# Biological Molecules

> Edexcel International GCSE Biology · 4BI1 2017 Spec (Issue 3)
> Source: https://www.owlsprep.com/study/edexcel-igcse-biology-s2-biological-molecules/

This guide covers all core content for biological molecules in Edexcel IGCSE Biology (4BI1), including molecular composition, food test practicals, enzyme function, and factors affecting enzyme activity, aligned to spec points 2.7–2.14B.

**Prerequisites:** [Basic cell structure (Edexcel IGCSE Biology S2_T01)](https://www.owlsprep.com/study/edexcel-igcse-biology-s2-cell-structure/); [Core practical biology skills](https://www.owlsprep.com/study/edexcel-igcse-biology-practical-skills/)

## Learning objectives

- Identify the chemical elements present in carbohydrates, proteins and lipids
- Describe the structure of large biological molecules from their basic sub-units
- Recall and interpret results of core food tests for glucose, starch, protein and fat
- Explain the role of enzymes as biological catalysts in metabolic reactions
- Analyse how temperature and pH affect enzyme activity, including the process of denaturation
- Describe practical methods for investigating enzyme activity changes with temperature and pH

## Composition and Structure of Biological Molecules

All living organisms are made of organic biological molecules, grouped into four main classes. The three assessed in this topic are carbohydrates, proteins and lipids, each made of specific chemical elements and built from smaller basic sub-units.

**Biological macromolecules** — Large organic molecules found in all living cells, formed by joining smaller repeating sub-units together.

| Molecule type | Elements present | Basic sub-units | Large molecule examples |
| --- | --- | --- | --- |
| Carbohydrate | Carbon, Hydrogen, Oxygen | Simple sugars | Starch, Glycogen |
| Lipid | Carbon, Hydrogen, Oxygen | Fatty acids + Glycerol | Fats, Oils |
| Protein | Carbon, Hydrogen, Oxygen, Nitrogen (sometimes Sulfur) | Amino acids | Enzymes, Antibodies |

**Worked example:** An unknown biological molecule contains carbon, hydrogen, oxygen and nitrogen. Identify the molecule type and its basic sub-units.

1. Step 1: Match the elements to the correct molecule group. Carbohydrates and lipids only contain C, H, O, so the molecule must be a protein.
2. Step 2: Recall that proteins are built from amino acid sub-units.

> **Exam tip:** Never state that lipids contain nitrogen: this is one of the most common mark-losing errors for this topic. Only proteins contain nitrogen (and sometimes sulfur) alongside C, H, O.

## Core Food Test Practicals

You will be assessed on your recall of reagents, methods and colour changes for four core food tests, as well as interpretation of results for unknown samples. All results are strictly defined in the mark scheme, so exact wording is required for full marks.

| Molecule tested | Reagent(s) | Method | Positive result | Negative result |
| --- | --- | --- | --- | --- |
| Starch | Iodine solution | Add 2–3 drops of iodine to the food sample | Blue-black colour | Stays orange/brown |
| Reducing sugar (glucose) | Benedict's solution | Add equal volume of Benedict's to sample, heat in 80°C water bath for 5 minutes | Blue → green → yellow → orange → brick red (darker = more sugar) | Stays blue |
| Protein | Biuret reagent (sodium hydroxide + copper(II) sulfate) | Add 1cm³ sodium hydroxide, then 2 drops copper(II) sulfate, shake gently | Blue → purple/lilac/mauve | Stays blue |
| Fat | Ethanol + water / filter paper | Emulsion test: add ethanol to sample, shake, add equal volume of water OR dab sample on filter paper, dry | White/cloudy emulsion / permanent translucent grease spot | Stays clear / no grease spot |

**Worked example:** A student tests a milk sample with Benedict's (heated), Biuret and iodine. They observe a brick red colour with Benedict's, purple with Biuret, and no colour change with iodine. State which molecules are present/absent.

1. 1. Brick red Benedict's result = glucose (reducing sugar) is present.
2. 2. Purple Biuret result = protein is present.
3. 3. No colour change with iodine = starch is absent.

> **tip**
>
> Always specify heating in a water bath for the Benedict's test: stating 'heat over a Bunsen' will lose marks, as this is unsafe and can boil the solution over.

*Calculator:* allowed

## Enzymes as Biological Catalysts

Enzymes are protein molecules that act as biological catalysts for all metabolic reactions in living organisms. A catalyst speeds up the rate of a reaction without being used up or changed in the process, so enzymes can be reused multiple times.

**Active site** — The specific region on an enzyme molecule where the substrate binds, with a shape complementary to the substrate (lock-and-key model).

*Example:* The enzyme amylase has an active site that only fits starch molecules, so it only catalyses starch breakdown.

Each enzyme is specific to one substrate, because only that substrate fits the shape of its active site. Enzymes lower the amount of energy needed for a reaction to occur, so reactions happen fast enough to support life processes.

**Worked example:** Explain why the enzyme catalase only breaks down hydrogen peroxide, and no other molecules.

1. Step 1: State that enzymes are specific to their substrate.
2. Step 2: Link specificity to active site shape: catalase has an active site with a shape that is only complementary to hydrogen peroxide, so no other substrate can bind.

> **Exam tip:** Never refer to enzymes as 'living' or 'killed' in exam answers: enzymes are protein molecules, not organisms, so these terms are not accepted. Use the term 'denatured' for permanent loss of function.

