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1.1.6Topic 1 · Lifestyle, Health and Risk

Investigating Heart Rate — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 23 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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1.1.6 Investigating Heart Rate — Edexcel A Level Biology A (SNAB) 9BI0 notes: Plan and evaluate an investigation into the effect of caffeine (or exercise) on the heart rate of Daphnia, and interpret results with appropriate statistics and ethical care.Daphnia (the water flea) is …

  • standard deviation
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3
  • Independent variable
  • Control variable

Part 1 of 7Why use Daphnia?

Daphnia (the water flea) is a small freshwater crustacean with a transparent body, so its heart can be seen under a low-power microscope.

Why use Daphnia?
Its heart rate is fast, about 200 beats per minute at room temperature,
  • and responds to chemicals dissolved in the surrounding water because they diffuse across the body surface.
It is therefore convenient for investigating factors such as caffeine, nicotine, alcohol or temperature.
The Edexcel specification includes the effect of caffeine on Daphnia as the core practical for this topic.

Part 2 of 7Method

1
Prepare solutions of caffeine of different concentrations (for example 0, 0.01, 0.05, 0.10, 0.50 and 1.0 g per 100 cm3) from a stock solution using serial dilution, using pond water or the culture water as the diluent.
2
Place a Daphnia in a few drops of water on a cavity slide.
  • A small piece of cotton wool fibres can restrict movement, and a drop of methyl cellulose can slow the animal without harming it.
3
Leave for two to five minutes to acclimatise under the microscope at low power with a lamp.
  • A heat shield prevents the lamp from warming the animal.
    • (a water-filled dish)
4
Count the number of heartbeats in a fixed time such as 20 or 30 seconds, then calculate beats per minute. Repeat three times and calculate a mean.
5
Replace the water with the lowest caffeine concentration, wait a set time (about 5 minutes) and count again.
6
Use a different Daphnia for each concentration,
  • or the same animal in increasing order only if recovery is allowed;
    • use a new animal if the response may be harmful.
7
Return animals to fresh water at the end.

Part 3 of 7Variables and controls

TypeDetail
IndependentConcentration of caffeine
DependentHeart rate (beats per minute)
Control variablesTemperature of solution (use a water bath or a thermometer to check), size and age of Daphnia, time of exposure, volume of solution, light intensity, pH
Control group0 g per 100 cm3 caffeine (water only)

Temperature must be controlled because heart rate of an ectotherm rises with temperature as enzyme and diffusion rates rise.

Part 4 of 7Results and analysis

Calculate the mean heart rate for each concentration and plot a graph with concentration on the x axis (independent variable) and mean heart rate on the y axis.

Results and analysis
Caffeine is a stimulant: expect heart rate to rise at lower concentrations.
At very high concentrations the heart may beat irregularly or stop, because the chemical is toxic.
Describe the trend, quote values, and compare with the control.

The standard deviation shows spread around the mean, and a statistical test such as a t-test can show whether the difference between two means is significant (probability less than 0.05). If the standard deviation bars of two points overlap, the difference may not be significant.

Part 5 of 7Further variables to investigate and using statistics

The same method can compare the effects of other substances, such as nicotine or alcohol, or of temperature.

Further variables to investigate and using statistics
In a temperature investigation the water is warmed or cooled in a water bath and the Daphnia is allowed to acclimatise for several minutes before measuring.
Heart rate increases with temperature up to an optimum because enzyme activity and diffusion increase; very high temperatures denature proteins and the heart rate falls or stops.

When processing data, give a table with units in the headings, calculate means to the same number of decimal places as the raw data, and include the standard deviation.

Further variables to investigate and using statistics
To use a t-test, state the null hypothesis, for example that there is no significant difference in heart rate between the control and 0.1 percent caffeine.
Compare the calculated t value with the critical value at p = 0.05; if it is greater, reject the null hypothesis.
Always say that a significant result gives evidence of a difference, not absolute proof.

Part 6 of 7Evaluation and ethics

1
Individual variation between animals is large, so a sample of at least ten is better than one.
2
Counting heartbeats by eye is difficult at high rates; recording a video and replaying it slowly improves accuracy.
3
Animals should be handled gently, not left under the lamp for long, and returned to culture water afterwards.
4
Daphnia are invertebrates, but ethical treatment is still necessary. Some people argue against any experimentation on animals.

Compare with human datacaffeine increases heart rate in humans and results may be extended to a hypothesis about people, though the mechanism of action differs.

Part 7 of 7Exam-style questions

  1. Describe how you would control temperature when investigating the effect of caffeine on Daphnia heart rate. [3]3 marks

  2. A Daphnia has 38 beats in 15 seconds. Calculate its heart rate in beats per minute. If heart rate at 0.5 percent caffeine is 40 percent higher than this, calculate that rate. [3]3 marks

  3. Suggest two ways of improving the reliability of the results. [2]2 marks

Key terms

TermMeaning
DaphniaA small transparent freshwater crustacean used to study heart rate.
Independent variableThe factor deliberately changed in an investigation.
Dependent variableThe factor measured as a result.
Control variableA factor kept constant to make the test fair.
Serial dilutionProducing a series of solutions of decreasing concentration.
StimulantA substance that increases the activity of the nervous system or heart.
Standard deviationA measure of the spread of data around the mean.
AcclimatiseTo allow an organism to adjust to new conditions before measurements.

Avoid these mistakes

1
Using the same temperature word for everything: state the actual control method.
2
Forgetting a control with no caffeine.
3
Counting beats for too short a time, which increases percentage error.
4
Concluding that caffeine will have the same effect on humans without caution.
5
Putting the dependent variable on the x axis.

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