Cellular Respiration
Autotrophs and Heterotrophs
Living organisms require a continuous input of energy to maintain cellular and organismal functions such as growth, repair…
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.
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.
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Q1: Why do organisms need cellular respiration?
Organisms need cellular respiration to harvest energy from glucose and other biomolecules for cellular functions like growth, repair, and movement. Cellular respiration breaks down glucose using oxygen to produce ATP, the energy currency cells can readily use. The process also releases heat as a byproduct, making energy available throughout the organism.
Q2: What is the difference between aerobic and anaerobic respiration?
Aerobic respiration requires oxygen and completely breaks down glucose into carbon dioxide and water, producing significant ATP. Anaerobic respiration occurs without oxygen and produces fewer ATP molecules. Fermentation, a type of anaerobic respiration, generates ethanol or lactic acid as byproducts instead of carbon dioxide and water.
Q3: How does a respirometer measure cellular respiration?
A respirometer measures oxygen consumption by organisms using the ideal gas law principle. Potassium hydroxide inside the respirometer traps carbon dioxide as potassium carbonate. As cells consume oxygen and release carbon dioxide, the total gas volume decreases, causing pressure to drop. A manometer attached to the respirometer detects this pressure change, allowing scientists to quantify oxygen used.
Q4: What role does the mitochondria play in cellular respiration?
In eukaryotes, the mitochondria is the primary site where cellular respiration occurs. Enzymes in the mitochondria carry out the breakdown of glucose through the Krebs cycle and oxidative phosphorylation, generating most of the ATP used by cells. Mitochondrial health directly affects an organism's energy production and overall cellular function.
Q5: How are photosynthesis and cellular respiration related?
Photosynthesis and cellular respiration are opposite processes. Photosynthesis produces glucose and oxygen from carbon dioxide and water using sunlight energy. Cellular respiration uses that glucose and oxygen to produce carbon dioxide, water, and ATP. The products of one process serve as reactants for the other, creating a continuous cycle of energy and matter in ecosystems.
Q6: Why do germinating seeds consume oxygen in a respirometer?
Germinating seeds are actively respiring because they lack green parts and cannot photosynthesize, so they depend entirely on stored energy. They break down glucose through cellular respiration to produce ATP for growth and development. This active respiration consumes oxygen and releases carbon dioxide, which is why oxygen levels decrease measurably in a sealed respirometer.
Q7: What does the ideal gas law tell us about gas molecules in a respirometer?
The ideal gas law states that the number of gas molecules in a container can be determined from pressure, volume, and temperature. In a respirometer, as organisms consume oxygen through respiration, the total number of gas molecules decreases, lowering pressure inside the sealed chamber. By measuring pressure changes, scientists can calculate exactly how much oxygen was consumed using this mathematical relationship.