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.
Living organisms require a continuous input of energy to maintain cellular and organismal functions such as growth, repair, movement, defense, and reproduction. Cells can only use chemical energy to fuel their functions, therefore they need to harvest energy from chemical bonds of biomolecules, such as sugars and lipids. Autotrophic organisms, namely plants, algae, and photosynthetic and chemosynthetic bacteria, convert inorganic materials into such biomolecules by harnessing energy from the environment, such as from sunlight during photosynthesis. Heterotrophic organisms are unable to synthesize high-energy biomolecules from inorganic materials, so they obtain energy by consuming carbon compounds produced by other organisms, primarily from autotrophs. When energy is needed, chemical bonds of carbon compounds are broken to harvest the energy stored in these bonds. The processes to harvest energy from biomolecules are called cellular respiration.
Cellular respiration occurs in both autotrophic and heterotrophic organisms, where energy becomes available to the organism most commonly through the conversion of adenosine diphosphate (ADP) to adenosine triphosphate (ATP). There are two main types of cellular respiration—aerobic respiration and anaerobic respiration. Aerobic respiration is a specific type of cellular respiration, in which oxygen (O2) is required to create ATP. In this case, glucose (C6H12O6) can be oxidized completely in a series of enzymatic reactions to produce carbon dioxide (CO2) and water (H2O).
Aerobic respiration occurs in three stages. A process called glycolysis splits glucose into two three-carbon molecules called pyruvate. This process releases energy, some of which is transferred to ATP. Next, pyruvate molecules enter the mitochondria to take part in a series of reactions called the Krebs cycle, also known as the citric acid cycle. This completes the breakdown of glucose, harvesting some of the energy into ATP and transferring electrons onto carrier molecules. In the last stage, known as oxidative phosphorylation, electrons pass through an electron transport system in the mitochondrial inner membrane, which maintains a gradient of hydrogen ions. Cells harness the energy of this proton gradient to generate the majority of the ATP during aerobic respiration.
Aerobic respiration requires oxygen, however, there are many organisms that live in places where oxygen is not readily available or where other chemicals overwhelm the environment. Extremophiles are bacteria that can live in places such as deep ocean hydrothermal vents or underwater caves. Rather than using oxygen to undergo cellular respiration, these organisms use inorganic acceptors such as nitrate or sulfur, which are more easily obtainable in these harsh environments. This process is called anaerobic respiration.
When oxygen is not present and cellular respiration cannot take place, a special anaerobic respiration called fermentation occurs. Fermentation starts with glycolysis to capture some of the energy stored in glucose into ATP. However, since oxidative phosphorylation does not occur, fermentation produces fewer ATP molecules than aerobic respiration. In humans, fermentation occurs in red blood cells that lack mitochondria, as well in muscles during strenuous activity generating lactic acid as a byproduct, therefore it is named lactic acid fermentation. Some bacteria carry out lactic acid fermentation and are used to make products such as yogurt. In yeast, a process known as alcoholic fermentation generates ethanol and carbon dioxide as byproducts, and has been used by humans to ferment beverages or leaven dough.
Cellular respiration together with photosynthesis is a feature of the transfer of energy and matter, and highlights the interaction of organisms with their environment and other organisms in the community. Cellular respiration takes place inside individual cells, however, at the scale of ecosystems, the exchange of oxygen and carbon dioxide through photosynthesis and cellular respiration affects atmospheric oxygen and carbon dioxide levels.
Interestingly, the processes of cellular respiration and photosynthesis are directly opposite of one another, where the products of one reaction are the reactants of the other. Photosynthesis produces the glucose that is used in cellular respiration to make ATP. This glucose is then converted back into CO2 during respiration, which is a reactant used in photosynthesis. More specifically, photosynthesis constructs one glucose molecule from six CO2 and six H2O molecules by capturing energy from sunlight and releases six O2 molecules as a byproduct. Cellular respiration uses six O2 molecules to convert one glucose molecule into six CO2 and six H2O molecules while harnessing energy as ATP and heat.
Scientists can measure the rate of cellular respiration using a respirometer by assessing the rate of exchange of oxygen. Understanding the Ideal Gas Law is of fundamental importance for knowing how the respirometer functions. The Ideal Gas Law states that the number of gas molecules in a container can be determined from the pressure, volume, and temperature. More specifically, the product of the volume and pressure of a gas equals the product of the number of gas molecules, the ideal gas constant and the temperature of the gas. Respirometers contain potassium hydroxide which traps carbon dioxide that is produced by respiration in solid form as potassium carbonate. When cells consume oxygen, the gas volume in the respirometer system decreases with no carbon dioxide to increase it back up, allowing scientists to calculate the amount of oxygen used using the ideal gas equation.
Cellular respiration is an important process that creates usable energy for organisms, therefore, studying the contexts in which it is improved or impeded is not only interesting, but also necessary. Especially, mitochondria are essential for cellular respiration and any conditions that affect mitochondrial health have immense consequences for the health of the organism. For instance, mitochondrial myopathies are a group of neuromuscular diseases which are caused by mitochondrial damage, affecting predominantly nerve and muscle cells, which require high levels of energy to function1. Moreover, many poisons work by inhibiting cellular respiration. For example, cyanide inhibits the production of ATP through oxidative phosphorylation, thus understanding the mechanisms cyanide or other metabolic poisons enables treatment of individuals who have been exposed to them2. Similarly, some medications such as certain antibiotics, chemotherapeutics, statins, and anesthetics can also interfere with mitochondrial function and may not be suitable to treat patients that have mitochondrial disorders3.
