POTATO/CATALASE COURSEWORK
- using different vegetables or substrate concentration PLANNING:
The independent variable is the factor that you control. Thus you need to include full
details of how you set about ensuring that the values you state are as accurate and reliable as
possible.
So:
Temperature – NB ‘Room Temperature’ does not exist in AS coursework – but you can use a
beaker with tap-water, by all means….. and measure the actual temperature,
allowing enough time for the ‘ingredients’ to reach the indicated temperature – state
in minutes how long.
Volume
- what volume of liquid are you going to use? Why? How are you going to
measure this? Why?
Vegetable
- How much are you going to use? Why? How are you going to cut them? Why?
Are the vegetables uniform? How are you going to randomise your disks? The
genetic / growing / storage variations in potatoes are nil (they are clones), but what
about in other veg?
Reaction vessels - What are you going to react the vegetables in? Why?
Time
- How long are you going to react them for? Why did you choose this time? How
are you going to measure time? Starting when? How are you going to ensure each
replicate is the same? Why use 6x 30 sec replicates, rather than 1x 3 mins?
Range
- your range of readings will be 0-100% of the strongest available peroxide (50vol)?
How many intermediate concentrations will you use? Will this be sufficient for a
reliable result? With a range of vegetables, what do they all have in common? Will
this range (assuming all other factors are fixed) be enough for you to draw any
general conclusions about catalase in veg?
Concentration - The possible range is sufficient (over 50 vol is explosive!)? But why did you
choose 20 vol for the ‘variable vegetable’ practical? Will this decline over the
course of the experiment? Will this matter?
Control
- There are two options here, one better than the other. No potato/veg at all or use
boiled veg (why – what will this do to the catalase?). For the ‘substrate
concentration’ option, you are going from 0% - 100% H2O2, so the control will be
part of the range of results taken.
The dependent variable is the one you measure.
So:
Oxygen volume - What are you going to measure this with? Why did you decide to use bubbles
rather than a gas syringe? What is the maximum error possible when counting
bubbles and how would this compare with a minute volume of gas in a syringe?
N. B. Bubble volume is affected only by the diameter of the tube and the depth of the
water (= pressure).
Replicates - How many readings are you going to generate per set of results? Clearly, you realise
that the minimum has to be 5. How many replicates will you actually get? Can you
combine your results with any other groups? Are you going to average them? Why
would this be a good idea?
DIAGRAM:
Have you done one? In pencil? Labelled?
COLLECTION OF DATA
Do ensure that you have collected your data on a decent bit of paper! These raw results
need to be included in your final submission (as an appendix), as well as the neat, fully titled, wordprocessed table(s) in the ‘Results’ section of your report.
Ensure your results are taken to an appropriate degree of accuracy and that you have enough
in each set (min 5) and enough repetitions (min 3). You can pool results with others and average
them, without fear, providing you state that they are pooled data.
GRAPHING DATA:
Titled. Lines labelled. Axes right way round – independent variable on the X axis, and
fully labelled -with units! Most of graph paper area used – don’t plot on computer – the
exam board don’t like them! Line of best fit optional; ‘joining up the dots’ = compulsory!
Take care not to extrapolate back to the origin, unless you are certain that that is correct (it
usually is not!)
Error bars might be a worthwhile inclusion – if a histogram/bar chart is used.
Calculations. As well as a bit of averaging, you might well be able to calculate a rate or two
– anything (simple!) like this helps to give an air of authority to your conclusions.
INTERPRETING
Describe the pattern of your results – using actual numbers! This seems pointless and even
offensive, given that the examiner can read and interpret the graph as well as you can. But
do it!
Conclusions need to be detailed. This means that you have to give an interpretation of your
results.
In this case, the catalase is found in the cytoplasm of the cell. It is water soluble. It is
surrounded by the cell membrane. The cellulose cell wall on the outside of the cell is fully
permeable and so no barrier to either the peroxide or the oxygen.
Fick’s Law states that diffusion is affected by surface area (think!); concentration difference
(think!) and by the distance (think!). Since gas and peroxide can’t be in the same place at the
same time, what happens when the surface of the disks are covered with a whole coat of
bubbles (as happens when the reaction is very vigorous)
ANALYSING
The main source of error in any biological experiment is usually the natural variation of
living things. What did you do to ensure that this variation was minimised?
The apparatus is accurate enough – but what about the disks themselves?
‘Experimental error’ counts for nothing – unless you detail the cause of the error and
(better) indicate how the experiment could have been improved to reduce/eliminate the error that
you have identified.
Clearly, even with pooled class results only a limited range of results were obtained; what
should be done to ensure that the results were reliable, repeatable and applicable to other
situations?
Simply repeating the experiment with the same apparatus and the same range of results will
improve the reliability but not the accuracy. For that, the experiment must be modified – perhaps
by using different apparatus, or by eliminating disk altogether and using ‘mashed’ (raw) veg, of a
fixed mass. Your anomalous results (or the class average’s) must be indicated on the graph(s)
then seek to explain them – i.e. what is the most probable cause(s) of these results? Once again
‘Because that idiot XXXX did it’ won’t get any credit at all!
Distinguish between biological factors (enzymes, active sites, E/S complexes etc) and scientific
factors (Fick’s Law, temperature affects on rate of diffusion etc)
 IHW March 2004