5.3.6Unit 5: Respiration, Internal Environment, Coordination and Gene Technology
The Effect of Temperature on Enzyme Reactions — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 6 parts Lesson 30 of 36 Unit quiz
Part 1 of 6Temperature and Reaction Rate
Low temperature
- Molecules have little kinetic energy, so collisions between enzyme and substrate are rare and have too little energy to change bonds.
Rising temperature
- More kinetic energy gives more frequent, more energetic collisions,
- so enzyme-substrate complexes form more often and the rate increases.
Optimum
- At the optimum temperature the enzyme catalyses the reaction at its maximum rate, and this value differs between enzymes.
Above the optimum
- The rate falls steeply because the enzyme begins to lose its shape.
Part 2 of 6Denaturation
1
Higher temperature makes the enzyme molecule vibrate more strongly, which strains the bonds holding its structure.
2
The weak hydrogen bonds and ionic bonds that maintain the tertiary structure begin to break.
3
The tertiary structure changes, so the shape of the active site is altered.
4
The substrate no longer fits the active site, because it is no longer complementary, and fewer complexes form.
5
The change is usually permanent, so the enzyme is denatured and cannot catalyse the reaction.
Part 3 of 6The Temperature Coefficient Q₁₀
The Temperature Coefficient Q₁₀
Definition
- Q₁₀ is the factor by which the rate of a reaction increases when the temperature rises by 10 °C.
Calculation
- Q₁₀ = rate at the higher temperature ÷ rate at the temperature 10 °C lower.
Meaning
- A Q₁₀ of 2 means that the rate doubles for a 10 °C rise, and a Q₁₀ of 3 means that it trebles.
Limit
- The calculation only applies below the optimum, because above it denaturation reduces the rate.
Part 4 of 6Why It Matters for Organisms
Faster photosynthesis
- Warm water raises enzyme activity in cyanobacteria, which can increase growth and produce harmful algal blooms.
Photorespiration
- At high temperature rubisco catalyses more photorespiration, which slows photosynthesis and can lower crop yields.
Egg development
- Some fish eggs develop more slowly at higher temperatures, and extreme changes can reduce hatching in some invertebrates.
Sex determination
- In some reptiles, such as alligators, incubation temperature affects the sex of the young, so warming can distort sex ratios.
Distribution
- Species may need to move to higher altitude or away from the equator to stay within the temperatures their enzymes tolerate.
Part 5 of 6Worked Example
Question
- A reaction produces 1.8 cm³ of gas per minute at 20 °C and 4.5 cm³ per minute at 30 °C.
- Calculate Q₁₀ and interpret it.
- Write the formulaQ₁₀ = rate at higher temperature ÷ rate at lower temperature.
- SubstituteQ₁₀ = 4.5 ÷ 1.8.
- Calculate4.5 ÷ 1.8 = 2.5.
Answer Q₁₀ = 2.5, so the rate increases by a factor of 2.5 for each 10 °C rise in this range.
Part 6 of 6Exam Checklist & Tips
5.3.6 – Key Points for the Exam
- Optimum temperature The temperature at which an enzyme catalyses a reaction at its maximum rate.
- Denaturation A permanent change in the tertiary structure of a protein that destroys the shape of its active site.
- Q₁₀ The factor by which a reaction rate increases for a 10 °C rise in temperature.
- Kinetic energy The energy of movement, which increases with temperature.
Common Mistakes
- Saying that the enzyme is killed by heat; enzymes are not alive, so say they are denatured.
- Writing that the active site is destroyed by heat without stating that its shape is no longer complementary to the substrate.
- Using the Q₁₀ formula with temperatures above the optimum, where the rate has fallen.
- Giving 37 °C as the optimum for every enzyme, when it differs between enzymes and organisms.
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