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Edexcel Biology AS Level - Core Practical 2 Summary Notes
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Summary notes on core practical 2 (Edexcel biology A). Covers variables, methid, data analysis, conclusion and validity check. [“A-Level Biology: Edexcel A Year 1 & 2 Complete Revision & Practice” (CGP, ISBN: 2986), “Salters-Nuffield AS/A level Biology Student Book 1” (Pearson, ISBN: 1007) ...

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  • June 13, 2023
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  • 2021/2022
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(4) The Effect of Changing Enzyme Concentration on Rate of Reaction
Independent Variable: Trypsin concentration (0.1%, 0.2%, 0.3%, 0.4%, 0.5%)

Dependent Variable: Initial Rate of Reaction (% transmission per second)

Control Variables:
• Temperature
• Volume of trypsin
• Volume of casein
• Concentration of casein
• pH

Method:
1. Dilute stock solutions of trypsin with distilled water
2. Make a control solution with no milk suspension and use this to set the colorimeter absorbance to
zero
3. Add the milk suspension and trypsin solution into a cuvette, mix and place in the colorimeter
4. Measure the % transmission at 15 second intervals for 5 minutes
5. Rinse the cuvette with distilled water and repeat for all trypsin concentrations

Data:
• Plot a graph of % transmission vs. time
• Draw a tangent to the curve at t=0 using a ruler and calculate its gradient – this represents the initial
rate of reaction

Theory:
• Casein is a white protein found in milk
• When casein is broken down, milk turns colourless
• Trypsin is a protease enzyme, which hydrolyses casein

Conclusion:

• As the trypsin concentration increases, the likelihood of a trypsin molecule colliding with a casein
molecule increases
• The number of enzyme-substrate complexes increases, so the rate of reaction increases

• The rate plateaus at the point where all substrates occupy an active site
• Increasing the enzyme concentration further will not affect the rate as the substrate concentration has
become the limiting factor

Validity:
• Difficult to maintain constant temperature
• Difficult to make up solutions of precise concentrations

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