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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...

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