Cellular Respiration

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19 March 2016

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The effect of nature of substrates on the rate of cellular respiration in yeast was determined by using the Smith fermentation tube method. Mixtures of 15ml distilled H2O, 10% yeast suspension and 15ml of the following solutions (all at 10% concentration):1- starch, 2 – lactose, 3 – sucrose, 4 – glucose, 5 – fructose, 6 – distilled water , were poured in six smith fermentation tubes. Cotton balls were plugged in the openings of the tubes and the tubes were kept upright and observed for 30 minutes. The mixture with the sucrose solution acquired the highest computed volume of gas evolved and the rate of CO2 evolution seconded by glucose and followed by fructose. This stated that the higher the amount of the CO2 evolved, the faster the rate of respiration. Mixtures with lactose, starch and dH2O solutions obtained zero result for the volume of gas evolved and rate of CO2 evolution. Thus, the nature of subtrate used slows down or fastens the rate of cellular respiration.

Cellular respiration is defined as an enzyme mediated process in which organic compounds such as glucose is broken down into simpler products with the release of energy (Duka, Diaz and Villa, 2009). It is a series of metabolic processes and oxidation-reduction reactions. Oxidation of substrates, such as glucose, is a fundamental part of cellular respiration (Mader, 2009). As a catabolic process, it may or may not require the presence of oxygen. The process that requires oxygen is called aerobic respiration while the process that does not require the presence of oxygen is called anaerobic respiration. (Duka, et.al. 2007) Despite of its low yield of only two ATP (energy used by the cells to perform its duties), anaerobic respiration is essential because it continuously synthesizes ATP albeit oxygen is temporarily in short supply. Although anaerobic respiration synthesizes a low yield of ATP (which is the energy used by the cell enables it to perform its duties), it is essential because it is a way to produce ATP even though oxygen is temporarily in short supply. Though this process brings benefits usually, these are accompanied by drawbacks. One of these downsides is the formation of lactate in the muscles because of “oxygen debt”, causing it to “burn” and eventually fatigue, until pyruvate is reduced from lactate (Madur, 2009). Anaerobic respiration can be further divided into two types; namely, alcohol fermentation and lactic acid fermentation. In alcohol fermentation, pyruvate (product of glucose in glycolysis) is converted to 2 molecules of ethanol (C2H5OH) and 2 molecules of carbon dioxide (CO2) while in lactic acid fermentation, pyruvate is reduced directly into lactic acid (Campbell and Reece, 2008). A good example of organism which produces ethyl alcohol and carbon dioxide through the process of alcohol fermentation is yeast (Madur, 2009). As a unicellular fungus, yeast is also an example of a facultative anaerobe, which depicts an organism with… [continues]


Materials and Methods
In determining the effect of the nature of substrates on rate of cellular respiration, smith fermentation tube method was done. In this method, six smith fermentation tubes were obtained. This special tube has a closed vertical arm which extends into a bulbous portion with tapered opening as seen in Figure 5b. 15 ml of the following solutions, all at 10% concentration, were poured to the respective tubes: 1- starch, 2 – lactose, 3 – sucrose, 4 – glucose, 5 – fructose, 6 – distilled water. 15 ml distilled water and 15 ml 10% yeast suspension was then added to each tube. The mixtures were shaked gently and assured to have no bubbles trapped at the closed end. If there were bubbles, it will be removed by covering the opening with the palm of one hand and tilting the tube horizontally. The openings of the tubes were plugged with cotton balls. The tubes were tied together at their vertical arms to keep them upright and were set aside where the will not be disturbed. All CO2 evolved will be trapped in the vertical arm and the height of the area occupied by the CO2 evolved were measured every five minutes for thirty minutes. The volume of the gas evolved and the rate of CO2 evolution were computed. The rate of CO2 evolution was computed by the amount of CO2 evolved over time and the volume of the gas evolved was computed by the formula:

Volume = pir2h whereas pi= 3.14
r= radius of the smith fermentation tube (cm)
h= height of the area occupied by the CO2 evolved (cm)

Computed results were tabulated. A graph of volume of CO2 evolved and time elapsed was then plotted and analyzed.

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