How to plan an experiment - First, make sure you know what you're trying to find out. In this case, it is...To measure the optimum temperature for respiration in yeast.

How to plan an experiment

First, make sure you know what you’re trying to find out. In this case, it is…

To measure the optimum temperature for respiration in yeast.

There is absolutely no point in stating this again as the “aim” or “introduction”. It merely wastes time, paper, ink and effort. What you should do first, however – before launching into a detailed description of how or why – is OUTLINE the procedure to be used so that the reader has some idea what is going on. You could include the diagram (which, if complete will get you P4b) at this stage. Something like this:

Outline

A sample of yeast will be suspended in glucose solution in a boiling tube. The carbon dioxide produced by respiration will be bubbled through water and the rate of bubbling used as a measure of the rate of respiration. The reaction tube will be kept in a water bath at a constant temperature for each run and the experiment will be repeated at a range of temperatures.

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Then consider the variables involved in the experiment. Remember,

the INDEPENDENT variable is what you deliberately change (you must specify how),

the DEPENDENT variable is what changes as a result (specify how it will be measured)

and everything else must be CONTROLLED, i.e. kept constant. Here, you must choose the most important variables to keep constant, say why they must be kept constant and how this will be done.

Variables

Independent: TEMPERATURE. This will be varied by carrying out the experiment in a water bath seven times at temperatures of 20, 25, 30, 35, 40, ...

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Then consider the variables involved in the experiment. Remember,

the INDEPENDENT variable is what you deliberately change (you must specify how),

the DEPENDENT variable is what changes as a result (specify how it will be measured)

and everything else must be CONTROLLED, i.e. kept constant. Here, you must choose the most important variables to keep constant, say why they must be kept constant and how this will be done.

Variables

Independent: TEMPERATURE. This will be varied by carrying out the experiment in a water bath seven times at temperatures of 20, 25, 30, 35, 40, 45 & 50°C.

Giving a suitable number and range of temperatures here also gets you P6b

Dependent: RATE OF RESPIRATION. This will be measured by counting the number of bubbles emerging from the end of the delivery tube in one minute.

Controlled: CONCENTRATION OF GLUCOSE SOLUTION. With more glucose available, the yeast will respire faster. Therefore the same concentration of glucose solution – 1g of glucose dissolved in 20cm3 water – will be used for all runs of the experiment.

CONCENTRATION OF YEAST SUSPENSION. If there are more yeast cells present in the mixture, more carbon dioxide will be given off and the rate of respiration will seem to be higher. Therefore the same concentration of yeast suspension – 2g of dried yeast in 20cm3 water – will be used for all runs of the experiment.

DIAMETER OF NOZZLE. A smaller nozzle will produce more bubbles for a given volume of gas and therefore appear to make the rate of respiration greater. The same nozzle will therefore be used for every run of the experiment.

This lot will score P6a(ii).

Then, describe the steps necessary to carry out the experiment. Since this is a plan, it is acceptable here to give instructions or even to say, “I will…”. Numbered steps are fine – in fact preferable since it helps you to think clearly and me to follow what you’re on about. Include safety precautions (needed for P2)in the appropriate place, not in a separate section.

Method

Measure out 7.0 g of glucose and dissolve it in 140cm3 of water in a 250cm3 beaker. (This solution will be used for all seven runs)
Add 2.0g of dried yeast to the first tube, assemble the apparatus as in the diagram and stand it in a water bath at room temperature (about 20°C). Leave it for 10 minutes to come to the same temperature as the water and for the yeast to rehydrate and start respiring.
Check that the yeast mixture has reached the correct temperature.
Count the number of bubbles emerging from the nozzle in one minute and record this result in the Results table.

Note the precision to which the masses are given.

The next two steps are what you should do ideally – in practice you won’t have time and you haven’t made up enough solution.

Repeat this measurement for a second minute to improve reliability.
If necessary, repeat again.
Wash out the tube containing the yeast suspension/glucose solution and repeat the whole experiment at the next higher temperature.
Continue in this way until the experiment has been carried out at all temperatures listed above.

In practice, you won't have time to do seven runs in succession, leaving each for 10 minutes to start bubbling. Instead, you can set up two or three at intervals of three minutes in separate water baths.

Calculate the mean rate of respiration at each temperature and plot this against temperature.

Stating this lot gets you P2 and P4a(i). Explaining why each step is carried out thus gets P6a(i).

Now it’s quite clear what’s going on, you can make your prediction of the results (P4a(ii)) and explain the scientific reasoning behind it. (P6a(ii)) "It says so in the book" or "It seems likely" are NOT scientific reasoning !