Cellular Respiration

Everything that is alive requires an energy source to fuel its activities. Ultimately, that source of energy is the sun. How then did the organisms on Earth harness this energy? It all starts with the photosynthesizers. These organisms are able to take carbon dioxide and water and then use energy captured from the sun as photons to force these molecules together, producing glucose and oxygen. The glucose is the key to the next critical step, one that nearly all organisms use in one form or another - cellular respiration. Cellular respiration is carried out by enzymes, either in the cell membrane in prokaryotes or in the mitochondria in eukaryotes.

The chemical reaction begins by breaking down the glucose using oxygen to create carbon dioxide and water again, but in the process, the energy that originally went into making the glucose molecule goes to two new places. One is into synthesizing ATP, or adenosine triphosphate molecules, an energy source that cells can readily use. The rest is lost as heat. So far, we have been talking about a kind of cellular respiration that uses oxygen, and this is referred to as aerobic respiration, but some organisms and cells are capable of doing respiration in the absence of oxygen. This is called anaerobic respiration. And instead of producing CO2 and water, this process makes ethanol as a byproduct. Fermentation is an example of this type of respiration. This is how yeast are able to produce alcohol in sealed containers. We can use a tool called a respirometer to measure aerobic respiration. Quite simply, this device measures the amount of oxygen used by an organism, germinating plant seeds in this case. Germinating seeds are respiring, which means we can expect them to be using up oxygen and releasing carbon dioxide. They don't have their green parts yet, so they aren't photosynthesizing.

Measuring respiration in a respirometer uses a clever method based on the ideal gas law, P times V equals n times R times T. P is the pressure of the system. V is the volume of the gas. n is the number of moles of gas present. R is the ideal gas constant…and T is the absolute temperature. Simply summarized, this means that you can figure out how many molecules of gas are present in a sample by measuring its volume, assuming you know the pressure and temperature, because R is a constant.

Respirometers contain potassium hydroxide, which traps carbon dioxide in solid form as potassium carbonate. So, due to cellular respiration, the oxygen in the sealed tube gets used up by the seeds, and they release carbon dioxide, which in turn gets trapped as potassium carbonate. Thus, as cellular respiration progresses, the total volume of gas inside the system decreases. We can quantify this by attaching the respirometer to a device called a manometer. Here, as the oxygen molecules are consumed, the pressure drops inside the respirometer chamber, and a colored liquid inside a tiny capillary tube is pulled in the direction of the decreased pressure. We can then estimate the quantity of gas remaining in the respirometer tube by reading off the value on the manometer level. This versatile system can be set up with many different variables, like different temperatures for example, and can be used to test the rate of respiration in many different types of living organisms.

In this lab, you will use a respirometer and manometer to measure the respiration rate of germinating seeds.