Back To CourseAP Biology: Exam Prep
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Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.
How long does it take you to get out of breath? Whether you start panting just from climbing the stairs or you can hold out until the fifteenth mile of a marathon, getting out of breath is your body's way of telling you that you need more oxygen. When we think of respiration, we tend to think of our respiratory system's organs, like the lungs and alveoli. But true respiration is much more fundamental than that. Cellular respiration is the oxygen-dependent process in which cells turn carbon compounds into energy.
Recall that the chemical 'currency' used by cells for energy is a molecule called adenosine triphosphate, or ATP. Although ATP is generated by the body in more than one way, the most common is cellular respiration and occurs through the oxidation of glucose, C6H12O6. This process can be summarized in the reaction: C6H12O6 + 6O2 --> 6CO2 + 6H2O + energy (ATP).
It's helpful to think of cellular respiration as a series of steps. Oxidation of glucose starts in the cytoplasm. During glycolysis, glucose is broken down into two molecules of pyruvate, and two molecules of ATP are created. These pyruvate molecules are transported to the mitochondria, where they are sent through the Krebs cycle and are used to generate a few more molecules of ATP. Most of the ATP molecules are made during the final step of cellular respiration, the electron transport chain, in which oxygen plays a vital role as electron acceptor.
Returning to the summary equation, you can see that for every molecule of glucose and set of six molecules of oxygen are consumed, six molecules of carbon dioxide are formed.
How can we measure this important process in a lab? Let's start by choosing a good experimental subject. What type of cells need a lot of energy, and therefore, would be home to a lot of cellular respiration? How about cells that are rapidly dividing? An easy way to obtain rapidly dividing cells is to use seeds that are in the process of germinating or growing. For this lab, we'll use germinating pea seeds.
We can measure the oxygen consumed by germinating pea seeds by using a respirometer, a system that measures changes in gas volume. If we place germinating seeds in a respirometer, we should be able to get a pretty good idea of how much cellular respiration is going on in those seeds if we compare it to a control condition.
Respirometers can be quite technologically complex, but a fairly simple respirometer can be made from a syringe, capillary tubing marked with graduations in microliters or milliliters, some cotton, and a little potassium hydroxide (KOH). If we seal the capillary tubing into the tip of the syringe (leaving the other end open to the air), pack the end of the syringe barrel with cotton, add a drop of KOH, and then put another protective layer of clean, non-absorbent cotton on top (to keep the caustic KOH from coming into contact with the organism), we've created a nice, airtight respirometer.
Let's make three respirometers. The first will have 0.5 mL of glass beads in it. This will be our control condition. Our second respirometer will have 0.5 mL of seeds that have been baked to ensure that they are completely dry. The third respirometer will have 0.5 mL of pea seeds that have been soaked overnight in warm water in order to start germination.
We'll put the plunger in each of the syringes, push them in until they reach the 1.0 mL mark, and then place each syringe in a water bath set for 20 degrees Celsius. We'll let everything reach equilibrium for a few minutes, and then put a drop of red-colored fluid into each capillary tube and mark its initial position, and sink the respirometers back into the water bath, allowing the capillary tubes to remain out of the water so we can read them easily. What do you think will happen as cellular respiration occurs?
The answer is that oxygen will be needed by the germinating seeds. As the oxygen is consumed, carbon dioxide will be released. This carbon dioxide will react with the KOH via the following equation: CO2 + 2 KOH --> K2CO3 + H2O.
This equation shows that carbon dioxide is removed from the air inside the syringe, forming solid potassium carbonate that settles into the cotton. With no carbon dioxide to take up space in the syringe, the oxygen consumed during respiration will move the fluid through the capillary tube toward the syringe.
Getting back to our setup, what do you predict will happen in the three respirometers? If we take readings every five minutes and mark the movement of the fluid on the capillary tube, we should see little to no movement in the baked peas and substantial movement in the germinating peas. What about the control? Although we expect to see no movement in the control respirometer, since glass beads are not respiring, we might see a change in the fluid level in the capillary due to changes in temperature or air pressure around the experimental setup. We'll use any changes to make sure we have accurate readings in the other two conditions.
If we graphed what occurred in each of our respirometers, what do you think we would see? The control condition, if everything's equal, shouldn't show any changes. If it does, though, we'll need to account for this and correct the readings on the other two respirometers by either adding or subtracting any changes in volume that the control condition shows. The baked pea seeds are dormant; that is, nothing's going on metabolically, so they should show little or no oxygen consumed. And finally, you would expect to see a pretty high amount on those germinating and growing peas.
To calculate the rate of respiration for each condition, you should take the total amount of oxygen consumed by respiration and divide it by the time of the experiment. A comparison of the rates of respiration for each tube should indicate that the germinating seeds show a greater rate of respiration when compared to baked seeds.
Do you have any ideas as to why the baked peas do show some evidence of respiration? Don't forget that the world around us is full of life. Even microscopic organisms, like bacteria and fungal spores, need oxygen to complete cellular respiration - even those busy dividing and growing on baked pea seeds.
Let's conclude by reviewing the major concepts covered in this lesson. Cellular respiration is the oxygen-dependent process in which cells turn carbon compounds into energy. This process is crucial to life because it produces adenosine triphosphate, or ATP, the compound that's used by cells for energy. The overall summary equation for cellular respiration is: C6H12O6 + 6O2 --> 6CO2 + 6H2O + energy (ATP). The process takes place in a series of steps and concludes with the electron transport chain, in which oxygen is used to generate most of the ATP that the body needs.
In this lab, we can measure the oxygen consumed by germinating pea seeds by using a respirometer, a system that measures changes in gas volume. If we place germinating pea seeds into a simple respirometer and take steps to prevent the CO2 formed from interfering with the respirometer's ability to measure the oxygen consumed, we can calculate the rate of respiration occurring in the pea seeds.
To calculate the rate of respiration for each condition, we can take the total amount of oxygen consumed by respiration and divide it by the time of the experiment. A comparison of the rates of respiration between baked pea seeds that were not germinating and germinating pea seeds will show a much higher rate of cellular respiration in germinating seeds.
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Back To CourseAP Biology: Exam Prep
29 chapters | 298 lessons