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Enzyme and substrate concentrations - rates of reactions CPAC 4
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Full CPAC 4 write up which achieved passing grade and fulfilled all necessary criteria for Biology Edexcel
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Name: Hiba Majid 12E (P:\Science\Mr Alim\Year 12\AS Biology)QUESTION TO BE INVESTIGATED: How does the concentration of enzymes affect rate of reaction?
SCIENTIFIC RESEARCH (REFERENCED):
● Enzymes are biological catalysts, which speed up the rate of chemical reactions happening in
organisms. They reduce the activation energy of a reaction and can be affected by factors
including temperature, pH, and the concentrations of substrates
● The induced fit model says that a substrate binds to an active site. The active site and substrate
are not exactly complementary so the active site changes shape to accommodate for the shape
● Milk powder has a protein called casein, which joins in clusters with calcium and phosphate to
form particles (micelles). The micelles reflect all light waves, making milk appear white
● Trypsin is an enzyme which hydrolyses proteins into peptides. A white suspension of milk powder
clears on the addition of the enzyme trypsin (Hamosh, 2007)
● The induced fit model says that a substrate binds to an active site, with the active site being
flexible and changing shape, fitting more closely around the substrate. However, only a
specifically shaped (Page 77, Salter Nuffield AS/A level Biology course book) (Biology online)
● As the reaction proceeds, the amount of substrate decreases and the amount of product
increases. Therefore the chance of enzyme-substrate collisions decreases, lowering the rate of
reaction. The substrate becomes the limiting factor. Therefore, the initial rate of the reaction is
the most valid measurement to take; it will give the rate of the reaction under the desired
conditions. This also makes comparisons between concentrations easier, as the limiting factor
will not act as a confounding variable
● To calculate the initial rate of reaction, a tangent is drawn at the beginning of the graph (before
it curves). The gradient of the tangent is calculated by dividing the change in y axis by the change
in x axis. (Page 78, Salter nuffield AS/A level Biology course book)
● A colorimeter measures the absorbance of particular wavelengths of light by a specific solution.
It compares the amount of light getting through a solution with the amount that can get through
a sample of pure solvent. It is used for identification and determination of concentrations of
substances that absorb light (Roy Choudhury, 2014)
● Serial dilution is a series of sequential dilutions that are performed to convert a dense solution
into a more usable concentration. (see preliminary method to see how serial dilution is used
experiment) (Microbenotes)
HYPOTHESIS: As the concentration of the trypsin increases, the initial rate of reaction will be faster
METHOD:
1. Carry out instructions given (have produced/researched) to complete practical (CPAC1) List
apparatus and describe techniques used when measuring quantities with it
Apparatus Purpose of apparatus How piece of equipment was used Appendix
accurately/appropriately/safely 5c tech.
, Standard To observe effect of enzyme (Low risk) Use a syringe to accurately measure.
Trypsin concentration on initial rate of Trypsin may be an allergen-avoid skin contact
solution (0.5%) reaction
Milk powder To observe the reaction when (Low risk) Use a syringe to accurately measure.
solution enzymes are added Casein may be an allergen-
Beaker Used to mix the solutions in. (Low risk) Ensure it is away from the edge so it
doesn’t break and cut as it is made of glass
Syringe and Used to accurately measure the (No risk) Use different syringes/pipette for the
pipette solutions to create different different solutions so they don’t get mixed up
concentrations
Make measurements with base of meniscus at
eye level
Stop watch To measure how long it takes for (No risk) Have a different person managing stop
the reaction to occur. watch while the other is putting in the cuvette to
reduce the time period between the 2
Test tubes Used to place and mix the enzyme (Medium risk) Keep away from the edge of the
concentration solutions in table, as it will fall and break leading to possible
injuries. Ensure all test tubes are labelled
correctly to avoid confusion and mixing up
results.
Test tube To hold test tubes in (Low risk) Keep away from edge of the table so
holder that it does not fall from the edge along with the
test tubes it is carrying
Distilled water To act as a control and as a (Low risk) Keep away from edge to avoid spillage
baseline for comparison purposes. (slipping hazard) and keep away from electrical
This will also be used to make the devices
different enzyme concentration
solutions.
Colorimeter Measure absorbance. This will be (Low risk) Electrical equipment means that it
calibrated using distilled water should be kept away from water sources and
and set to measure absorbance other solutions. To use accurately, it should be on
and the same wavelength the correct setting (wavelengths) and calibrated
properly
Cuvette This will be placed into the To ensure the other light does not interfere with
colorimeter (on the correct side- results, lid of the colorimeter should be closed
transparent side)
(CPAC a, 2a, 2b, 2d, 3a, 3b)
2. Summarise the method used, pointing out any difficulties in carrying out the practical and any
changes that were necessary from the original plan and why they were made. Draw an
appropriate diagram as necessary. (CPAC 2c)
Preliminary method:
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