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.

Learning Objectives¶

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).

• 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 water bath at 60–70 °C for five minutes. 3. A positive result (glucose present) turns the solution brick red. 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. When 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.

Effect of temperature: - Increasing temperature increases reaction rate up to the optimum (around 37 °C for most human enzymes). - 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. - At pH values far above or below the optimum, the forces holding the protein structure together are disrupted. The active site shape changes, the enzyme is denatured, and activity is lost.

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.
• Denaturation: Denaturation changes the shape of the active site permanently. 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; used in energy storage and release.
• Protein: a biological molecule made from amino acids; contains carbon, hydrogen, oxygen and nitrogen.
• Lipid: a biological molecule made from fatty acids and glycerol; used for energy storage and insulation.
• 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 carbohydrate made from many glucose units, used as an energy store in animals and fungi.
• Starch: a carbohydrate made from many glucose units, used as an energy store in plants.

Connected Pages¶

• 2b Cell Structure
• 2d Movement of Substances into and out of Cells
• 2e Nutrition
• IGCSE Biology Key Terms Glossary
• Practical Investigation Reference