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: How do enzymes work, and what is the induced-fit model?
Enzymes are proteins that act as biological catalysts: they speed up reactions by lowering the activation energy, without being used up. Each enzyme is specific to its substrate because of the shape of its active site. In the induced-fit model, the active site is not a rigid match; when the substrate binds, the enzyme changes shape slightly to fit it more closely, forming an enzyme–substrate complex that puts strain on bonds and helps the reaction occur.
Q2: How do temperature, pH, and enzyme and substrate concentration affect enzyme activity?
Temperature: activity increases up to an optimum as molecules collide more often; above the optimum, the enzyme denatures — its active site changes shape and activity falls sharply. pH: each enzyme has an optimum pH; values away from it alter bonds in the protein and can denature it. Substrate concentration: rate increases until all active sites are occupied (saturation), then levels off. Enzyme concentration: more enzyme increases the rate if substrate is in excess.
Q3: How do enzyme inhibitors work, and how are they used?
Inhibitors reduce enzyme activity. Some compete with the substrate for the active site (competitive inhibitors); others bind elsewhere and change the shape of the active site (non-competitive inhibitors). Understanding inhibitors helps scientists design solutions to problems: the herbicide glyphosate inhibits an enzyme plants need to make certain amino acids, and the drug ritonavir inhibits an HIV enzyme (protease) the virus needs to mature. Some chemotherapy drugs also target enzymes.
Sample Quiz Questions
Q1: Enzymes speed up reactions by increasing the activation energy required.
Answer: FALSE
Enzymes lower the activation energy, which allows reactions to proceed faster at body temperature.
Q2: In the induced-fit model, the active site changes shape slightly when the substrate binds.
Answer: TRUE
The enzyme moulds around the substrate to form a closer fit, unlike the rigid lock-and-key model.
Q3: Increasing substrate concentration always increases the rate of an enzyme-controlled reaction.
Answer: FALSE
The rate levels off once all active sites are occupied; enzyme concentration then becomes the limiting factor.
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: 9 October 2026 · 3 sample flashcards · 3 sample quiz questions