Does caffeine affect heart rate?

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Biology – Does caffeine affect heart rate?

 Hypothesis

The higher the concentration of caffeine, the higher the heart rate of Daphnia.

Caffeine:

Caffeine can block a receptor on the surface of heart muscle cells called an A1 adenosine receptor, which releases adenosine that has the effect of decreasing heart rate. Also, caffeine has the effect of inhibiting a group of enzymes called cyclic nucleotide phosphodiesterases. This group of enzymes is needed to degrade a stimulatory signal transmitted by excitatory neurotransmitters. Caffeine dissolves in water and alcohol.

Daphnia:

Daphnia are small, mostly planktonic, crustaceans, between 0.2 and 5 mm in length. The body is covered by a carapace, which is translucent.

Daphnia have simple heart with no pacemaker.

Summary:

Caffeine, being soluble in water, diffuses from the water surrounding the Daphnia into their bodies directly. Caffeine reaches the simple heart and give rise to a series of reactions. Firstly, blockage of the A1 adenosine receptor stops the release of adenosine, so heart rate cannot be slowed down. Secondly, when cyclic nucleotide phosphodiesterases are inhibited by caffeine, nerve signals remain active for a longer time, resulting in higher heart rate.

Apparatus/Materials

  • Culture of Daphnia
  • 0.5% caffeine
  • Distilled water
  • Microscope
  • Cavity slides
  • 6 beakers
  • 2 10cm3 syringes
  • 7 pipettes
  • Stopwatch
  • Cotton wool
  • Filter paper

Procedure

  1. Prepare caffeine solutions of different concentrations by adjusting the proportion of distilled water and the provided 0.5% caffeine solution. Label the beakers:  0%, 0.1%, 0.2%, 0.3%, 0.4%, and 0.5%.
  2. Put strands of cotton wool on a cavity slide to restrict the movement of Daphnia.
  3. Use a pipette to obtain Daphnia from the bucket provided.
  4. Place the Daphnia on a cavity slide.
  5. Place the cavity slide under the microscope.
  6. Adjust the microscope so that the Daphnia’s heart is clearly visible, and use a stopwatch to measure the time taken for the Daphnia’s heart to beat 20 times.
  7. Use filter paper to suck water from the cavity slide.
  8. Use a different pipette to transfer 5 drops of 0.1% caffeine to the cavity slide with Daphnia.
  9. Adjust the microscope and meanwhile allow 3 minutes for caffeine to diffuse into the Daphnia
  10. Measure time taken for the Daphnia’s heart to beat 20 times, and then record the time.
  11. Repeat step 10 for three times.
  12. Use a filter paper to absorb caffeine solution from the cavity slide, and replace with the next concentration(0.2%) using a new pipette.
  13. Repeat steps 8 to 12 for all concentrations of caffeine.
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Variables

Independent variable: Caffeine concentration (0.1%, 0.2%, 0.3%, 0.4%, 0.5%)

Reason: Independent variables are what the experimenter changes. Since I am investigating how concentration of caffeine affects heart rate of Daphnia, changing this variable can effectively test my hypothesis.

Dependent variable: Heart rate (time taken for 20 beats)

Reason: Dependent variables are what the experimenter measures. Measuring the heart rate helps me to test my hypothesis – whether increasing the concentration of caffeine increases heart rate of Daphnia.

Controlled variables: Temperature of water, temperature of caffeine, light intensity, amount of water, amount of liquid, and stress of ...

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