Enzymes — IGCSE Science - Combined (0653) questions by topic
pH-activity graphs and substrate-active-site matching sit at the heart of this topic: identifying which substrate fits a given enzyme's active site, explaining how pH beyond the optimum affects activity, and naming enzymes as biological catalysts made of protein. Explain, identify and state are the leading command words, and explain wants active-site shape change stated explicitly, not just an activity decrease.
This set holds 10 real questions from 2023 to 2025 papers, worth 1 to 13 marks, so single-word recall sits beside multi-part explanations of an activity-pH graph. Submitting each explanation for instant marking is useful here, since it checks quickly whether both the rise before pH 8 and the fall after it have been described, as the mark scheme requires both.
What examiners look for
- A frequent gap states only the increase before pH 8 or the decrease after it, when both parts of the trend are needed for credit.
- Reports flag confusion between elements, compounds and other substances when identifying what enzymes are made of; proteins is the required answer.
- Enzyme function is often confused with growth and repair, which are functions of proteins generally rather than enzymes specifically.
- Strong answers state that maximum enzyme activity occurs at pH 8 and describe both the rise before this point and the fall after it.
May–June 2025, Paper 11
Enzymes are ......Z...... that function as biological catalysts. What is Z? A fats B proteins C sugars D vitamins
Answer this question and get it marked →May–June 2025, Paper 13
What function as biological catalysts? A antibodies B enzymes C hormones D platelets
Answer this question and get it marked →May–June 2025, Paper 21
Which statement explains how an increase in pH affects enzyme activity? A The active site changes shape. B The substrate is denatured. C There is an increase in the kinetic…
Answer this question and get it marked →May–June 2025, Paper 22
Which statement describes enzyme action in humans? A Enzymes become denatured at 0 °C. B The activity of enzymes always increases with an increasing temperature. C The enzyme…
Answer this question and get it marked →May–June 2025, Paper 23
The graph shows the activity of an enzyme at different temperatures. The curve of enzyme activity (vertical axis) against temperature/°C (horizontal axis, 0 to 60) rises from a…
Answer this question and get it marked →May–June 2025, Paper 61
Invertase is an enzyme. Invertase breaks down the non-reducing sugar sucrose into reducing sugars: non-reducing sugar sucrose --(invertase)-- reducing sugars Benedict's solution…
Answer this question and get it marked →May–June 2024, Paper 62
A student investigates the effect of enzyme concentration on the breakdown of a substance. Catalase is an enzyme found in peas that breaks down hydrogen peroxide into water and…
Answer this question and get it marked →October–November 2024, Paper 42
(a) Fig. 7.1a shows a model of an enzyme (with an active site). Fig. 7.1b shows four possible substrates, A–D, for this enzyme. Identify which substrate, A–D, is the substrate for…
Answer this question and get it marked →October–November 2023, Paper 32
Carbohydrates are part of a balanced diet for humans. Large carbohydrates are digested by enzymes in the alimentary canal. The graph in Fig. 7.1 shows the effect of pH on the…
Answer this question and get it marked →October–November 2023, Paper 33
Carbohydrates are part of a balanced diet for humans. Large carbohydrates are digested by enzymes in the alimentary canal. The graph in Fig. 7.1 shows the effect of pH on the…
Answer this question and get it marked →Other IGCSE Science - Combined topics
- Practical skills (96)
- Motion, forces and energy (53)
- Electricity and magnetism (38)
- Waves (32)
- Human nutrition (23)
- Acids, bases and salts (22)
- Organisms and their environment (21)
- Organic chemistry (21)
- Reproduction (20)
- Thermal physics (20)
- Plant nutrition (19)
- The Periodic Table (18)