We've learned that energy exists both as potential energy and as kinetic energy. The foods that we eat are a source of potential energy. This is the energy stored in the chemical bonds of food molecules, primarily carbohydrates, proteins and fats. The potential energy contained in the chemical bonds of a simple sugar like glucose (C₆H₁₂O₆), for example, can be converted into carbon dioxide (CO₂), water (H₂O) and energy as the following chemical reaction shows:

However, the body doesn't use glucose directly, instead the cells of the body (in a process called cellular respiration) breakdown glucose capturing much of the energy in the high-energy phosphate bonds of a molecule called adenosine triphosphate or "ATP" for short. {When converting from one energy source to another, some of the energy is always lost as heat. This is in accordance with the second law of thermodynamics.}

ATP is a more useful form of energy for cells to meet their many needs. ATP can be made directly from the breakdown of sugars and fats. Mitochondria are known as the "powerhouses" of cells because this is where large quantities of ATP are produced.

Mitochondria are shaped like a peanut. They have both an inner and an outer membrane (called a double-membrane) with the inner membrane being highly folded (to greatly increase the surface area) into partitions called cristae. The cristae are finger-like projections whose walls are studded with special proteins (called cytochromes) used in cellular respiration and an important enzyme called ATP synthase that is needed to make ATP from ADP (adenosine diphosphate). The mitochondrial matrix contains strands of DNA, ribosomes and small granular material. Mitochondria are more concentrated in skeletal muscles than in other tissues, because muscles require large amounts of energy for mechanical work.

If we think of ATP as a kind of "energy currency" for cellular processes, then we might also consider mitochrondria to behave as a kind of change machine. However, instead of converting one-dollar bills to coins, like a change machine does, mitochondria convert glucose, for example, into ATP.

The goal here is to get as much ATP as is possible out of glucose. Most of a cell's ATP is produced inside of the cell's mitochondria, while a much smaller amount is made via anaerobic glycolysis in the cell's cytosol (the internal fluid of the cell).

Before glycolysis can take place glucose must first enter the cell. This feat is accomplished with the help of special protein molecules called “glucose transporters." In the cell cytosol, glucose can undergo glycolysis, which is an anaerobic process (meaning that oxygen is not required).

In what is called the "preparatory phase" of glycolysis, two molecules of ATP are used to phosphorylate glucose (meaning that two phosphate PO₄^3- molecules have been added) creating fructose 1,6-bisphosphate (bisphosphate means "two phosphates"). Unlike glucose, which can pass through the cell membrane, fructose 1,6-bisphosphate cannot pass through the membrane, thus confining it to the cell and keeping it on its journey through glycolysis.

The 6-carbon fructose 1,6-bisphosphate molecule is cleaved (cut in two) by a special enzyme called fructose 1,6-bisphosphate aldolase. In the end, two molecules of the 3-carbon glyceraldehyde 3-phosphate are formed.

In a series of steps, requiring inorganic phosphate (PO₄^3-), magnesium ion Mg²⁺, adenosine diphosphate (ADP), the coenzyme nicotinamide adenine dinucleotide (NAD⁺) and several important enzymes, glyceraldehyde 3-phosphate is oxidized to a molecule called pyruvic acid.

__Pyruvic Acid

This is known as the "payoff phase" of glycolysis because it results in the production of ATP. For each glucose molecule entering glycolysis, two molecules of pyruvic acid are formed along with four molecules of ATP. However, because two ATP molecules were required in the preparatory phase, the overall result is that glycolysis produces two molecules of ATP.

Oxidation is a term used by scientists to describe a process in which electrons are taken away from an atom or molecule. Oxidation is also referred to as a "loss of electrons."

