OCR AS Biology Planning Exercise 2008

When I first started doing my preliminary investigation, I used 5cm3 of rennin solution. I quickly found out this was too much as I did not have time to put the slide with the solution and rennin onto the microscope before it had started to coagulate. I tried many different amounts of rennin, but lots of them coagulated too quickly for me to measure. I finally settled on using 0.1cm3 of rennin, as this usually took more than two or three minutes to coagulate, so was easiest to measure.

Variables

Input variable: The concentration of calcium ions (calcium chloride) in the solution

Output variable: The time taken for the milk to coagulate

Control variables: - The volumes and concentrations of the different solutions

- The apparatus used

- The concentration of rennin solution

- Temperature and pH of solution (this could affect enzyme)

Safety

* Normal lab precautions (tying hair back, stools under desks etc.) should be observed

* The experiment would involve liquids, which are easily spilt. Therefore, you should be careful when moving around the lab as there could be spillages on the floor which you could slip on.

* Rennin is an enzyme which could be harmful to the eyes, so goggles should be worn at all times.

* You will be using glass beakers which could be smashed, so you should be careful not to cut yourself on shards of glass if this happens.

Apparatus

* Rennin solution

* Milk

* Sodium citrate

* 1.0 mol dm-3 calcium chloride solution

* Microscope (and lamp, if necessary)

* 5cm3 syringe (for measuring out milk, calcium chloride and water)

* 1cm3 syringe (for measuring out sodium citrate and rennin solution)

* Boiling tube

* Slide and cover slip

* Pipette

* Electronic stopwatch

Method

. Ensure that your work area is clean and safe

2. Measure out 10cm3 of milk with a syringe and put it into a boiling tube

3. Add 1cm3 of sodium citrate solution to get rid of any calcium ions already present in the milk

4. Next, measure out the correct amounts of water and calcium chloride using the table below to make a solution using different 5cm3 syringes.

Concentration

Amount of Water (cm3)

Amount of Calcium chloride (cm3)

0M

5

0

0.2M

4

0.4M

3

2

0.6M

2

3

0.8M

4

M

0

5

5. Add this solution to the milk

6. Measure out 0.1cm3 of rennin in the 1cm3 syringe

7. Add this to the boiling tube too

8. Start the stopwatch

9. Using the pipette, add one drop of the milk solution to a slide

0. Let the milk drip down the slide slightly, before carefully placing a cover slip on top

1. Place the slide under a microscope until you see the first specks of coagulation appear

2. Stop the stopwatch

3. Record the time taken and wash the apparatus carefully

4. Repeat the experiment with each of the different concentrations twice for accuracy

Results Table

Concentration of calcium ions

Coagulation time (s)

2

3

Average (s)

0

0.2

0.4

0.6

0.8

Analysis

From my results you will be able to see if there is any correlation between the concentration of calcium ions and the coagulation of milk. More calculations would also be able to be carried out. You could work out the rate of reaction by the equation where t is 'time in seconds' and then also be able to plot a graph using this. You would be able to tell if there was a point at which the maximum rate of reaction was reached or not.

A graph would be plotted with concentration along the x-axis and time rate of reaction up the y-axis. From this you would be able to see if the coagulation rate increases or decreases as concentration is increased. The graph would be able to show us if there was a linear change or not and would also be able to show clearly if there were any anomalous results.

Evaluation

Overall, I think my experiment is quite precise. The apparatus is class B (accurate to 0.1), and although you could get more precise measurements from class A (accurate to 0.01 such as a graduated pipette instead of a syringe) apparatus, I think the results that you could get are good enough for this experiment.

I have chosen to use electronic equipment in a couple of places (e.g. stopwatch) as these are more accurate. Although, the stopwatch could be a problem as it is impossible to add the rennin and start the stopwatch at the same time which could cause a slight delay and make the results out by a second or two.

The fact that human judgment is used to tell when the coagulation starts could be a problem as people have different ideas about when the coagulation has started. A device such as a colorimeter could be used instead as this would give an accurate measurement that everyone could keep to and no room for human error at this stage would be left.

I think that the experiment is definitely valid as it shows exactly what has been asked (a comparison between concentration of calcium ions and time of coagulation) and the data can be shown in many ways (e.g. a graph or a table).

To make the results reliable you could repeat the experiment more than twice. Overall however, I think my experiment is reliable enough and quite fair. There are variables beyond our control, of course (such as air temperature or pressure) which could affect the rate of coagulation, and the experiment would have to be carried out in a sealed environment such as a laboratory to rectify this.

References

[1] Encyclopaedia Britannica article on Rennin

http://www.britannica.com/eb/article-9063204/rennin

30 March 2008, 11:53

[2] Wikipedia article on Rennin/Chymosin

http://en.wikipedia.org/wiki/Rennin

28 March 2008, 20:23

Total Word Count: 1,223

Emily Clare Taylor | 2218

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