Glucose oxidation is a process that breaks down glucose into carbon dioxide and water, releasing energy stored in ATP. It consists of three stages: glycolysis, the citric acid cycle, and the electron transport system. ATP is essential for cellular metabolism and various cellular functions.
Glucose oxidation is a chemical process that provides energy for an organism to perform all of its required activities. During this process glucose, a simple sugar molecule obtained from food, is broken down into carbon dioxide and water. This reaction releases energy and stores it in a chemical form that the cell can use. There are three separate stages of glucose oxidation: glycolysis, the citric acid cycle, and the electron transport system.
Glucose
Glucose molecules are used to build more complex carbohydrates, such as starch and cellulose. The chemical formula of this molecule is C6H12O6, which means that it is made up of six carbon atoms, 12 hydrogen atoms and six oxygen atoms. Found in plants and many types of foods, glucose is absorbed into the bloodstream during digestion.
Oxidation
The oxidation of glucose is an aerobic process, a chemical reaction that requires oxygen. In fact, the term “oxidation” refers to any reaction in which oxygen is combined with another molecule, which is then said to be oxidized. In the process, one glucose molecule combines with six oxygen molecules to produce six carbon dioxide molecules, six water molecules, and adenosine triphosphate (ATP), a molecule that cells use to store or transfer energy.
Glycolysis
The first step in the oxidation process is glycolysis, which occurs within a cell’s cytoplasm, the gelatinous substance that fills the cell and surrounds other cellular organs. During this phase, the glucose molecule is broken down into two molecules of pyruvate, an organic acid capable of providing cells with energy. This breakdown also releases energy, which is used to add a phosphate ion to adenosine diphosphate (ADP) to create ATP. ADP, in turn, is formed with ATP which is broken down to release its energy.
Glycolysis of a single glucose molecule consumes two ATP molecules and produces four total, leading to a net energy gain of two ATP. Energy from the process is also used to produce two NADH, a form of enzyme used to transfer electrons to fuel cellular chemical reactions.
The citric acid cycle
To initiate the citric acid cycle, also called the Krebs cycle, the pyruvate molecules produced by glycolysis are transferred to the mitochondria, a cellular organ involved in metabolic processes. Once there, the molecules are converted into acetyl CoA, the molecule that powers the citric acid cycle. Acetyl CoA is made up of the carbon of pyruvate and coenzyme A, a molecule that assists in biological processes. The conversion process produces a NADH.
Acetyl CoA releases the carbon portion of the molecule into the citric acid cycle, which runs constantly, producing ATP, high-energy electrons, and carbon dioxide. Most of the energy produced is stored in the form of high energy electrons and one round of the cycle will result in three NADH and one FADH2. Like NADH, FADH2 stores captured electrons. The cycle also produces two ATP and releases the rest of the energy as heat.
The electron transport system
The final stage of glucose oxidation also occurs within the mitochondria, where a group of proteins, called the electron transport system, help transform the energy of the electrons captured by NADH and FADH2 into ATP. This process is modeled by the chemiosmotic theory, which describes how these electrons pass through the transport system, releasing energy as they move.
The energy released is used to move positively charged hydrogen ions back and forth across the membrane that separates two parts of the mitochondria. The energy of this movement is stored in ATP. This process is called oxidative phosphorylation because oxygen is required for the final step, accepting electrons and hydrogen atoms to become H2O, or water. The energy yield of this phase ranges from 26 to 28 ATP.
Energy gained
When a single glucose molecule is oxidized, the cell gains approximately 30-32 ATP. This number can vary, because often a mitochondrion is not functioning at full capacity. Some energy can be lost when the NADH molecules formed in glycolysis transfer their electrons across the membrane that separates the mitochondria and the cytoplasm.
ATP
ATP is present in all living organisms and plays a vital role in cellular metabolism, as it is the primary way cells store and transfer energy. Plants produce it by photophosphorylation, a process that converts sunlight into energy. ATP can also be produced in an anaerobic process, a reaction that does not require oxygen. Fermentation, for example, can occur with no oxygen present, but this and other anaerobic metabolic processes tend to be much less efficient ways to produce this molecule.
A large number of cellular functions require ATP. The cell breaks these molecules down into ADP and phosphate ions, releasing the stored energy. This energy is then used to do things like move large molecules in and out of the cell or to help make proteins, DNA and RNA. ATP is also involved in muscle movement and is essential for maintaining the cell’s cytoskeleton, the structure within the cytoplasm that supports the cell and holds it together.
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