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NATIONAL 5 BIOLOGY

EXPERIMENTAL PRACTISE

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Experimental Questions

  • VALIDITY (VARIABLES)
  • CONTROLS
  • RELIABILITY
  • POSSIBLE IMPROVEMENTS
  • GRAPHS

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Validity & Variables

  • Only 1 variable should be changed per investigation

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  • All the others should be controlled to ensure the investigation is VALID

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  • If more than 1 variable has been changed, the investigation is INVALID

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Variables

    • Volumes of solutions
    • Concentrations of solutions
    • Mass of substance
    • pH
    • Temperature
    • Surface area/size

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State 2 variables which should be controlled to make this experiment valid

REMEMBER YOU CAN’T STATE THE WHAT HAS ALREADY BEEN MENTIONED AS BEING CONTROLLED OR THE VARIABLE BEING CHANGED

Temperature

Volume of pepsin/solution

Concentration of pepsin

Spacing of holes

Size/depth/diameter/volume of holes/wells

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Identify a variable that would have to be kept the same between the two fields to ensure the results were valid

  • Number of potatoes/plants;
  • Spacing between potatoes/plants;
  • pH of soil;
  • Nutrient content of soil;
  • Moisture content of soil;
  • Fertility of soil;
  • Type of soil

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Explain why experiment 2 is invalid

There are two variables altered

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For a valid conclusion to be drawn, which two tubes should be compared to show the effect

of temperature on germination?

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All flasks at same pH 1

Yeast — different types of yeast in each flask

OR

Temperature — different temperatures 1

OR

Glucose — different glucose concentrations used

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Control Experiments

  • A control is set up to show that the variable you are investigating (have changed) is causing the results

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  • It is set up to allow a comparison to those with the variable you are investigating (have changed)

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  • To create a control, set up the investigation exactly as you have before, except remove the variable you are investigating

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Explain the purpose of the control

  • To allow a comparison to those where carbon dioxide and/or light have been removed OR
  • To show that carbon dioxide and/or light are required

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Respirometers B and D were set up as control experiments.

Describe the purpose of the controls in this investigation.

To show it is the germinating peas that are producing the result/using oxygen/respiring

OR

To show that dead peas do not respire

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Describe a suitable control for this experiment

Exactly the same set up but without the plant

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  • To show any difference is due to the predator
  • To allow it to be compared to the one with the predator
  • To compare the number of (red spider) mites with and without the predator

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Conclusions

  • Always make sure your conclusion relates back to the aim of the investigation

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  • For example, if the aim was ‘to investigate the effect of temperature on the rate of photosynthesis’, you should word your conclusion ‘as the temperature increased, the rate of photosynthesis...’

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Tomato growers aimed to investigate whether a predator would reduce the spider mite numbers in their greenhouses. Two identical greenhouses were used and the predator was released into only one greenhouse

With reference to the aim of this investigation, give the conclusion that the tomato growers would have drawn from these results

When predators are present (the number of red spider) mites

decrease

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With reference to the aim of this investigation, give the conclusion from these results

As competition is increased, growth decreased

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The aim of the experiment was to investigate catalase activity in a variety of tissues.

Using the information given, write an appropriate conclusion for this experiment

  • Liver has the highest catalase activity
  • Apple has the lowest catalase activity
  • Different tissues have different catalase activity

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Reliability

  • This is ensured by repeating the investigation several times and calculating an average

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Describe how the reliability of these results could be increased

Repeat the investigation

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Drawing a Graph

  • Labels copied exactly from the table – do not shorten them
  • Scale must be equal
  • You don’t have to start at 0 at the origin
  • Use a RULER!
  • You must use 50% of the axes

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Drawing a Line Graph

  • Remember to label the origins of each axis
  • Plot points with neat crosses and join with a single line
  • Do not start line at origin unless you have data for this and have plotted a point there

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Line graphs are for 2 sets of numbers in the your table

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Drawing a Bar Graph

  • For bar graphs, use a ruler to draw straight lines at the top of the bars
  • Do not colour in your bars!
  • Remember to label the origin of the x axis (side)

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Bar graphs are for 1 set of words and 1 set of numbers in the your table

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Describing a Graph

  • Describe what is happening on the x-axis (bottom) first using data e.g. “as the temperature increases from 0-40oC”
  • Then describe what is happening on the y-axis (side) at the same time using data eg “the activity decreased from 90% to 50%”

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As the distance increases, number of lugworms increases up to 12 metres

After that, numbers stays the same

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Good Luck!!!!