Describe how the effect of caffeine on heart rate in Daphnia can be investigated practically

SNAB Core practical

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CORE PRACTICAL 1
Describe how the effect of caffeine on heart rate in Daphnia can be
investigated practically, and discuss whether there are ethical issues in the use
of invertebrates.
Daphnia, the water flea, is a small freshwater crustacean; it is relatively
transparent and its heart can be seen quite easily under the low power
of the microscope, therefore it is ideal for this experiment.
Daphnia needs a heart and circulation because compared with a single
celled organism it is relatively big, so its surface area volume ratio is too
small for diffusion alone to supply oxygen.
Setting up the experiment
1. Select a large specimen and, with a pipette, transfer it to a cavity slide
and immobilise it with cotton wool.
2. Place either solution A or B (with caffeine or without) onto the cavity
slide (this was a double blind test ­ in order to reduce observer bias)
3. Place the cavity slide on the stage of a microscope and observe the
animal under low Power.
4. Record the daphnia's cardiac frequency by making a mark on a piece of
paper every beat for 15 seconds, then multiply this by 4 in order to get
the beats per minute. To make this more reliable you could record it and
record the beats in slow motion.
Factors to be controlled:
- Size of daphnia
- Habitat from which daphnia is obtained
- Temperature of the surrounding water (Daphnia lacks physiological
methods of maintaining a constant body temperature. This means that if the
environmental temperature changes, its body temperature does so too and its
metabolic rate will be expected to rise or fall accordingly)
- Oxygen concentration of the water surrounding the daphnia
Independent variable : Caffeine concentration
Dependent variable: heart rate.

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