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2c Biological Molecules

Part of 2 Structure and Functions in Living Organisms.

Biological molecules supply energy, build structures and allow reactions to happen at useful rates. The main focus here is the relationship between basic chemical building blocks, food tests and enzyme function.

What You Need to Learn

Further detail: Pearson Edexcel International GCSE Biology specification.

On this page you'll learn about carbohydrates, and proteins and lipids and food tests. You'll also cover enzymes and the active site. The notes bring these ideas together into one clear overview of biological molecules.

Carbohydrates, Proteins and Lipids

All three main groups of biological molecules contain carbon, hydrogen and oxygen. Proteins are distinguished by also containing nitrogen (and sometimes sulfur and phosphorus). These three groups are often described as macromolecules because they are large biological molecules.

A polymer is a large molecule made from many repeating smaller units joined together. In this topic, carbohydrates such as starch and glycogen, and proteins, are polymers. Lipids are also large molecules, but they are built differently from polymers.

  • Carbohydrates are polymers that break down into simple sugars. Examples include starch (the main storage carbohydrate in plants) and glycogen (the storage carbohydrate in animals and fungi). Glucose is the simple sugar unit released when these are digested.
  • Proteins are polymers made from amino acids joined together. The specific sequence of amino acids determines each protein's shape and function. Proteins are used for growth, repair and making enzymes.
  • Lipids (fats and oils) are large molecules made from one glycerol molecule joined to three fatty acid molecules. They are used for energy storage and insulation.

Food Tests

The following practical tests detect specific biological molecules:

Test for glucose (Benedict's test):

  1. Add Benedict's solution to the sample in a test tube.
  2. Heat in a hot water bath at 60–70 °C for five minutes.
  3. A positive result (glucose present) produces an orange or brick-red precipitate. The mixture becomes cloudy rather than staying clear. If glucose is absent, the solution remains blue.

Test for starch (iodine test):

  1. Add drops of iodine solution to the sample on a tile or in a well.
  2. A positive result (starch present) turns the solution blue-black. If starch is absent, the solution remains brown/orange.

Test for protein (Biuret test):

  1. Add Biuret solution to the sample.
  2. Leave for one minute.
  3. A positive result (protein present) turns the solution purple. If protein is absent, the solution remains blue.

Test for fat (ethanol emulsion test):

  1. Add 2 cm³ of ethanol to the sample.
  2. Add 2 cm³ of distilled water.
  3. A positive result (fat present) produces a milky white emulsion. If fat is absent, the solution remains colourless.

The value of these tests is not just memorising colours. They let you link invisible chemical content to observed evidence from a sample.

Enzymes and the Active Site

Enzymes are proteins that act as biological catalysts — they speed up chemical reactions in cells without being used up themselves. Because each enzyme is a protein, its three-dimensional shape is critical to its function.

Lock and Key model: Each enzyme has a uniquely shaped active site that is complementary in shape to a specific substrate. Specific means that an enzyme normally works with one substrate, or a very small group of closely related substrates. Complementary means that the shapes fit together. When the substrate binds to the active site, an enzyme-substrate complex forms, the reaction is catalysed, and products are released from the enzyme. The enzyme is then free to catalyse the same reaction again.

Use the interactive below to watch the lock-and-key model step by step, from substrate binding to product release. It keeps the focus on the enzyme-substrate complex and the idea that the enzyme can be reused. Open full interactive.

Effect of temperature:

  • Increasing temperature increases reaction rate up to the optimum (around 37 °C for most human enzymes). The particles have more kinetic energy, so enzyme and substrate molecules collide more often and more enzyme-substrate complexes form each second.
  • Above the optimum, the bonds holding the enzyme in shape begin to break. The active site changes shape so the substrate no longer fits. The enzyme is said to be denatured and activity stops.

Effect of pH:

  • Most enzymes have an optimum pH of around 7, though some that work in acidic environments (such as the stomach enzyme pepsin) have a lower optimum.
  • Moving above or below the optimum pH reduces activity because the active site changes shape and fewer substrate molecules fit correctly.
  • At more extreme pH values, the forces holding the protein structure together are disrupted further. The active site shape changes enough for the enzyme to become denatured and lose activity.

Use the interactive below to compare the two rate-factor graphs you need here. This version keeps the focus on temperature and pH only, so the key thing to notice is that temperature first raises collision rate before denaturation takes over, whereas pH changes the active-site shape on either side of the optimum. Open full interactive.

Investigating enzyme activity in practical work: Amylase can be used to investigate how temperature or pH affects enzyme activity. Amylase breaks down starch, so iodine solution is used to monitor starch disappearance — the solution changes from blue-black (starch present) to orange-brown (starch absent). The time taken for starch to disappear is a measure of enzyme activity: shorter time = higher activity.

Common Confusions

  • Catalyst vs reactant: A catalyst speeds a reaction up without being used up by the reaction.
  • Specific vs complementary: Specific describes which substrate an enzyme works on; complementary describes how the substrate shape matches the active site.
  • Substrate binding: The substrate binds to the active site, not just vaguely to the enzyme as a whole.
  • Benedict's test: A positive Benedict's result requires heating and gives a precipitate, not just a slight colour tint.
  • Denaturation: Denaturation changes the shape of the active site. It does not mean the enzyme simply slows down for no structural reason.
  • Starch vs glucose: Starch is a polymer made from many glucose units. Only glucose gives a positive Benedict's test; starch does not.

Key Terms

  • Carbohydrate: a biological molecule made from carbon, hydrogen and oxygen.
  • Protein: a biological molecule made from amino acids.
  • Lipid: a biological molecule made from fatty acids and glycerol.
  • Amino acid: the monomer building block from which proteins are assembled.
  • Enzyme: a protein that acts as a biological catalyst, speeding up reactions without being used up.
  • Active site: the region of an enzyme that is complementary in shape to a specific substrate.
  • Substrate: the molecule that binds to an enzyme's active site and is converted into products.
  • Enzyme-substrate complex: the temporary combination formed when a substrate binds to the active site.
  • Denature: to irreversibly change an enzyme's shape so it can no longer catalyse reactions.
  • Optimum: the temperature or pH at which an enzyme works at its fastest rate.
  • Glycogen: a storage carbohydrate in animals and fungi made from many glucose units.
  • Starch: a storage carbohydrate in plants made from many glucose units.

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