Core Practicals Biology AS level
CORE PRACTICAL ONE
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 which 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. So the temperature of the organism must be kept constant during the procedure.
In this investigation we shall test the hypothesis that as the concentration of caffeine changes, the heartbeat rate (cardiac frequency) of Daphnia also changes. Fortunately Daphnia is relatively transparent and its heart can be seen quite easily under the low Power of the microscope.
Setting up the experiment
1. Select a large specimen and, with a pipette, transfer it to the centre of a small, dry Petri dish. With filter paper remove excess water from around the specimen so that it is completely stranded.
2. With a seeker place a small blob of silicone grease onto the floor of the Petri dish. Then wipe the needle clean and use it to gently push the posterior end of the animal into the grease so that it is firmly anchored. Now fill the Petri dish with water at 300C.
3. Place the Petri dish on the stage of a microscope and observe the animal under low Power. The figure above shows the position of the heart, watch it beating. Don't confuse the beating of the heart with the flapping of the legs.
4. Surround the animal with a circular heating coil and fix it in position as shown in the figure below. Also clamp a small mercury thermometer, or the temperature probe of a digital thermometer, into position.
Estimating the cardiac frequency
A convenient way of doing this is to time how long it takes for the heart to beat 50 times. If it is beating too frequently for every beat to be counted,
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 which 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. So the temperature of the organism must be kept constant during the procedure.
In this investigation we shall test the hypothesis that as the concentration of caffeine changes, the heartbeat rate (cardiac frequency) of Daphnia also changes. Fortunately Daphnia is relatively transparent and its heart can be seen quite easily under the low Power of the microscope.
Setting up the experiment
1. Select a large specimen and, with a pipette, transfer it to the centre of a small, dry Petri dish. With filter paper remove excess water from around the specimen so that it is completely stranded.
2. With a seeker place a small blob of silicone grease onto the floor of the Petri dish. Then wipe the needle clean and use it to gently push the posterior end of the animal into the grease so that it is firmly anchored. Now fill the Petri dish with water at 300C.
3. Place the Petri dish on the stage of a microscope and observe the animal under low Power. The figure above shows the position of the heart, watch it beating. Don't confuse the beating of the heart with the flapping of the legs.
4. Surround the animal with a circular heating coil and fix it in position as shown in the figure below. Also clamp a small mercury thermometer, or the temperature probe of a digital thermometer, into position.
Estimating the cardiac frequency
A convenient way of doing this is to time how long it takes for the heart to beat 50 times. If it is beating too frequently for every beat to be counted,