Study of Oxygen Consumption to Measure Energy Metabolism in Mammals

In this experiment the oxygen consumption of a mouse in environments of varying temperatures was studied. The hypothesis behind this experiment was that more oxygen would be consumed at a lower temperature to maintain body heat. To measure the rate of oxygen consumption a mouse was given a specific amount of oxygen. The amount consumed in a specific time was measured and compared to the rates at a colder temperature. Just as expected, oxygen was consumed more rapidly at a lower temperature.

This would lead to the conclusion that more oxygen is required to maintain homeostasis in a system. Introduction Metabolic rate is a measurement of energy used by an animal within a specific period of time. This can be measured in a number of ways: recording an animal’s heat loss with a calorimeter, determining the amount of carbon dioxide an animal produces, recording an animal’s food intake compared to the food’s chemical energy potential and the amount of waste the animal creates, and finally by measuring the amount of oxygen a test subject consumes during a given period of time1.

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The mouse used in the experiment is an endotherm. Endotherms use energy produced through metabolism to maintain and generate consistent body heat2. Endotherms also generally have a higher metabolic rate than ectotherms (organisms that gain heat from their surroundings) 1. Factors other than endothermic or ectothermic status also influence metabolic rate, including size and activity. Size is inversely proportional to metabolic rate. Higher metabolic rates have been found in small endothermic animals; however there is no concrete explanation for this finding1. Activity also affects metabolic rate.

The longer and activity lasts the lower the maximum metabolic rate becomes. In the absence of activity an organism is said to be either at its basal metabolic rate (for endotherms) or is standard metabolic rate (for ectotherms). These rates can be affected by many outside factors including time, temperature, nourishment, and physical characteristics1. Metabolic rate is also a way of maintaining homeostasis in an organism. Homeostasis is the relationship between the body’s internal regulatory factors (for example, metabolic rate) and outside factors that affect an organism’s internal systems1.

Multicellular organisms cannot survive without a homeostatic internal environment3. Without thermoregulation brought about by a constant metabolic rate reactions and interactions between bodily chemicals would not function properly and homeostasis would no longer exist. In other words, an endothermic organism cannot carry on normal functions if it internal temperature varies greatly from the homeostatic norm1. One method of thermoregulation is through evaporation.

Water on the surface of an organism absorbs a great deal of heat as it evaporates, thus carrying heat away from the body and maintaining the body’s internal temperature. Other methods of thermoregulation include conduction, convection, and radiation1. In order for the metabolic rate to be maintained there must be a supply of energy to meet fluctuating demands. This energy is produced by the controlled oxidation of sugars, starches and other energy yielding organic molecules. In short, oxygen and an organic molecule react to produce water, carbon dioxide and energy2.

The energy produced is used to maintain the metabolic rate. Therefore, if the amount of oxygen consumed can be determined the average metabolic rate can be estimated as can fluctuations therein. Therefore, concerning the oxygen consumption experiment; as the need for energy increases with lowering surrounding temperatures, the consumption of oxygen will increase in order to maintain homeostasis. Materials and Methods A respirometer was the primary device used in this experiment. This piece of equipment consisted of a jar with a rubber stopper that sealed the top.

The rubber stopper was equipped with a system of tubing that helped measure oxygen consumption within the jar without allowing air into or out of the jar. Before the experiment could begin several preparatory steps had to be taken. The respirometers had to be charged by placing a test tube of KOH pellets into the jar. The apparatus had to then be tested before the mouse could be placed inside and the data collection portion of the experiment commenced. The jar was dried and a piece of dry paper towel was placed in the bottom. The pipette end of the tubing was placed into a beaker of ink.

The pinch clamp on the other end of the tubing was opened completely and the pipetor (blue handled) was attached to the end. The handle of the pipetor was pulled until ink on the other end of the tubing reached the bent portion of the pipette. The pinch clamp was pinched closed. The ink was studied and when it maintained a constant level after thirty seconds the preliminary testing went into its final stages. The sensitivity of the apparatus was then tested by warming the jar with body heat from the hands. The ink went down showing that the apparatus was correctly prepared.

The apparatus was then taken apart. A mouse was weighed and placed inside the jar by the tail with the KOH pellets remaining in the test tube inside the apparatus. The respirometer was reassembled and a table was prepared to compare the movement of ink to the passage of time during the experiment. The respirometer was sealed and the ink was allowed to rise to just above the bottom of the pipette. Once operational ink level readings were taken every minute for six minutes, at which time the five minutes at which time the ink reached the end mark (this can take more than ten minutes).

This procedure was repeated to ensure the consistency of the results. To measure oxygen consumption at lower temperatures the entire respirometer, with the mouse, was placed inside a bucket of ice and allowed to cool. Readings were taken of the ink levels every thirty seconds until the top mark was reached. This procedure was done twice, again to ensure consistency. The mouse was then removed and placed back in the container it was obtained from. The KOH was removed, still in the tube. Feces and other waste materials were removed from the jar. The jar was thoroughly rinsed and dried.

The data obtained from this experiment was made into graphs and submitted for approval to the lab instructor. Results On the following page the graphs representing oxygen consumption as a function of time have been modified for accuracy using Microsoft Excel. The pink and blue squares represent the readings obtained from the room temperature experiment. While the green circles and orange triangles are a representation of the reading from the low temperature portion of the experiment. Oxygen consumption at room temperature was fairly steady and gradual.

At the lower temperature consumption was steady but much more rapid compared to room temperature. It also varied more as time passed and the temperature stabilized within the jar. See tables. Discussion Oxygen consumption should have increased as the temperature lowered as a direct result of the need for more energy. The results, as represented by the graphs on the previous page, would seem to support this hypothesis. The rate of oxygen consumption increased as the temperature dropped. This was because the mouse needed to metabolize more organic molecules to obtain energy to be used to maintain body heat and continue homeostasis.

In order to produce energy a constant supply of oxygen was needed to oxidize the sugars and starches in the mouse’s system. As the demand for more energy went up so did the demand for and oxidizing reagent. There was a noticeable difference in the rate of oxygen consumption between the two low temperature readings. This was most likely the result of a large increase in oxygen intake just after the respirometer was added to the ice. The sudden extreme temperature change may have spiked the metabolic rate in order to regain homeostasis more rapidly.

The second reading is a more accurate representation because the jar was given time to reach a state of equilibrium. The difference was caused by human error. Had the jar been allowed some time to cool and the mouse’s metabolism some time to initiate a homeostatic response the two lower temperature readings would have been quite similar. Another way to display changes in oxygen consumption as a result of external temperature fluctuations might be to place the respirometer (along with the mouse) under a heat lamp and observe the effects.

Also a way to improve upon this experiment would be to use stop watches to improve timing accuracy and possibly obtain more modern form of the respirometer as some students experienced difficulty in the setting-up procedures. References 1 Campbell, N. and Reece, J. Biology Seventh Edition. Pearson Education, Inc. , San Francisco. 2005. pp. 828-838 2 Frankel, J. , et al. Principles of Biology I Laboratory Manual Fifth Edition. Pearson Custom Publishing, Boston. 2005. pp. 37-42 3 Information obtained from the internet at: http://en. wikipedia. org/wiki/Homeostasis


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