Abstract
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.
Introduction
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).
Results
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
Discussion
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.
References
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.