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 The Production of Carbon Dioxide
During Aerobic Respiration

The purpose of this experiment was to measure the respiration rate of a snail, and two Elodea cuttings—one that was placed in the dark and one that was placed in the light.  The rate of respiration was not directly measured, instead a byproduct of respiration, carbon dioxide, was measured to determine the rate.  The slightly acidic solutions that the organisms were placed into became even more acidic as a result of the release of carbon dioxide from respiration—CO2 molecules bind to H2O molecules forming carbonic acid.  Sodium hydroxide was then added to each of the solutions, for it reacts by raising the pH level, the solution becomes less acidic, and ultimately is neutralized at a pH of 7.  The varying amounts of sodium hydroxide added to the individual solutions are a determinant of respiration rate.  It was found that the rate of respiration is greater the larger the volume of the organism.  The results also found that the rate of respiration was greater in the Elodea that respired in the dark, and this is because the light plant is photosynthesizing in addition to respiring.

In this experiment the rate of respiration was indirectly measured.  During respiration organisms oxidize pyruvate to carbon dioxide through a series of chemical reactions in what is known as the Krebs cycle.  Respiration releases energy that is then stored in the form of ATP to be readily used for cellular metabolism.  When organisms respire they release carbon dioxide, the carbon dioxide released into the dechlorinated water makes it more acidic—in the form of carbonic acid.  By using phenolphthalein it can be determined whether a solution is acidic or basic.  Adding sodium hydroxide to the solution, after the organisms have been allowed to respire, makes the solution less acidic, and ultimately neutralizes it (pH 7).  By knowing this fact one can determine the rate of respiration of each of the organisms by measuring the amount of sodium hydroxide required to neutralize each of the solutions.  This experiment was conducted to determine if the volume of an organism has any relation to its respiration rate.  Also tested was whether a plant respires more in the light or in the dark.

Materials and Methods
Four beakers were filled with 75 ml of dechlorinated water—a slightly acidic solution—and numbered one through four.  The volume of each of the organisms used in this experiment was then measured.  Two cuttings of Elodea and a snail were each placed in separate 50 ml graduated cylinders containing exactly 25ml of water.  The increase in the water above the 25 ml mark was recorded as the volume of the organism (see Table 1).  After determining the volume of each of the organisms they were removed from their respective graduate cylinders and were placed into the beakers of dechlorinated water.  Beaker one contained no organism, and thus was the control.  Beaker two contained the snail, and beakers three and four held a cutting each of Elodea.  They were then covered with a petri dish and allowed to sit for 15 minutes to allow the process of respiration to occur; additionally, beaker four was also covered with a coffee tin.  At the end of 15 minutes the organisms were removed from their beakers, and a drop of phenolphthalein was added to each beaker to test that each of the solutions were still acidic.
Next, a burette was filled with a certain amount of sodium hydroxide—this amount was recorded.  Using the burette, sodium hydroxide was added to the control beaker (beaker number 1)  drop by drop, and only when the control changed color to pink was the addition of sodium hydroxide stopped.  The amount of sodium hydroxide released from the burette was then determined from the original measurement, and recorded in Table 2.  This procedure—the adding sodium hydroxide to the solutions with the use of a burette—was then repeated for the remaining beakers (beakers 2 through 4).  The color of the control beaker was then used to match those of the remaining beakers, and once this was achieved the amount of sodium hydroxide was recorded in Table 2.  The relative respiration rate for the organism in beaker 2 was determined by subtracting the amount of sodium hydroxide added to the control beaker from the amount added to beaker 2.  This same process was then done to determine the respiration rate of the organisms in beakers 2 and 3 (the results are found in Table 3).   The respiration rate was then determined for each of the organisms by dividing the relative respiration rate by the volume of the organism (the results in Table 4).

