Glucose is the source for most of the energy that fuels biochemical reactions in the human body. It is converted through a series of metabolic pathways into energy-producing molecules. The levels of glucose in cells are maintained through a balance of breaking down glucose and synthesizing new glucose as necessary, by the glycolysis and gluconeogenesis pathways. Glucose can also be stored by the cells for later use.
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Glucose is converted through a series of metabolic pathways into energy-producing molecules called ATP, which are vital for most of the biochemical reactions in living organisms.
When cells need energy, they use glycolysis to break down a glucose molecule into two molecules of pyruvate, two molecules of ATP and two molecules of NAD. Further breakdown of the pyruvate and the NAD yield a total of 36 molecules of ATP from one molecule of glucose.
During periods of low carbohydrate ingestion, the body can synthesize glucose for energy through a process called gluconeogenesis, using two molecules of pyruvite. In times when there is sufficient glucose, cells can store it to use later by making long glucose chains called glycogens.
Glucose Is Energy
Glucose is obtained by breaking down carbohydrates from ingested food. Through a series of metabolic reactions, glucose is broken down into various intermediate products, before eventually producing molecules of adenosine triphosphate, or ATP. ATP is responsible for driving most of the biochemical reactions in a living organism. Cells in critical organs, such as the brain and muscles, require high amounts of energy, and therefore high amounts of glucose, to perform their normal functions.
Breaking Down Glucose
Glycolysis is the initial metabolic pathway through which glucose is broken down. Each molecule of glucose is broken down into two molecules of pyruvate, two molecules of ATP and two molecules of the coenzyme NAD. The pyruvate molecules are further broken down during another series of metabolic reactions known as the Krebs cycle. The Krebs cycle yields more ATP and NADH molecules, as well as another coenzyme, FADH2. The coenzymes can enter the electron transport chain, where they are converted into ATP. Each molecule of glucose yields a total of 36 ATP molecules.
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Gluconeogenesis is essentially the reverse of glycolysis, involving the synthesis of glucose from two pryuvate molecules. Gluconeogenesis occurs primarily in the liver and to a lesser extent, in the kidneys. During times of carbohydrate starvation, such as fasting conditions, there is not enough glucose to power the needs of the cells. Protein in muscle tissue can be broken down to help power the conversion of pryuvate to glucose and fat can be broken down to glycerol to also help power the reactions. Often, gluconeogenesis occurs so that the glucose can be transported to cells with greater energy needs, such as those in the brain and muscles.
When a cell has a sufficient level of ATP, it does not require that glucose be broken down to provide more ATP. In this case, the glucose is stored in the cell by joining together several molecules of glucose into long chains, known as glycogen. The formation of glycogen, known as glycogenesis, primarily occurs in the liver and muscle cells. Glycogen can be rapidly broken down into single molecules of glucose during times of low glucose and low energy in the cell by a process called glycogenolysis.