Caffeine is found in many plant species, where it acts as a natural pesticide. It is found most commonly in cocoa, tea and coffee, but is also artificially added to some soft drinks such as cola to act as a flavour enhancer. When consumed by humans, caffeine works as a stimulant causing amounts of released neurotransmitters to be increased. High use of caffeine has been related with raised blood pressure, restlessness, insomnia and anxiety which, in the long term, can lead to heart and circulation problems. The aim of this experiment is to observe whether caffeine has any significant effect on heart rate. To test this, daphnia will be placed in water and then caffeine solution, so the difference in heart rates can be compared. Daphnia are small invertebrates that are found in aquatic environments, more commonly known as ‘water fleas’. They are approximately 3mm in length and have simple internal structures. They have transparent skin that allows you to view their internal organs, making them ideal subjects for scientific experiments.
It is expected that the daphnia’s heart rate will be greater in the caffeine solution than in the water. This prediction can be justified with the fact that caffeine is a stimulant drug that affects the central nervous system, speeding up your metabolism and increasing alertness.
The independent variable in this experiment is the use of caffeine. The dependent variable in this experiment is the heart rate of the daphnia.
* Pipette | * Cotton wool|
* Concaved microscope slide| * Water (20cm²)|
* Microscope slide cover| * Caffeine Solution (20cm²)| * Microscope| * 1 daphnia|
* Thermometer| * Stopwatch/timer|
* Petri dish * Two experimenters | * Pen * Paper|
1. A concaved microscope slide is placed in a lidless Petri dish. 2. A tiny strand of cotton wool is placed in the concave of the microscope slide. 3. A pipette is used to pick out a single daphnia from a beaker of water. 4. The daphnia is then placed on the cotton wool on microscope slide. 5. A microscope slide cover is then put over the concaved slide. 6. The Petri dish is topped up with 20cm² of water (20°C). 7. The Petri dish is placed on the microscope platform.
8. The microscope is adjusted so that the daphnia can be observed. 9. Experimenter 1 counts the daphnia’s heart rate for 20 seconds, dotting on a piece of paper with each beat whilst experimenter 2 keeps track of time with the stopwatch. This is repeated four times and then an average is calculated. 10. The microscope slide is removed.
11. The Petri dish is emptied of water.
12. The slide is placed back in the Petri dish.
13. The Petri dish is topped up with 20cm² of caffeine solution (20°C). 14. The Petri dish is placed on the microscope platform.
15. Experimenter 1 counts the daphnia’s heart rate for 20 seconds, dotting on a piece of paper with each beat whilst experimenter 2 keeps track of time with the stopwatch. This is repeated four times and then an average is calculated.
Controlled Variable| Reason|
Same Daphnia| The same daphnia must be used for both parts of the experiment as every individual daphnia’s metabolism works at different speeds. | Same volume of liquid (20cm²)| The same volume of both water and caffeine solution must be used in both parts of the experiment as the temperatures two different volumes of water would be dissimilar.| Water temperature| Daphnia do not thermoregulate as they are cold-blooded animals, meaning they will be at the same temperature as the water they are placed in. As the temperature of the water is increased, so is the metabolism of the daphnia, consequently causing it’s heart rate to increase. So, to ensure a balanced experiment the temperature of the water and the caffeine solution must be about...