In the case of glyceraldehyde 3-phosphate being oxidized to pyruvic acid, high-energy electrons in the form of hydride ions H^- (hydride ions carry two electrons) are removed from glyceraldehyde 3-phosphate by the coenzyme NAD⁺, which serves as an electron acceptor (also called an electron carrier). In accepting H^-, which is a gain of two electrons, NAD⁺ is converted to its reduced form NADH. NADH transfers its two electrons to several types of electron carriers in the electron transport chain, which takes place inside the cell's mitochondria (within the inner membrane). In aerobic respiration, oxygen (O₂) is the final acceptor of these electrons. When NADH transfers two electrons to the electron transport chain (also called the respiratory chain) it regenerates NAD⁺, otherwise glycolysis will stop.

In the B-vitamin section of this webpage we discuss the importance of vitamin B₃, which is also called Niacin (nicotinic acid or nicotinamide). Niacin is a water-soluble B-vitamin that plays an essential role in energy metabolism by serving as a component of the coenzyme NAD and its reduced form NADH. Check it out !!

In glycolysis, only about 5% of the energy from glycolysis, ends up in the production of ATP and in the high-energy electrons of NADH. Most of the energy is stored in pyruvic acid, which is later used to make much more ATP in the cell's mitochondria.

Pyruvic acid enters the mitochondrion where in the presence of oxygen (O₂) it is converted to a compound called acetyl coenzyme-A before taking part in the Kreb's Cycle. The Kreb's Cycle, which is also known as the Citric Acid Cycle and the Tricarboxylic Acid Cycle (TCA) is named in honor of the biochemist Sir Hans Krebs who worked out the details of the Citric Acid Cycle.

Interestingly, the Citric Acid Cycle is so named because the initial reaction of the cycle occurs when acetyl-CoA transfers its two-carbon acetyl group to a four-carbon compound called oxaloacetate, thus forming a six-carbon compound called citrate. Citrate undergoes a number of changes via the chemical reactions that make up the Citric Acid Cycle, ending in oxaloacetate, which can then react with another molecule of acetyl-CoA and start the cycle all over again. Each full turn of the cycle produces a molecule of guanosine triphosphate (GTP), which is then converted to ATP.

Coenzyme-A (CoA) is an important catalyst for the activation of organic molecules like pyruvic acid. In one turn of the Kreb's Cycle two carbons enter as acetyl-CoA and two carbons leave as CO₂. Since two acetyl-CoA molecules are produced from each glucose molecule, the Kreb's cycle must turn twice to process each molecule of glucose.
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Animal cells in the presence of oxygen further metabolize the products of glycolysis via the Kreb's Cycle and the Electron Transport Chain to produce carbon dioxide and water as end products along with a substantial amount of ATP. This is referred to as aerobic respiration. Simply put, it is the oxidation of foods for the purpose of acquiring energy. In this three-stage process, which includes glycolysis, the citric acid cycle and oxidative phosphorylation, 32 molecules of ATP are produced from one molecule of glucose.

Metabolism is a term used to describe all of the chemical reactions taking place in living beings. Metabolism is generally divided into two basic reaction types: anabolic reactions, those reactions involving the synthesis or construction of compounds and catabolic reactions, reactions concerned with the breakdown of compounds.

Anabolic reactions need energy to occur and they are known as endothermic reactions. ATP comes in handy for anabolic reactions because the release of inorganic phosphate from ATP is a useful source of energy. (See the figure below.)

The mole concept is a very useful one in chemistry. One mole is a very large number, about 6.0 x 10²³ molecules. This number is called Avogadro's number in honor of the Italian chemist Amedeo Avogadro. Avogadro's Hypothesis (now known as Avogadro's Law) dictates that equal volumes of pure gases at the same temperature and pressure contain the same number of gas molecules regardless of the type of gas.

Catabolic reactions release energy and they are known as exothermic (exergonic) reactions. The energy released from various catabolic reactions taking place in the body can be used to turn the lower energy ADP into the higher energy ATP.

Catabolic reactions release energy and are known as exothermic reactions. A catabolic reaction can be used, for example, to convert ADP to ATP. This is the reverse of the reaction shown in the above figure.

The ATP molecule can therefore, be thought of as a kind of "chemical battery," storing energy in high-energy phosphate bonds when energy is not needed but always available to release this energy when it is needed.