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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
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.