SACE Biology — Stage 2
Enzymes — Flashcards & Quiz
Enzymes are protein catalysts that speed up biochemical reactions by lowering activation energy. SACE Biology Stage 2 expects you to describe enzyme mechanism using the induced-fit model, explain how temperature, pH and substrate concentration affect activity, and distinguish competitive from non-competitive inhibition. Graph interpretation is a frequent exam skill.
Key Points
- Enzymes are globular proteins that catalyse reactions by lowering activation energy — they are not consumed and can be reused.
- Active site fits a specific substrate (lock-and-key); induced-fit model shows the site adjusts slightly for a better fit.
- Factors affecting activity: temperature (optimum ~37°C for mammals; denatures above that), pH (optimum varies by enzyme), substrate concentration (plateaus at Vₘₐₓ).
- Competitive inhibitors bind the active site and compete with substrate; effect reversed by increasing substrate concentration.
- Non-competitive inhibitors bind an allosteric site, changing the enzyme's shape; effect not reversed by adding more substrate.
- Denaturation: high temperature or extreme pH breaks the tertiary structure irreversibly, destroying the active site.
Common Mistakes to Avoid
- Stating enzymes are used up in reactions — they are catalysts and are reused.
- Confusing denaturation (shape change, irreversible) with inhibition (can be reversible).
- Mixing up competitive (active site) and non-competitive (allosteric) inhibitors.
- Claiming enzyme activity increases indefinitely with temperature — there's an optimum, then denaturation.
- Forgetting that each enzyme has a specific optimum pH — e.g. pepsin (pH 2) vs trypsin (pH 8).
Exam Strategy
SACE Stage 2 enzyme questions usually give you a graph (activity vs temperature, pH, or substrate concentration) and ask you to interpret or explain the trend. Method: (1) identify the axis and variable, (2) describe the shape (increase, optimum, plateau), (3) explain in terms of enzyme structure (induced fit, denaturation, saturation). For inhibitor questions, always state competitive vs non-competitive and how the graph shifts.
Sample Flashcards
Q1: Explain how enzymes function as biological catalysts.
Enzymes are proteins that lower the activation energy of biochemical reactions, increasing the reaction rate without being consumed. They have a specific 3D shape with an active site complementary to their substrate (lock-and-key or induced-fit model). The enzyme-substrate complex forms, the reaction proceeds, and products are released.
Q2: What factors affect enzyme activity?
Temperature: increasing temperature increases kinetic energy and reaction rate up to the optimum; beyond this, the enzyme denatures (tertiary structure unfolds). pH: each enzyme has an optimal pH; extreme pH causes denaturation. Substrate concentration: rate increases until all active sites are saturated (Vmax). Enzyme concentration: more enzyme = more active sites = faster rate (if substrate is not limiting).
Sample Quiz Questions
Q1: Enzymes are consumed in the reactions they catalyse.
Answer: FALSE
Enzymes are biological catalysts that are NOT consumed — they lower activation energy, speed up reactions, and are recycled to catalyse further reactions.
Q2: Increasing temperature beyond an enzyme's optimum causes denaturation and loss of function.
Answer: TRUE
Excessive heat disrupts hydrogen bonds and other interactions maintaining the enzyme's tertiary structure, altering the active site shape and preventing substrate binding.
Revision Tip
Enzyme graphs are pattern recognition — build a Revizi deck with graphs for each factor (temperature, pH, substrate, inhibitor types) and drill the interpretation until automatic.
Related Concepts
Last updated: March 2026 · 2 flashcards · 2 quiz questions