Investigating Factors That Affect The Action of Pepsin on Photographic Film
This is an experiment to investigate the factors that affect the
action of pepsin on photographic film. The photographic film is made
up of three layers, plastic, photographic pigment and gelatine in
between. See diagram:
The protease enzyme digests the gelatine section, causing the other
two layers to fall apart.
Pepsin is a digestive enzyme found in the gastric juice of the
stomach. It changes proteins in food into substances called peptides.
In chemical composition, pepsin is like other enzymes, but its effects
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Varying the temperature will change the rate of reaction due to how
much energy the molecules have. When there is a higher temperature the
molecules have more energy, so they move around faster. This causes
more collisions and so more reactions will take place per minute.
Making the temperature too high will denature the molecules; their
active site will become deformed. This causes the lock and key
reaction to not work. See diagram:
The lock and key do not 'fit' any more.
Before I could start the main experiment (varying the temperature), I
needed to investigate the other factors as well. I needed to
investigate the concentration of the pepsin, and the amount of acid
added to the mixture.
When doing the preliminary experiment, I used a water bath set at
37ºC. I first varied the amount of pepsin to water. The total amount
had to add up to 5ml. This meant I could use the following ratios of
pepsin to water: 5:0, 4:1, 3:2, 2:3, 1:4, 0:5. The ratio 0:5 (all
pepsin) worked best so I decided to use this.
I then needed to decide how many drops of acid to use. I did the
experiment at 37ºC with 5ml of pepsin. I did the experiment with no
drops of acid and then from 1-4 drops of acid. I found that 4 drops of
acid worked best so I used this for my main experiment.
The main experiment
I measured out 5ml of pepsin using a 5ml syringe and placed it into a
test-tube. I then added 4 drops of acid to the pepsin. I shook the
mixture and put it in a water bath to reach the desired temperature.
Once the pepsin was at the desired temperature I dropped a piece of
photographic film into the test tube and started the stop-clock. At
regular intervals I shook the test tube a set amount. Once the pigment
had fallen off I stopped the stop-clock and noted down the time. I
repeated this three times for accuracy.
After one temperature was done I used the same method to do another
Once I had done a range of temperatures I then did small increments to
find the optimum temperature.
This was done at 37ºC with 5ml of pepsin.
Drops of Acid
Time in seconds
This was done with 4 drops of acid and 5ml of pepsin.
Time 1 (s)
Time 2 (s)
Time 3 (s)
Av. Time (s)