Biological Molecules and Enzymes

IGCSE Edexcel Biology
2.7 Biological Molecules
2.8 Structure of Molecules
2.10–2.13 Enzymes
Key Concepts: Carbohydrates, proteins, and lipids are biological molecules made from specific elements and building blocks. Enzymes are protein catalysts and their activity is affected by temperature and pH.

Section A: Biological Molecules

1. State the elements found in carbohydrates, proteins and lipids. [4]
2. Describe how the following molecules are built from their subunits. [6]

a) Starch and glycogen from simple sugars

b) Protein from amino acids

c) Lipid from fatty acids and glycerol

Section B: Enzymes as Biological Catalysts

3. Define an enzyme and explain its role in metabolic reactions. [3]
4. Describe how temperature affects enzyme function. [4]
5. Describe how pH affects enzyme function. [3]
Total marks: 20

Mark Scheme

1. Carbohydrates: carbon, hydrogen, oxygen [1]; Proteins: carbon, hydrogen, oxygen, nitrogen (and sometimes sulfur) [1]; Lipids: carbon, hydrogen, oxygen [1]; Allow any correct element listing [4]
2. a) Many glucose/simple sugars joined to form starch/glycogen [2]; b) Many amino acids joined to form proteins [2]; c) Glycerol + fatty acids form lipid [2]
3. Enzyme is a protein catalyst [1]; speeds up metabolic reactions [1]; without being used up [1]
4. Rate increases with temperature up to an optimum [2]; above optimum enzyme denatures/active site changes shape so activity falls [2]
5. Each enzyme has an optimum pH [1]; pH change alters active site shape [1]; extreme pH reduces/halts activity [1]