## Effect of Temperature on Enzyme Activity

Temperature follows a predictable pattern of effect on enzyme activity, with three distinct regions of activity as temperature changes:

- **Low temperatures**: Enzyme and substrate molecules move slowly, so there are fewer successful collisions between active sites and substrates. Reaction rate is low, but enzymes are not denatured.
- **Optimum temperature**: The temperature at which enzyme activity is highest (around 37°C for most human enzymes). Molecules move fast, so there are the maximum number of successful collisions, with no damage to enzyme structure.
- **Above optimum temperature**: High heat breaks the bonds holding the enzyme's shape together, so the active site changes shape permanently (denaturation). The substrate can no longer fit, so reaction rate drops rapidly to zero.

**Worked example:** A student investigates amylase activity at 10°C, 37°C and 80°C. Predict the relative reaction rate at each temperature, explaining your answer.

1. 10°C: Low rate. Molecules move slowly, so few collisions between amylase and starch. Amylase is not denatured.
2. 37°C: Highest rate. This is the optimum temperature for human amylase, so maximum successful collisions occur, with no denaturation.
3. 80°C: Zero rate. High temperature denatures amylase, so the active site shape changes, and starch can no longer bind.

Practical investigation: To test temperature effect, you can measure how quickly amylase breaks down starch at different temperatures, using iodine to test for starch presence at 1-minute intervals. The time taken for iodine to stay orange/brown (no starch left) is used to calculate reaction rate.

> **warning**
>
> Low temperatures do NOT denature enzymes, only slow them down: this is one of the most frequently tested misconceptions for this topic.

## Effect of pH on Enzyme Activity

pH measures how acidic or alkaline a solution is. Each enzyme has an optimum pH at which its activity is highest. Extremes of pH (too acidic or too alkaline) denature enzymes by altering the shape of the active site permanently, so reaction rate drops to zero.

For example, pepsin (a protein-digesting enzyme in the acidic stomach) has an optimum pH of ~2, while lipase (a fat-digesting enzyme in the alkaline small intestine) has an optimum pH of ~8.

**Worked example:** Pepsin is added to a protein solution at pH 10. Explain why no reaction occurs.

1. Step 1: Recall pepsin's optimum pH is ~2, very acidic.
2. Step 2: pH 10 is far too alkaline for pepsin, so it denatures: the active site shape changes, so protein molecules cannot bind.

> **note**
>
> **Biology-only (Paper 2)**: You must be able to describe the practical to investigate pH effect on enzyme activity: use buffer solutions to maintain fixed pH levels, test rate of starch breakdown by amylase (using iodine) or protein breakdown by pepsin. This is not assessed for Double Award students.

## Common pitfalls

- **Wrong:** Stating lipids contain nitrogen.
  - Why it fails: Lipids only contain carbon, hydrogen and oxygen. Nitrogen is only present in proteins and nucleic acids (not assessed here).
  - Correct: Only state that proteins contain nitrogen (and sometimes sulfur) alongside C, H, O.
- **Wrong:** Claiming low temperatures denature enzymes.
  - Why it fails: Low temperatures only slow molecular movement, reducing collision frequency. The active site shape remains intact, so enzymes are not denatured.
  - Correct: Only state that high temperatures and extreme pH cause denaturation; low temperatures only inactivate enzymes temporarily.
- **Wrong:** Referring to enzymes as 'killed' or 'alive'.
  - Why it fails: Enzymes are non-living protein molecules, so terms referring to life or death are not accepted by the mark scheme.
  - Correct: Use the term 'denatured' to describe when an enzyme's active site changes shape permanently and stops working.
- **Wrong:** Forgetting to specify heating in a water bath for the Benedict's test.
  - Why it fails: Heating is a required step for the Benedict's colour change, and specifying a water bath (not Bunsen heating) is required for full practical marks.
  - Correct: Always include 'heat in an 80°C water bath' when describing the Benedict's test method.
- **Wrong:** Stating the Biuret test requires heating.
  - Why it fails: The Biuret test for protein produces a colour change at room temperature, so no heating is needed.
  - Correct: Only mention heating for the Benedict's test, not the iodine, Biuret or emulsion food tests.

## Cheatsheet

| Topic | Exam Key Fact |
| --- | --- |
| Carbohydrates | C, H, O; made of simple sugars |
| Lipids | C, H, O; made of fatty acids + glycerol |
| Proteins | C, H, O, N (sometimes S); made of amino acids |
| Starch test | Iodine → blue-black positive |
| Glucose test | Benedict's, heat → brick red positive |
| Protein test | Biuret → purple positive |
| Fat test | Ethanol + water → white emulsion positive |
| Enzyme definition | Biological catalyst, protein, specific, not used up |
| Temperature effect | Low = slow, optimum = fastest, high = denatured |
| pH effect | Optimum pH = fastest, extremes = denatured |

## What's next

Now that you have mastered biological molecules and enzyme function, you can move on to the next topics in the *Structure and Functions in Living Organisms* unit. First, you will learn about movement of substances into and out of cells, which relies on enzyme-controlled reactions for active transport processes. Next, you will study human nutrition, where you will learn how these biological molecules are broken down and used by the body, including the role of digestive enzymes. You can also use the practical skills from this guide to revise for practical assessment questions across both Paper 1 and Paper 2, including planning and evaluating enzyme investigation experiments. Be sure to practice past paper questions on this topic to familiarize yourself with exact mark scheme wording for food test and enzyme questions.

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