Biological Molecules
Edexcel International GCSE BiologyΒ· 2.7β2.14BΒ· 25 min read
1. Composition and Structure of Biological Moleculesβ β ββββ± 5 min
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 |
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.
2. Core Food Test Practicalsβ β β βββ± 7 min
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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 2β3 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 80Β°C water bath for 5 minutes | Blue β green β yellow β orange β brick red (darker = more sugar) | Stays blue |
Protein | Biuret reagent (sodium hydroxide + copper(II) sulfate) | Add 1cmΒ³ sodium hydroxide, then 2 drops copper(II) sulfate, shake gently | Blue β 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 |
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
- Brick red Benedict's result = glucose (reducing sugar) is present.
- 2
- Purple Biuret result = protein is present.
- 3
- No colour change with iodine = starch is absent.
3. Enzymes as Biological Catalystsβ β β βββ± 4 min
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.
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.
4. Effect of Temperature on Enzyme Activityβ β β β ββ± 5 min
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 37Β°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.
A student investigates amylase activity at 10Β°C, 37Β°C and 80Β°C. Predict the relative reaction rate at each temperature, explaining your answer.
- 1
10Β°C: Low rate. Molecules move slowly, so few collisions between amylase and starch. Amylase is not denatured.
- 2
37Β°C: Highest rate. This is the optimum temperature for human amylase, so maximum successful collisions occur, with no denaturation.
- 3
80Β°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.
5. Effect of pH on Enzyme Activityβ β β β ββ± 4 min
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.
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.
6. Common Pitfalls
Wrong move:
Stating lipids contain nitrogen.
Why:
Lipids only contain carbon, hydrogen and oxygen. Nitrogen is only present in proteins and nucleic acids (not assessed here).
Correct move:
Only state that proteins contain nitrogen (and sometimes sulfur) alongside C, H, O.
Wrong move:
Claiming low temperatures denature enzymes.
Why:
Low temperatures only slow molecular movement, reducing collision frequency. The active site shape remains intact, so enzymes are not denatured.
Correct move:
Only state that high temperatures and extreme pH cause denaturation; low temperatures only inactivate enzymes temporarily.
Wrong move:
Referring to enzymes as 'killed' or 'alive'.
Why:
Enzymes are non-living protein molecules, so terms referring to life or death are not accepted by the mark scheme.
Correct move:
Use the term 'denatured' to describe when an enzyme's active site changes shape permanently and stops working.
Wrong move:
Forgetting to specify heating in a water bath for the Benedict's test.
Why:
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 move:
Always include 'heat in an 80Β°C water bath' when describing the Benedict's test method.
Wrong move:
Stating the Biuret test requires heating.
Why:
The Biuret test for protein produces a colour change at room temperature, so no heating is needed.
Correct move:
Only mention heating for the Benedict's test, not the iodine, Biuret or emulsion food tests.
7. Quick Reference 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 β blue-black positive |
Glucose test | Benedict's, heat β brick red positive |
Protein test | Biuret β purple positive |
Fat test | Ethanol + water β 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 |
8. Frequently Asked
What is the difference between denaturation and low-temperature enzyme inactivation?
Denaturation is a permanent change to the shape of an enzyme's active site caused by extreme temperature or pH, so substrates can no longer bind. Inactivation at low temperatures is temporary: molecules move slower so fewer collisions occur, but the active site shape remains intact, so activity will recover if temperature increases.
Do I need to heat the Biuret test for protein?
No, the Biuret test does not require heating. Only the Benedict's test for reducing sugars requires heating in a water bath to produce a colour change.
Is the enzyme pH practical assessed in Paper 1?
No, the 2.14B enzyme pH practical is a Biology-only topic, so it is only assessed in Paper 2 of the Edexcel IGCSE Biology (4BI1) exam. Double Award students do not need to learn this practical.
Going deeper
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.