Caffeine is a natural component of coffee which directly involves in causing the increase in Daphnia heart rate after the addition of coffee solution. Caffeine binds to adenosine receptors (adenosine A(1), A(2A) receptors) to prevent adenosine from binding to these receptors of its own. As adenosine is an inhibitory neurotransmitter, this effect of caffeine will prevent the inhibitory effect, and thus increase neural discharge. As a consequent, cardiac muscle will be more active and heart rate will increase (Docshare, 2010 [online]). Likewise, for human, caffeine is a strong stimulant which helps stimulate the nervous system and increase the cell metabolism (M.Kent, 2000). Also, it works by a similar mechanism to Daphnia; it releases the neurotransmitter which makes the heart and breathing rate increase. Therefore, caffeine increases human heart rate as well. However, human has mechanism to alter their enzymes levels to compensate for a higher heart rate, so the effect of coffee (even concentrated) happens only in a short period of…