Aerobic cellular respiration refers to the process by which living organisms convert nutrients into energy for the body to use via the oxidization of nutrients. During aerobic respiration, catabolic reactions convert larger complex organic molecules into ATP, the chemical that drives most physiological processes in the body. In other words, respiration is the key way that a cell gets chemical energy to drive cellular activity. The process of aerobic respiration involves 4 main steps: glycolysis, production of acetyl-CoA, the citric acid cycle, and oxidative phosphorylation.
Each step involves the conversion of one or more chemical substances to utilize the chemical energy stored in their bonds.
“No taxation without respiration.” — Tom Feeney
Most commonly, the substances utilized in cellular respiration are simple sugars, amino acids, and fatty acids. Aerobic cellular respiration in eukaryotes requires the presence of oxygen as an oxidizing agent. Other forms of cellular respiration that do not use oxygen are fermentation and anaerobic respiration. The relatively large amount of energy yielded from oxidative reactions allows for complex multi-cellular life, so aerobic respiration occurs in virtually all eukaryotic organisms.
Steps Of Cellular Respiration
Glycolysis is the first step in the chain of catabolic reactions the comprise the process of cellular respiration. During glycolysis, monosaccharides (simple sugars) such as glucose, sucrose, or fructose are converted into pyruvic acid. Incidentally, the word “glycolysis” literally means “splitting sugar.” The whole sequence of glycolysis is comprised out of 10 individual reactions, each of which is catalyzed by a different enzyme. Glycolysis takes place in the cytoplasm, the jelly-like substance that fills the inside of cells. For every 1 molecule of glucose, glycolysis produces 2 molecules of pyruvate, 2 molecules of NADH, and 2 molecules of ATP.
The first 5 steps of glycolysis are called the “preparatory phase” as they are energy-consuming reactions that produce 2 three-carbon sugar phosphates. Afterward comes the “pay-off” phase in which the three-carbon sugar phosphates are broken down, resulting in a net gain of 2 molecules of pyruvate, 2 molecules of ATP and 2 molecules of NADH.
(2) Pyruvate Decarboxylation
Once pyruvate is formed from glycolysis, the body still needs to process the pyruvate to access the chemical energy stored in its bonds. In the second step of cellular respiration, pyruvate molecules produced by glucose are transported to the cell’s mitochondria and are oxidized to produce acetyl-CoA, an enzyme the provides the acetyl base for the next step in cellular respiration. One molecule of pyruvate is oxidized into acetyl-CoA, so two molecules of acetyl-CoA are produced for every initial molecule of glucose.
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(3) Citric Acid Cycle
Once acetyl-CoA has been produced by pyruvate oxidization, the next step in cellular respiration occurs. Infamous to intro biology students, the citric acid cycle, (also called the Krebs cycle), is extremely important as it provides the lion’s share of energy used to produce ATP during oxidative phosphorylation. It also creates the molecule NADH which is required for the phosphorylation of ADP into ATP. The Krebs cycle consists of 8 definite enzyme-catalyzed reactions and occurs within the mitochondrial matrix, tiny compartments created by the folded inner membrane of the mitochondria.
During the Krebs cycle, two molecules of acetyl-CoA are each completely oxidized into 3 molecules of NADH and 2 molecules of carbon dioxide and water. Since one molecule of glucose produces two molecules of acetyl-CoA, one molecule of glucose ultimately produces 6 molecules of NADH and 4 molecules of carbon dioxide and water.
(4) Oxidative Phosphorylation
The final step in cellular respiration consists of the oxidization of NADH molecules to release energy used to form the majority of ATP produced by cellular respiration. NADH produced from the Krebs cycle has a high electron transfer potential, meaning that a large amount of energy is stored in its chemical bonds. NADH will donate electrons to oxygen molecules and release this stored energy. That energy is then used to add a phosphate group to ADP to create ATP, the fundamental energy currency of living organisms. These oxidization and reduction reactions are also known as the “electron transport chain” and occur in the cristae of the mitochondria. The reactions are driven by enzymes embedded in the surface of the inner membrane.
The oxidization of NADH is a high energy event and can synthesize a number of ATP molecules. For one molecule of glucose, the maximum theoretical yield of the entire process of cellular respiration is 36 molecules of ATP. In actual cells though, energy is always lost due to heat dissipation and proton leakage, making the average total yield around 29-30 molecules of ATP per molecule of glucose. Oxidative phosphorylation marks the terminal point of the cellular respiration and the main sequence that accounts for the high ATP yield of aerobic cellular respiration.
Although necessary for multicellular life, in an ironic twist of fate aerobic cellular respiration is thought to also be responsible for the processes that end multicellular life. Oxidative phosphorylation produces highly reactive species of oxygen like superoxides, peroxides, and hydroxyls. These atoms that have unpaired electrons, called “free radicals,” build up over time and can wreak havoc on cellular structures such as chromosomes. This damage leads to the mechanical and functional decline characteristic of the aging process.
“Aging is not lost youth but a new stage of opportunity and strength.” — Betty Friedan
It is generally accepted that free radical production is responsible in part for aging, but there is some debate over the exact nature of the degradation caused by oxidative stress. Some scientists hold that free radical buildup damages mitochondrial structures, causing increased production of reactive oxygen species. The result is a positive feedback loop where cellular degradation gets progressively worse, leading to the functional failures symptomatic of aging. Others hold that it is the body’s ability to stabilize levels of free radicals that determine lifespan, as free radicals are signaling molecules used for maintaining normal cell functioning.
Aerobic cellular respiration is the most basic metabolic pathway found in eukaryotic organisms. Aerobic respiration is fundamental as it allows for the production of ATP, the molecule that drives every physiological process in every known living organism. The high energy yield of aerobic respiration allows for complex multicellular life and is occurring all the time in every cell of the body.