Effect of Caffeine on the Heart Rate of Daphnia

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In 1819, caffeine (IUPAC nomenclature: 1,3,7-trimethyl- 1H-purine- 2,6(3H,7H)-dione) was discovered by a German chemist, Friedrich Ferdinand Runge. It is a bitter white crystalline xanthine that acts as a psychoactive stimulant drug and a mild diuretic  in humans and other animals. Caffeine, an odourless and slightly bitter alkaloid, is found in coffee, tea ,kola nuts, ilex plants (the source of the Latin American drink maté ), and, in small amounts, in cocoa, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants. It is also known as guaranine when found in guarana, mateine when found in mate, and theine when found in tea; all of these names are synonyms for the same chemical compound. It is most commonly consumed by humans in infusions extracted from the cherries of the coffee plant and the leaves of the tea bush, as well as from various foods and drinks containing products derived from the kola nut. Other sources include yerba mate, guarana berries, and the Yaupon Holly.

Caffeine is absorbed rapidly into the bloodstream from the gastro-intestinal tract in human whenever consumed. It reaches maximum concentration in circulatory system within about an hour. The blood distributes it throughout the body. It even manages to pass through the blood-brain barrier.

Early experiments showed that low concentrations of caffeine may produce small decreases in heart rate in human, whereas higher concentrations may make the heart beat abnormally fast. In the brain it constricts the cerebral blood vessels. If one is used to drinking several cups of coffee a day but quit drinking later, those blood vessels will dilate, maybe enough to give that person a powerful headache. It is one of the best known withdrawal symptoms.

In humans, caffeine is a central nervous system (CNS) stimulant, having the effect of temporarily warding off drowsiness and restoring alertness. It is the most commonly used mind-altering drug in the world. When used in moderation, caffeine acts as a mild stimulant to the nervous system, blocking the neurotransmitter adenosine and resulting in a feeling of well-being and alertness. Also, it is found that caffeine binds to receptors on the surface of heart muscle cells which leads to an increase in the level of cAMP inside the cells (by blocking the enzyme that degrades cAMP), mimicking the effects of epinephrine (which binds to receptors on the cell that activate cAMP production). cAMP acts as a "second messenger," and activates a large number of protein kinase A (PKA; cAMP-dependent protein kinase). This has the overall effect of increasing the rate of glycolysis and increases the amount of ATP available for muscle contraction and relaxation. According to one study, caffeine in the form of coffee, significantly reduces the risk of heart disease in epidemiological studies. However, the protective effect was found only in participants who were not severely hypertensive (i.e. patients that are not suffering from a very high blood pressure). Furthermore, no significant protective effect was found in participants aged less than 65 years or in cerebrovascular disease mortality for those aged equal or more than 65 years.

Beverages containing caffeine, such as coffee, tea, soft drinks and energy drinks enjoy great popularity. Caffeine is the world's most widely consumed psychoactive substance, but unlike many other psychoactive substances it is legal and unregulated in nearly all jurisdictions. In North America, 90% of adults consume caffeine daily. The U.S. Food and Drug Administration lists caffeine as a "Multiple Purpose Generally Recognized as Safe Food Substance".

        Excessive intake of caffeine can result in restlessness, insomnia, and heart irregularities. The effects of caffeine vary from person to person, as people excrete it at different rates. Physical dependence and unpleasant symptoms upon withdrawal (headache, fatigue and depression) are common in regular caffeine users.

        A study of effect of caffeine in human heart rate will be very valuable. However, there are many difficulties in conducting an experiment that involves human as “guiney pig”. Thus, an alternative is to use a model organism to represent human. The results of the experiment can then be used as a reference to the effect of caffeine on human. Daphnia are selected as the model as they can be easily handled in many ways in the investigation.

        Daphnia sp. are small, planktonic crustaceans. They, between 0.2 to 5mm in length, are common freshwater cladocerans, often classified with other tiny crustaceans as “microcrustaceans.” Cladocerans are commonly known as water fleas not only due to their salutatory swimming style, but also because of their resemblance to real fleas, though real fleas are insects and share only an extremely distant common ancestry with Daphnia, since both crustaceans and insects are arthropods. Most species in the Order Cladocera are freshwater species, although there are some marine species. The classification of Cladocera is as an order within the Subclass Diplostraca within the Class Branchiopoda within the Subphylum Crustacea. All species of Daphnia occur in different strains - sometimes the same species can look completely different, both in terms of size and shape, depending on its origin, and environmental factors at that location. There are approximately 150 known species in North America, and a similar number in Europe (many of these species are found on both continents, either through accidental introduction by man, or nature). Many foreign species have been introduced to America and Europe from Asia and Africa (the most notorious of which is Daphnia lumholtzi, which is native to Africa). It is not uncommon to collect 20 or more species in one small area of lake bottom! Daphnia sp. live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.

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The figure below shows the anatomy of a typical Daphnia sp.

        In terms of nutritional information, Daphnia have a protein content of around 50% dry weight and a fat content of 20-27% for adults (4-6% for juveniles), nevertheless some species have been reported to have a higher protein content. Daphnia tend to be almost kidney shaped, possessing only a single compound eye (though they have an ocellus, a simple eye), two doubly-branched antennae (frequently half the length of the body or more), and leaf-like limbs inside the carapace that produce a current of water which carries ...

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The quality of writing in terms of grammar, spelling and vocabulary is adequate. At times, some terms could be better handled such as the use of “independent variable” instead of “manipulated variable”. Overall, the use of biological terms however is well-handled. The organization of the report is very good and this is an added advantage to this report.

The depth of the report is brought out well in the procedure, discussion and evaluation. The procedures are very detailed in describing each step, especially in handling the daphnia. The evaluation and discussion section are very meticulous in describing the results collected and each observation is explained using background biological information. The evaluation section is excellent in that it lists of shortcoming of a specific procedure, explains how this might affect data, recommends other ways to improve and justifies how this might not affect data negatively. While the writer lists many ways of improvement, the writer also manages to score on the quality of these improvements. Overall, the writer’s analysis of the data and the sophistication in writing is excellent.

The writer has designed a logical experiment to determine the effect of caffeine on the heart rate of Daphnia, collected adequate results to establish a justified conclusion and evaluated the data collected and experimental procedures as well. The writer has provided a very elaborate introduction to provide background knowledge on relevant details, but there are many unnecessary details as well. The introduction can definitely be more succinct and comprehensive. The report clearly states the aim of the report, the hypothesis with a “cause and effect” statement, the variables of the experiment and the procedures of the experiment. The data collected is presented in tabular and graphical form in neat and elegant manner. More consistency on the significant figures and decimal places could have been achieved though. The precautionary steps taken are very detailed. The discussion of the report highlights the trends observed and the writer attempts to explain the data using the biology explained in the introduction. The evaluation section provides realistic ways to modify the experiment to improve it, and these modifications can actually enhance the experiment by a large margin, so the report definitely answers the question overall. Overall, the report is very detailed in approach.