

## Common pitfalls

- **Wrong:** Stating Benedict's test detects all sugars
  - Why it fails: Benedict's only detects reducing sugars like glucose; sucrose is a non-reducing sugar and gives a negative result unless first hydrolyzed
  - Correct: Always specify 'reducing sugar' when describing Benedict's test positive results
- **Wrong:** Claiming lipids contain nitrogen
  - Why it fails: Lipids only contain C, H, O; only proteins and nucleic acids contain nitrogen
  - Correct: Only link nitrogen to proteins or DNA in biological molecule element questions
- **Wrong:** Pairing Adenine with Guanine or Cytosine in DNA sequences
  - Why it fails: Complementary base pairing rules strictly state A pairs with T, C pairs with G
  - Correct: Write down A-T and C-G at the start of base pairing questions to avoid mixing up pairs
- **Wrong:** Describing positive lipid test results as a precipitate
  - Why it fails: The lipid test produces a stable emulsion, not a solid precipitate
  - Correct: Always use the term 'milky white emulsion' for positive lipid test results to get full marks
- **Wrong:** Stating carbohydrates are made of amino acid monomers (Extended only)
  - Why it fails: Carbohydrate monomers are glucose; amino acids are the monomers of proteins
  - Correct: Map each polymer to its specific monomer when answering extended level questions

## Cheatsheet

| Concept | Core Requirement | Extended Requirement |
| --- | --- | --- |
| Carbohydrates | Elements: C, H, O; energy store | Monomer = glucose |
| Proteins | Elements: C, H, O, N (S optional); structural/immune function | Monomer = amino acids (20 types) |
| Lipids | Elements: C, H, O; long-term energy store | Made of glycerol + 3 fatty acids |
| Benedict's test | Heat to 80°C; positive = brick red precipitate | Same as core + non-reducing sugar hydrolysis prep |
| Iodine test | Positive = blue-black colour change | Same as core |
| Biuret test | Positive = lilac/purple colour change | Same as core |
| Lipid test | Positive = milky white emulsion | Same as core |
| DCPIP test (vitamin C) | Positive = blue DCPIP decolourised (goes colourless) | Same as core |
| DNA Structure | Double helix, A-T / C-G base pairing | Polymer of nucleotide monomers |

## What's next

Now that you have mastered biological molecules, food tests and DNA structure, the next step is to study enzyme function, the second key part of Unit 3. Enzymes are protein molecules that catalyze all biological reactions in cells, and are a high-frequency exam topic for both Core and Extended papers. You will learn how temperature and pH affect enzyme activity, and how to interpret experimental data on enzyme reaction rates, which is commonly tested in 3-5 mark structured questions. You can also practice applying your food test knowledge to practical exam questions, which often make up 10-15% of Paper 3/4 marks.

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