Aerobic respiration, a process that uses oxygen, and anaerobic respiration, a process that doesn't use oxygen, are two forms of cellular respiration. Although some cells may engage in just one type of respiration, most cells use both types, depending on an organism's needs. Cellular respiration also occurs outside of macro-organisms, as chemical processes — for example, in fermentation. In general, respiration is used to eliminate waste products and generate energy.
|Aerobic Respiration||Anaerobic Respiration|
|Definition||Aerobic respiration uses oxygen.||Anaerobic respiration is respiration without oxygen; the process uses a respiratory electron transport chain but does not use oxygen as the electron acceptors.|
|Cells that use it||Aerobic respiration occurs in most cells.||Anaerobic respiration occurs mostly in prokaryotes|
|Amount of energy released||High (36-38 ATP molecules)||Lower (Between 36-2 ATP molecules)|
|Stages||Glycolysis, Krebs cycle, Electron Transport Chain||Glycolysis, Krebs cycle, Electron Transport Chain|
|Products||Carbon dioxide, water, ATP||Carbon dixoide, reduced species, ATP|
|Site of reactions||Cytoplasm and mitochondria||Cytoplasm and mitochondria|
|Reactants||glucose, oxygen||glucose, electron acceptor (not oxygen)|
|Production of Ethanol or Lactic Acid||Does not produce ethanol or lactic acid||Produce ethanol or lactic acid|
Aerobic vs. Anaerobic Processes
Aerobic processes in cellular respiration can only occur if oxygen is present. When a cell needs to release energy, the cytoplasm (a substance between a cell's nucleus and its membrane) and mitochondria (organelles in cytoplasm that help with metabolic processes) initiate chemical exchanges that launch the breakdown of glucose. This sugar is carried through the blood and stored in the body as a fast source of energy. The breakdown of glucose into adenosine triphosphate (ATP) releases carbon dioxide (CO2), a byproduct that needs to be removed from the body. In plants, the energy-releasing process of photosynthesis uses CO2 and releases oxygen as its byproduct.
Anaerobic processes do not use oxygen, so the pyruvate product — ATP is one kind of pyruvate — remains in place to be broken down or catalyzed by other reactions, such as what occurs in muscle tissue or in fermentation. Lactic acid, which builds up in muscles' cells as aerobic processes fail to keep up with energy demands, is a byproduct of an anaerobic process. Such anaerobic breakdowns provide additional energy, but lactic acid build-up reduces a cell's capacity to further process waste; on a large scale in, say, a human body, this leads to fatigue and muscle soreness. Cells recover by breathing in more oxygen and through the circulation of blood, processes that help carry away lactic acid.
When sugar molecules (primarily glucose, fructose, and sucrose) break down in anaerobic respiration, the pyruvate they produce remains in the cell. Without oxygen, the pyruvate is not fully catalyzed for energy release. Instead, the cell uses a slower process to remove the hydrogen carriers, creating different waste products. This slower process is called fermentation. When yeast is used for anaerobic breakdown of sugars, the waste products are alcohol and CO2. The removal of CO2 leaves ethanol, the basis for alcoholic beverages and fuel. Fruits, sugary plants (e.g., sugarcane), and grains are all used for fermentation, with yeast or bacteria as the anaerobic processors. In baking, the CO2 release from fermentation is what causes breads and other baked products to rise.
The Krebs Cycle is also known as the citric acid cycle and the tricarboxylic acid (TCA) cycle. The Krebs Cycle is the key energy-producing process in most multicellular organisms. The most common form of this cycle uses glucose as its energy source.
During a process known as glycolysis, a cell converts glucose, a 6-carbon molecule, into two 3-carbon molecules called pyruvates. These two pyruvates release electrons that are then combined with a molecule called NAD+ to form NADH and two molecules of adenosine triphosphate (ATP).
These ATP molecules are the true "fuel" for an organism and are converted to energy while the pyruvate molecules and NADH enter the mitochondria. That's where the 3-carbon molecules are broken down into 2-carbon molecules called Acetyl-CoA and CO2. In each cycle, the Acetyl-CoA is broken down and used to rebuild carbon chains, to release electrons, and thus to generate more ATP. This cycle is more complex than glycolysis, and it can also break down fats and proteins for energy.
As soon as the available free sugar molecules are depleted, the Krebs Cycle in muscle tissue can start breaking down fat molecules and protein chains to fuel an organism. While the breakdown of fat molecules can be a positive benefit (lower weight, lower cholesterol), if carried to excess it can harm the body (the body needs some fat for protection and chemical processes). In contrast, the breaking down of the body's proteins is often a sign of starvation.
Aerobic and Anaerobic Exercise
Aerobic respiration is 19 times more effective at releasing energy than anaerobic respiration because aerobic processes extract most of the glucose molecules' energy in the form of ATP, while anaerobic processes leave most of the ATP-generating sources in the waste products. In humans, aerobic processes kick in to galvanize action, while anaerobic processes are used for extreme and sustained efforts.
Aerobic exercises, such as running, cycling, and jumping rope, are excellent at burning excess sugar in the body, but to burn fat, aerobic exercises must be done for 20 minutes or more, forcing the body to use anaerobic respiration. However, short bursts of exercise, such as sprinting, rely on anaerobic processes for energy because the aerobic pathways are slower. Other anaerobic exercises, such as resistance training or weightlifting, are excellent for building muscle mass, a process that requires breaking down fat molecules for storing energy in the larger and more abundant cells found in muscle tissue.
The evolution of anaerobic respiration greatly predates that of aerobic respiration. Two factors make this progression a certainty. First, the Earth had a much lower oxygen level when the first unicellular organisms developed, with most ecological niches almost entirely lacking in oxygen. Second, anaerobic respiration produces only 2 ATP molecules per cycle, enough for unicellular needs, but inadequate for multicellular organisms.
Aerobic respiration came about only when oxygen levels in the air, water, and ground surfaces made it abundant enough to use for oxidation-reduction processes. Not only does oxidation provide a larger ATP yield, as much as 36 ATP molecules per cycle, it can also take place with a wider range of reductive substances. This meant that organisms could live and grow larger and occupy more niches. Natural selection would thus favor organisms that could use aerobic respiration, and those that could do so more efficiently to grow larger and to adapt faster to new and changing environments.