Edexcel A (SNAB) A Level Biology:复习笔记5.3.6 Temperature & Enzyme Activity

• Changing air temperature can have a significant impact on the metabolism of living organisms due to the effect of temperature on enzyme activity
• Enzymes have a specific optimum temperature
  • This is the temperature at which they catalyse a reaction at the maximum rate
• Lower temperatures either prevent reactions from proceeding or slow them down
  • Molecules move relatively slowly as they have less kinetic energy
  • Less kinetic energy results in a lower frequency of successful collisions between substrate molecules and the active sites of the enzymes which leads to less frequent enzyme-substrate complex formation
  • Substrates and enzymes also collide with less energy, making it less likely for bonds to be formed or broken
• Higher temperatures cause reactions to speed up
  • Molecules move more quickly as they have more kinetic energy
  • Increased kinetic energy results in a higher frequency of successful collisions between substrate molecules and the active sites of the enzymes which leads to more frequent enzyme-substrate complex formation
  • Substrates and enzymes also collide with more energy, making it more likely for bonds to be formed or broken

Denaturation

• If temperatures continue to increase past a certain point, the rate at which an enzyme catalyses a reaction drops sharply as the enzymes begin to denature
  • The increased kinetic energy and vibration of an enzyme puts a strain on its bonds, eventually causing the weaker hydrogen and ionic bonds that hold the enzyme molecule in its precise shape to start to break
  • The breaking of bonds causes the tertiary structure of the enzyme to change
  • The active site is permanently damaged and its shape is no longer complementary to the substrate, preventing the substrate from binding
  • Denaturation has occurred if the substrate can no longer bind

At high temperatures enzymes can denature

The rate of an enzyme catalysed reaction is affected by temperature. Note that 35 °C is not the optimum temperature for all enzyme-controlled reactions.

Enzyme activity and living organisms

• Changes to enzyme activity that result from changing global temperatures can affect living organisms
• Some chemical reactions take place faster at higher temperatures
  • Photosynthesis is essential for converting carbon dioxide into carbohydrates, the process which produces food for producers and other organisms higher up the food chain; it relies on the function of proteins in the electron transport chain and that of enzymes such as rubisco
    • E.g. blue-green algae, also known as cyanobacteria, photosynthesise at a higher rate in warmer water due to increased enzyme activity; this increases the formation of potentially harmful algal blooms
• Some chemical reactions are slowed down at higher temperatures
  • At high temperatures plants carry out a reaction called photorespiration at a faster rate; this reaction uses the enzyme rubisco and so slows down photosynthesis
    • This can reduce crop yields as temperatures rise
  • Some fish eggs have been shown to develop more slowly at higher temperatures
    • Many species' successful egg development is dependent on temperature, with impacts such as
      • Extreme temperature fluctuations can reduce hatching rates in some invertebrates
• The sex of the young inside the egg of some species is determined by temperature, so increasing temperatures can affect the sex ratios in a species
  • E.g. in alligators
• Species may have to change their distribution in response to changing temperatures in order to survive
  • Species may migrate to higher altitudes or further from the equator to find cooler temperatures

• The progress of enzyme-catalysed reactions can be investigated by measuring the rate of formation of a product
• This can be carried out using the enzyme catalase
  • Hydrogen peroxide is a common but toxic by-product of cell metabolism which must be broken down quickly
  • Catalase is an enzyme found in the cells of most organisms that breaks hydrogen peroxide down into water and oxygen
  • The rate at which oxygen is produced can be recorded to give a measure of enzyme activity

Investigating the effect of temperature on catalase activity

Apparatus

• Boiling tubes or flasks
• Hydrogen peroxide solution
• Buffer solution
• Measuring cylinder or syringe
• Bung and delivery tube
• Water basin
• Pipettes
• Catalase enzyme solution
• Water baths at a range of temperatures
• Stopwatch

Method

1. Set up a series of water baths at different temperatures e.g. 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C
   • This provides the varying independent variable
2. Set up a series of boiling tubes each containing 5 cm3 of hydrogen peroxide solution
   • Each tube should contain the solution at the same concentration
   • Equal volumes of a buffer solution can be added to each tube to control pH
3. Place boiling tubes into water baths at different temperatures and leave for 10 minutes
   • This allows the tubes to reach the required temperature
4. Use a pipette to add a sample of catalase solution to a boiling tube containing hydrogen peroxide
5. Immediately place the bung onto the tube and use the water basin and measuring cylinder to collect the oxygen produced during each 10 second period over the course of 1 minute
   • The volume of oxygen produced is the dependent variable
6. Repeat steps 4-5 twice more at the same temperature, ensuring that the same volume of catalase solution is added each time
   • This allows any anomalous results to be identified
   • An average volume during each time period can be calculated from the set of results for each temperature
7. Repeat steps 4-6 using boiling tubes at the remaining temperatures and the same volume of catalase solution
8. Plot a graph of volume of oxygen produced against time for each temperature tested
9. Use the graph to calculate the initial rate of reaction for each temperature

• Rate of reaction can be calculated using the following equation

rate of reaction = change in volume ÷ time taken

The rate of catalase activity can be measured by recording the volume of oxygen produced.

A tangent can be used to calculate the initial rate of an enzyme controlled reaction

Calculating the temperature coefficient

• The temperature coefficient, represented by Q10, calculates the increase in rate of reaction when the temperature is increased by 10 ºC
• Q10 can be calculated using the following equation

Q10 = rate at higher temperature ÷rate at lower temperature

• A Q10 value of 2 indicates that the reaction rate doubles with an increase in temperature of 10 ºC, while a value of 3 indicates that it trebles with every 10 °C increase

Step 1: Write out the relevant equation

Q10 = rate at higher temperature ÷ rate at lower temperature

Step 2: Substitute numbers into the equation

Q10 = 6.8 ÷3.5

Step 3: Complete calculation

Q10 = 1.94

This value is close to 2, indicating that the rate of reaction has almost doubled