Biological Molecules & Enzymes

IGCSE Edexcel Biology
2.7–2.13 Biological molecules, food tests, and enzymes
Key Concepts: The main biological molecules are carbohydrates, proteins, and lipids. Enzymes are biological catalysts made of protein. Their activity is affected by temperature and pH.

Section A — Biological Molecules

1. Complete the table for the three main biological molecules. [9]
Molecule Building blocks (monomers) Elements present Function(s) in the body
Carbohydrates
Proteins
Lipids (fats/oils)
2. State the difference between a simple sugar (monosaccharide) and a complex carbohydrate (polysaccharide). [2]
3. Name the molecule formed when two glucose molecules join together. [1]

Section B — Food Tests

4. Complete the table for food tests. [10]
Food substance Reagent used Positive result (colour change)
Reducing sugars (e.g. glucose)
Starch
Protein
Lipids (fats)
Vitamin C
5. A student tests a food sample with Benedict's reagent and gets a blue colour. What does this tell the student about the food? [1]
6. Describe how you would test a food sample for protein, including safety precautions. [4]

Section C — Enzymes

7. Define the term enzyme. [2]
8. Explain the lock-and-key model of enzyme action. [3]
9. Explain the effect of increasing temperature on enzyme activity, referring to the optimum temperature and what happens above it. [4]
10. Sketch a graph showing how enzyme activity changes with pH. Mark the optimum pH. Label the axes. [3]
11. Name three digestive enzymes and state the substrate each breaks down. [6]
EnzymeSubstrate
12. Explain why enzymes are described as specific. [2]

Total marks: 47

Mark Scheme

1. Carbohydrates: glucose/monosaccharides; C, H, O; energy source, energy store (starch/glycogen), structural (cellulose); Proteins: amino acids; C, H, O, N (S); enzymes/hormones/antibodies/structural; Lipids: fatty acids + glycerol; C, H, O; energy store, thermal insulation, cell membranes [9 — 3 per row]
2. A monosaccharide is a single sugar unit (e.g. glucose); a polysaccharide is made of many monosaccharide units joined together (e.g. starch, glycogen, cellulose) [2]
3. Maltose (a disaccharide) [1]
4. Reducing sugars: Benedict's reagent, blue → brick red/orange (heated); Starch: iodine solution, yellow-brown → blue-black; Protein: Biuret reagent, blue → purple/violet; Lipids: ethanol emulsion test, white milky emulsion forms (or Sudan III → red); Vitamin C: DCPIP, blue → colourless [10 — 2 per row]
5. No reducing sugars present in the sample (negative result) [1]
6. Add Biuret reagent (dilute NaOH + dilute copper sulfate / Biuret solution) to the food sample [1]; purple/violet colour indicates protein present [1]; wear eye protection [1]; handle NaOH with care as it is corrosive [1] [4]
7. A biological catalyst [1] made of protein [1] that speeds up chemical reactions without being used up [2]
8. The enzyme has a specific active site [1]; the substrate (reactant) fits into the active site like a key into a lock [1]; the enzyme-substrate complex forms, then the product is released and the enzyme is unchanged [1] [3]
9. As temperature increases, enzyme and substrate molecules have more kinetic energy [1]; more frequent/successful collisions → rate increases up to the optimum [1]; above the optimum, the enzyme is denatured [1]; the shape of the active site changes so the substrate can no longer fit [1] [4]
10. Bell-shaped curve; x-axis = pH, y-axis = enzyme activity / rate of reaction; optimum clearly marked at peak; activity falls on either side [3]
11. Any three correct pairs: amylase — starch; protease — protein; lipase — lipid/fat; maltase — maltose; pepsin — protein; sucrase — sucrose [6 — 2 per correct pair]
12. Each enzyme has an active site with a specific shape [1]; only one type of substrate (complementary shape) can fit into the active site [1] [2]