LEARNING OBJECTIVES
What you will be able to do
- define catalyst and enzyme
- explain enzyme specificity
- describe enzyme-substrate complexes
- interpret effects of temperature and pH
AT A GLANCE
INTRODUCTION · THE BIG IDEA
Why biological reactions need specific catalysts and how conditions alter their activity.
Most reactions in cells would be too slow at normal temperatures without enzymes. Their three-dimensional active sites make reactions both rapid and selective.
Temperature and pH affect enzyme activity for different reasons: one changes collision rate, while extreme conditions can alter the active site's shape.
SECTION 01
Enzyme action
Enzymes are proteins that act as biological catalysts. Their active site has a three-dimensional shape complementary to a particular substrate.
DETAILED EXPLANATION
- The reaction produces products, which leave; the unchanged enzyme can be reused.
- Specificity arises because only suitable substrates fit the active site.
- Enzymes make metabolic reactions fast enough to sustain life.
SECTION 02
Temperature
At low temperature, molecules have less kinetic energy and fewer collisions are successful. Warming increases collision frequency until the optimum is reached.
DETAILED EXPLANATION
- The active site changes so the substrate no longer fits; this is denaturation.
- Cooling slows an enzyme but usually does not permanently alter its shape.
SECTION 03
pH and investigation
Each enzyme has an optimum pH. Conditions far from it alter bonds and active-site shape, reducing successful binding and possibly denaturing the enzyme.
DETAILED EXPLANATION
- Measure rate using product formed per unit time or time for substrate disappearance.
- Repeat readings and calculate a mean.
STEP-BY-STEP EXAM EXAMPLE
Explaining an enzyme-rate graph
- From 20 °C to 40 °C, increasing kinetic energy produces more frequent successful collisions.
- The maximum rate at 40 °C identifies the optimum under these conditions.
- Above 40 °C, bonds maintaining the enzyme's shape begin to break.
- The altered active site forms fewer enzyme–substrate complexes.
Answer: The rate rises to an optimum, then falls because the enzyme denatures.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Enzymes are reusable protein catalysts.
- Complementary shape explains specificity.
- Temperature affects collisions and structure.
- Extreme temperature or pH can denature an enzyme.
QUICK REVISION CHECKLIST
Can you do each of these without your notes?
- define catalyst and enzyme
- explain enzyme specificity
- describe enzyme-substrate complexes
- interpret effects of temperature and pH