Aerobic cellular respiration

Aerobic cellular respiration is one of two processes the cells of living organisms use to break down food and convert it into energy. Aerobic respiration occurs in the presence of oxygen. The four-step process takes the molecular sugars stored in food and transforms them into chemicals that cells use as energy. This energy is what allows cells to perform their basic functions. Due to the presence of oxygen, aerobic cellular respiration produces a larger amount of energy. Anaerobic respiration, another type of respiration, occurs without oxygen. This process also creates energy used to power cells but is less efficient and produces less energy.

Background

Cells are the fundamental building blocks that make up all living things. While hundreds of types of cells serve various functions within an organism, they are generally divided into two basic types. Prokaryotes are typically single-celled organisms that lack a central nucleus. The cell’s genetic material, or DNA, floats in a gel-like substance called cytoplasm found within the cell. Bacteria are prime examples of prokaryotic cells.

Eukaryotic cells are found in more complex life forms such as humans, animals, plants, and fungi. These cells have a central nucleus that houses the cell’s genetic material. They also have various membrane-enclosed structures within the cytoplasm called organelles. Organelles perform different functions within the cell. For example, ribosomes use the cell’s genetic instructions to reproduce proteins. Mitochondria are organelles that act as the cell’s power plant, taking the nutrients from food and converting it into energy the cell can use.

Overview

Aerobic cellular respiration is the primary way that eukaryotic cells produce energy. These cells run on a form of chemical energy provided by a chemical compound known as adenosine triphosphate (ATP). Because ATP molecules are larger, delivering them directly to the body’s cells would be a difficult and an inefficient task. The smaller sugar molecule glucose acts as the “fuel tank” that carries the stored energy to the cell. Cellular respiration is the process of converting the glucose into the energy-producing ATP. In aerobic cellular respiration, oxygen is used as a catalyst to transfer the energy stored in glucose into ATP.

The molecular formula of glucose is written as C₆H₁₂O₆, meaning that each molecule is made up of six carbon atoms, twelve hydrogen atoms, and six oxygen atoms. In the first step of cellular respiration, each glucose molecule is broken down into two molecules of pyruvate acid, a compound with the molecular formula C₃H₆O₃. This process is called glycolysis—a term meaning “sugar splitting”—and takes place in the cell’s cytoplasm. To create the energy needed during glycolysis, each glucose molecule also produces two molecules of ATP and two molecules of nicotinamide adenine dinucleotide (NADH). NADH is another energy-producing molecule and a chemical compound used as a catalyzing agent. The glycolysis process occurs in both aerobic and anaerobic cellular respiration.

For aerobic cellular respiration to proceed, the remaining steps of the process require the presence of oxygen. Through a process called oxidation, oxygen strips away electrons from atoms or molecules by way of a chemical reaction. After glycolysis, the pyruvate acid is transferred into the cell’s mitochondria, specifically, an interior section of the structure known as the mitochondrial matrix. There, the pyruvate acid reacts with an enzyme called coenzyme A to form a two-carbon molecule called acetyl-CoA. During this part of the process, one carbon atom is stripped away from the pyruvate acid and combines with oxygen to form carbon dioxide (CO₂), which is released as a waste product. In addition, more NADH molecules are also produced.

The next step of the process is called the citric acid cycle or the Krebs cycle, named after Hans Krebs, the biologist who discovered it in the 1930s. The acetyl-CoA reacts with a molecule called oxaloacetate (OAA), which produces citric acid. The citric acid then goes through a series of reactions to produce energy and carbon dioxide. The end of each cycle produces additional OAA to begin the cycle over again. Because the process to this point has produced two molecules of acetyl-CoA for every glucose molecule, the cycle must repeat itself twice for each glucose molecule. At the end of the cycle, each glucose molecule has been completely broken down. It has produced six carbon dioxide atoms—which are expelled as waste—four ATP molecules, ten NADH, and two molecules of flavin adenine dinucleotide (FADH₂), another type of energy-producing molecule.

The last step of aerobic cellular respiration is called the electron transport chain. This occurs in the christae, a folded area in the inner membrane of the mitochondria. During this step, energy from NADH and FADH₂ is transferred into ATP. The electron transport chain is a series of proteins that transfers high-energy electrons along the inner membrane of the mitochondria. This process creates hydrogen ions—hydrogen atoms that have lost their electrons. As the hydrogen ions build up, a protein known as ATP synthase channels them through the membrane, capturing their kinetic energy and transforming it into chemical energy found in ATP. At the end of the aerobic cellular respiration process, each glucose molecule can produce from thirty-six to thirty-eight molecules of ATP. The leftover electrons that have passed through the chain combine with the oxygen to form water (H₂O).

Cells can also produce energy without oxygen, but this process is far less efficient. Anaerobic cellular respiration shares the process of glycolysis with its aerobic counterpart. However, without oxygen, the process splits off into fermentation in which the glucose is broken down by microorganisms such as bacteria or yeast. Fermentation is the same process used to make beer, wine, or cheese. In humans, a lack of cellular oxygen can break down glucose into lactic acid. During strenuous exercise, in which the body uses oxygen very quickly, a buildup of lactic acid can cause a burning feeling in the body’s muscles.

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