Before doing this laboratory you should understand:
After doing this laboratory you should be able to:
Early experiments by Joseph Priestly showed a candle burning in a closed container would soon be extinguished. When he placed a mouse in a closed container of air, the mouse soon died. Working and talking with Antoine Lavoisier furthered more experiments. If a mouse was placed in a container where a candle had burned until extinguished, the mouse died quickly. Perhaps the candle and the mouse both required the same thing in the air. Oxygen was about to be discovered!Oxygen gas is required when the aerobic form of cellular respiration occurs. There are four main parts to aerobic respiration: glycolysis, pyruvic acid being converted to acetyl CoA, the citric acid cycle, and the electron transport system/chemiosmosis. Energy in the form of adenosine triphosphate molecules are formed as a result of electrons moving down the electron transport system. The final electron acceptor in the chain is oxygen, making the process aerobic. Without oxygen, the reactions in the electron transport system cannot occur. Thus, living things that break down glucose by means of aerobic respiration are dependent on the intake of oxygen for energy. The starting glucose molecules are broken down to release energy in the form of ATP molecules. The equation for the oxidation of glucose is
Because oxygen is required for the complete oxidation of glucose, the consumption of oxygen is a good measure of an organism's metabolic rate. The equation shows that for every molecule of glucose to be metabolized, six molecules of oxygen gas are consumed. Six molecules of carbon dioxide gas will also be formed. There is no net change in the volume of gas in the container, only the identity of the gas has changed. In Priestley's experiments, the flame was extinguished in the carbon dioxide produced as the candle burned and carbon combined with oxygen in the air. The mouse died as the oxygen in the air was replaced by carbon dioxide produced by respiration.In this experiment, carbon dioxide is produced as crickets undergo respiration in a sealed container or respirometer. By adding a solution of potassium hydroxide to the respirometer, generated carbon dioxide is removed from the air when it reacts with the hydroxide to form a carbonate. The decreasing gas volume causes the internal air pressure to drop and water is pushed up the graduated pipette by the larger force of the atmospheric pressure outside the tube. The rate at which oxygen moves into the graduated pipette is a measurement of the cricket's respiration. Insects cannot regulate their body temperatures. Their respiratory rates tend to be directly related to the temperature of their environment. To accurately measure this effect, the respirometers are equilibrated in a water bath. This allows the gas volume inside the tubes to adjust to the water temperature. Cooling gases occupy smaller volumes while heated gases expand. These changing gas volumes can alter the amount of water being pushed into the pipette and affect experimental measurements. Equilibrating the tubes in the water bath minimizes changes in gas volumes during the course of the experiment that would be attributed to temperature changes. Any gas volume changes that do occur because of temperature changes can be corrected for by using a control tube. Other measured volumes are attributed to the respiration of the cricket.
By studying the equation above, you will notice there are three ways cellular respiration could be measured. One could measure the:
1. Consumption of O2 ( How many moles of oxygen are consumed in cellular respiration?)
2. Production of CO2 ( How many moles of carbon dioxide are produced by cellular respiration?)
3. Release of energy during cellular respiration.
When an living thing carries on aerobic respiration, oxygen is removed from the
atmosphere and an equal volume of carbon dioxide is produced and released into
the atmosphere. In a closed system, removing the carbon dioxide and recording
the oxygen consumed provides a way
of measuring the rate of respiration.
Problem: Will temperature affect the rate of respiration for a cricket?
test tubes (20 x 150 or larger),
one-hole stopper with tubing attached
calibrated pipette, cotton, KOH, water baths
Procedure:1. Prepare a water bath at room temperature (20 -25 C) and record the temperature. Additional experiments will be conducted at 10 C above and below room temperature.
|Respirometer Setup Containing a Cricket|
3 . Select a cricket or two and place it each of two respirometers. (obtain the total mass of your crickets before doing this) Into each of the test tubes, insert the stopper fitted with the tubing and the pipette. Your instructor may have prepared the respirometers for you. Place a mass of glass beads approximately equal to the mass of the crickets in a third respirometer.
4. Place the test tube portion of the apparatus into the water bath. If the test tube is completely submerged, be sure to check for leaks. If necessary, seal with petroleum jelly. Allow the gas volumes to adjust to the water temperature for 10 minutes.
5. Submerge the tip of the pipette under the surface of the water to trap a volume of air in the pipette.
6. Readings are made by noting the movement of the water-air interface in the pipette. Readings can be taken at 5-minute intervals. Volumes are often difficult to estimate; another option is to record the time to intake a set increment of water, as illustrated in the tables for this laboratory.
7. Repeat the experiment at the different temperatures. One table will be needed for each temperature.
8. Each volume reading should be corrected for any expansion or contraction of gases due to changes in the temperature of the air in the tubes. This is reflected by the movement of the air-water interface in the control tube. For example, if the gas in the control tube has contracted a total of 0.02 mL, then a total of 0.02 mL should be subtracted from the experimental reading. If the gas expanded, the volume would be to added to the experimental reading in order to obtain the true value of oxygen used during that time interval. This value should be divided by the mass of the glass beads to obtain a true value before adding or subtracting it from the value obtained from the respiring crickets.
Remember the rate of respiration for the crickets will be equal to their oxygen consumption divided by their body mass.
Respiration Measurement Chart for the Crickets
|Time (min)||0 min||5 min||10 min||15 min||20 min||25 min||30 min||35 min||40 min|
Correction Factor _________
** Construct a line graph of the above collected data as well.