Pearson Edexcel International GCSE Human Biology 2: Biological molecules

Biological molecules provide structure, energy storage and the chemistry of metabolism. Pearson Edexcel International GCSE Human Biology (4HB1) Topic 2 covers carbohydrate, protein and lipid composition; their smaller units; food tests; vitamin C and food-energy investigations; enzyme action and four controlling factors; and immobilised-enzyme applications. This theory chapter explains molecular and enzyme principles plus how evidence is interpreted. The practical hub owns executable protocols, safety, apparatus, variables, data processing and evaluation.

1. Elements and molecular groups

Carbohydrates contain carbon, hydrogen and oxygen. Lipids also contain carbon, hydrogen and oxygen, but their elemental proportions differ from carbohydrates. Proteins contain carbon, hydrogen, oxygen and nitrogen, and some contain sulfur.

These elemental descriptions do not mean the molecules have identical structures. Structure determines solubility, energy density, catalytic behavior and biological function. Avoid defining a food group only by the elements it contains.

Water is also biologically essential, serving as a solvent, transport medium and reactant, but the official molecular-building emphasis here is carbohydrates, proteins and lipids.

2. Large molecules and smaller units

Simple sugars are smaller carbohydrate units. Starch and glycogen are large carbohydrate molecules built from many glucose units. Starch is a plant storage carbohydrate; glycogen is the main storage carbohydrate in animals and is stored notably in liver and muscle.

Proteins are polypeptides built from amino acids. The amino-acid sequence influences folding and therefore function. Proteins include enzymes, receptors, antibodies, transport proteins and structural components.

Lipids are formed from glycerol and fatty acids. Many familiar fats and oils are triglycerides, with three fatty acids joined to one glycerol. Lipids store energy, contribute to membranes, provide insulation and protect organs.

Digestion breaks large dietary molecules into absorbable smaller units, while cells can assemble smaller units into larger molecules. Do not describe starch as a single glucose molecule or protein as “made of enzymes”; enzymes are themselves mostly proteins.

3. Food-test evidence

Food tests detect chemical groups through observable changes. Required interpretations include:

• reducing sugar such as glucose: Benedict's reagent changes from blue toward green, yellow, orange or brick-red precipitate after controlled heating, depending on amount;
• starch: iodine solution changes from orange-brown to blue-black;