Table 1 shows the volume measured of each of the organisms.  In beaker 1, the control, no organisms were used, therefore no volume of a organism was found.  In beaker 2 the volume of the snail was found to be 6.0 ml.  The volume of the Elodea for the beaker that respired in the light was also 6.0 ml.  Beaker 4, containing the Elodea that was covered with a coffee tin, and thus respired in the dark, was measured at 5.0 ml.
Table 2 depicts the amount of sodium hydroxide that was added to each beaker in order to attain a neutral pH (a pH of 7).  For the control beaker only 0.5 ml of sodium hydroxide was added.  Beaker 2, which contained the snail, required 1.8 ml of sodium hydroxide, and beakers 3 and 4 had 1.5 ml each added to them.
The relative respiration rate of each of the organisms is shown in Table 3.  The control beaker was recorded as having no relative respiration rate simply because no organisms were used.  For beaker 2 a relative respiration rate of 1.3 ml was calculated, and for beakers 1 and 2 it was recorded as 1.0 ml for both.
Table 4 depicts the respiration rate per milliliter of each of the organisms.  The control beaker was recorded at zero.  Beaker 2 (the snail containing beaker) had a respiration rate of 0.216 ml.  The respiration rate per milliliter of the organism in beakers 3 and 4 were 0.166 ml and 0.2 ml, respectively.

Table 1
Organisms                         Total Volume of Organisms (ml)
Beaker 1 (control)    0
Beaker 2 (snail)    6.0
Beaker 3 (Elodea, light)   6.0
Beaker 4 (Elodea, dark)   5.0

Table 2
Organisms   Milliliters of NaOH to Reach Endpoint (ml)
Beaker 1 (control)    0.5
Beaker 2 (snail)    1.8
Beaker 3 (Elodea, light)   1.5
Beaker 4 (Elodea, dark)   1.5

Table 3
Organisms   Relative Respiration Rate for Organisms
Beaker 1 (control)    0
Beaker 2 (snail)    1.3
Beaker 3 (Elodea, light)   1.0
Beaker 4 (Elodea, dark)   1.0

Table 4
Organisms  Respiration Rate per Milliliter of Organism (ml NaOH/ml)
Beaker 1 (control)    0
Beaker 2 (snail)    0.216
Beaker 3 (Elodea, light)   0.166
Beaker 4 (Elodea, dark)   0.2

The results show, firstly, that the control was necessary to conduct the experiment, for without it there would have been no standardization—the results would have nothing to substantiate them.  Secondly, it is clearly discernible that the greater the volume of the organism the greater was the rate of respiration, and the greater the rate of respiration the more CO2 was released.  The volume of the snail, at 6.0 ml was greater than those of both the Elodea which were at 5.0 ml.  After respiration, beaker 2 (the snail) required 0.3 ml more of sodium hydroxide, to reach the endpoint, than beakers 3 and 4 (which contained Elodea cuttings).  Since a greater amount of sodium hydroxide was needed to bring the solution in beaker 2 to a neutral pH, the beaker 2 solution was therefore more acidic than beakers 3 and 4.  An acidic solution indicates a greater presence of CO2, and thus, in this experiment, indicates a greater rate of respiration.  All of the organisms in this experiment respired aerobically.
Another notable occurrence was that the plant that respired in the dark did so at a greater rate to that of the plant that respired in the light.  This was because the light plant not only conducted respiration but also photosynthesis.  Respiration of the plant released carbon dioxide into the water, and, at the same time, photosynthesis was taking up some of the carbon dioxide released during respiration.  Therefore the amount of carbon dioxide did not change as much as that of the dark plant which only performed respiration.  It should be noted, though, that despite the fact that the results showed the dark plant to have respired more than the light plant this may not actually be the case.  The light plant could just have likely respired more than or equal the amount that the dark plant, but this would be difficult to determine simply because photosynthesis is removing some of the carbon dioxide produced by the process of respiration.

Vodopich, D.S. and R. Moore.  1996.  Biology Laboratory Manual, 4th ed. Wm. C. Brown  Publishers, Dubuque, IA.
Purves, W.K., G. Orians, C. Heller, and D. Sadava.  1998.  Life The Science of Biology, 5th ed. Sinauer Associates, Inc., Sunderland, MA.