Although many people fail to realize this, every day that you wake up and experience existence is rather incredible. Every action of our bodies, from pumping blood around our organs and inhaling oxygen to flexing our fingers and climbing a flight of stairs, requires energy. For every living creature, the acquisition, transformation and expenditure of energy is critical to survival. The generation of that energy through chemical processes in an organism is known as cellular metabolism.
What is Cellular Metabolism?
Essentially, in the case of humans, when the body consumes carbohydrates, proteins and fats, those substances must be converted into the proper form (usable energy). Cellular metabolism is the series of steps and cycles that lead from macromolecules of food to more basic molecules of functional energy, similar to a currency, that can be “spent” in countless different ways throughout the body.
While most people have some idea or inkling about cell metabolism, what they often don’t know – or have forgotten since their high school science classes – is that there are two different types of cell metabolism: aerobic and anaerobic. These two sides of the metabolic coin are quite different, despite beginning with the same raw materials, but both forms are also essential for survival!
Aerobic Metabolism vs Anaerobic Metabolism
This heading may be slightly misleading, suggesting that these two forms of metabolism are in opposition, when in fact, they are closely linked and interdependent in certain ways. The key difference between the two is that aerobic metabolism occurs in the presence of oxygen, while anaerobic metabolism does not.
Although aerobic metabolism is the slower of the two processes, it is far more prevalent and essential for our daily functioning. Representing about 90% of all cellular metabolism, aerobic metabolism is the process by which fats and carbohydrates are converted into usable energy in the form of ATP (adenosine triphosphate). Aerobic metabolism is counted on for the production of energy that can support sustained energetic needs, e.g., walking, jogging, daily exertions.
The conversion of carbohydrates into energy is required every day, for all of your basic functional needs. This is why aerobic metabolism occurs constantly in the body. Aerobic metabolism is also the only means by which you body can extract energy from fat. Proteins can also be broken down and used as energy, through a process called ketosis, but this process is not nearly as efficient as the energetic conversion of carbohydrates.
Despite being the slower of the two processes, aerobic metabolism is very efficient, and is able to squeeze 34 molecules of ATP from a single molecule of glucose. The additional byproducts include carbon dioxide and water. Aerobic metabolism consists of two different pathways, the Krebs cycle and the electron transport chain, both of which occur in the mitochondria, the energy factories of cells. The raw materials required for these two stages include water and oxygen, but the step preceding these aerobic processes is none other than anaerobic glycolysis, which we will explain below.
Unlike the form explained above, anaerobic metabolism does not require the presence of oxygen to convert raw materials into energy. However, anaerobic glycolysis is far less efficient, producing only two molecules of ATP, in comparison to aerobic metabolism’s impressive 34.
The body relies on anaerobic respiration when a sudden burst of energy is required in a short amount of time. For example, imagine that you are a sprinter or a weightlifter; your physical demands are usually intense, but only for a limited amount of time. Since aerobic metabolism takes more time, the body uses anaerobic metabolism to generate energy for immediate use from carbohydrates, but not fat or protein. Beginning with a single molecule of glucose, the glycolysis process unfolds in the cytoplasm of a cell, and does not require any organelles.
Unfortunately, one of the byproducts of anaerobic metabolism is lactic acid, which can cause fatigue. A rapid buildup of lactic acid is what causes cramps in athletes who push themselves too hard without properly warming up – or when the body fails to balance aerobic and anaerobic metabolism. For this reason, and the lack of efficiency, the body tends to avoid using anaerobic metabolism unless absolutely necessary.
Not So Different After All…
While the volume and variety of the end products vary, aerobic and anaerobic metabolism are intimately connected. As mentioned earlier, glycolysis is the first step that leads into aerobic respiration, and is therefore happening all the time, just like aerobic metabolism. Essentially, the body needs to “warm up” into the aerobic metabolism of fats, so it begins with pure carbohydrate conversion in the cytoplasm and then transitions into aerobic metabolism.
You don’t notice the negative byproducts of anaerobic metabolism in most cases because your body can eliminate the byproducts faster than you can produce them. Imagine that you take a long, leisurely walk through the park. Your body will be undergoing anaerobic and aerobic metabolism, but provided you have eaten enough food that morning, are moving at a normal pace, and not overly exerting yourself, you probably won’t break a sweat or develop a lactic acid cramp. So long as the body’s aerobic metabolic pathways can keep up with the body’s demands, anaerobic metabolism can take a backseat. When the energy produced by aerobic processes is insufficient, the anaerobic processes take on a bigger role to fill the demand.
The two processes are clearly intertwined, and equally important. Without aerobic respiration, we would lack the constant sources of energy needed to walk, breathe, work, speak and drive a car. Without anaerobic respiration, our ability to snap into action, such as during a fight-or-flight scenario, would be severely compromised. All in all, we should be eternally grateful for both sides of the metabolic pathway, and the clever path of evolution that allows us to live!