Metabolism (from the Greek metabole, which means “change”) is the word for the myriad chemical reactions that happen in the body, particularly as they relate to generating, storing, and expending energy. All metabolic reactions are either catabolic or anabolic. Catabolic reactions break food down into energy (memory tip: it can be catastrophic when things break down). Anabolic reactions require the expenditure of energy to build up compounds that the body needs. The chemical alteration of molecules in the cell is referred to as cellular metabolism. Enzymes can be used as catalysts, accelerating chemical reactions without being changed by the reactions. The molecules that enzymes react with are called substrates.
Adenosine triphosphate (ATP) is a molecule that stores energy in a cell until the cell needs it. As the tri– prefix implies, a single molecule of ATP is composed of three phosphate groups attached to a nitrogenous base of adenine. ATP’s energy is stored in high energy bonds that attach the second and third phosphate groups. (The high-energy bond is symbolized by a wavy line.) When a cell needs energy, it removes one or two of those phosphate groups, releasing energy and converting ATP into either the two phosphate molecule adenosine diphosphate (ADP) or the one-phosphate molecule adenosine monophosphate (AMP).
Later, through additional metabolic reactions, the second and third phosphate groups are reattached to adenosine, reforming an ATP molecule until energy is needed again. Oxidation-reduction reactions are an important pair of reactions that occur in carbohydrate, lipid, and protein metabolism. When a substance is oxidized, it loses electrons and hydrogen ions, removing a hydrogen atom from each molecule.
When a substance is reduced, it gains electrons and hydrogen ions, adding a hydrogen atom to each molecule. Oxidation and reduction occur together, so whenever one substance is oxidized, another is reduced. The body uses this chemical-reaction pairing to transport energy in a process known as the respiratory chain, or the electron transport chain.
Carbohydrate metabolism involves a series of cellular respiration. All food carbohydrates are eventually broken down into glucose; therefore, carbohydrate metabolism is really glucose metabolism. Glucose metabolism produces energy that is then stored in ATP molecules. The oxidation process in which energy is released from molecules, such as glucose, and transferred to other molecules is called cellular respiration. It occurs in every cell in the body and it is the cell’s source of energy. The complete oxidation of one molecule of glucose will produce 38 molecules of ATP. It occurs in three stages: glycolysis, the Krebs cycle, and the electron transport chain:
From the Greek glyco (sugar) and lysis (breakdown), this is the first stage of both aerobic (with oxygen) and anaerobic (without oxygen) respiration. Using energy from two molecules of ATP and two molecules of NAD+ (nicotinamide adenine di-nucleotide), glycolysis uses a process called phosphorylation to convert a molecule of six-carbon glucose — the smallest molecule that the digestive system can produce during the breakdown of a carbohydrate — into two molecules of three-carbon pyruvic acid or pyruvate, as well as four ATP molecules and two molecules of NADH (nicotinamide adenine dinucleotide). Taking place in the cell’s cytoplasm, glycolysis doesn’t require oxygen to occur. The pyruvate and NADH move into the cell’s mitochondria, where an aerobic (with oxygen) process converts them into ATP.
2. Krebs cycle
Also known as the tricarboxylic acid cycle or citric acid cycle, this series of energy producing chemical reactions begins in the mitochondria after pyruvate arrives from glycolysis. Before the Krebs cycle can begin, the pyruvate loses a carbon dioxide group to form acetyl coenzyme A (acetyl CoA). Then acetyl CoA combines with a four-carbon molecule (oxaloacetic acid, or OAA) to form a six carbon citric acid molecule that then enters the Krebs cycle. The CoA is released intact to bind with another acetyl group. During the conversion, two carbon atoms are lost as carbon dioxide and energy is released. One ATP molecule is produced each time an acetyl CoA molecule is split. The cycle goes through eight steps, rearranging the atoms of citric acid to produce different intermediate molecules called keto acids. The acetic acid is broken apart by carbon (or decarboxylated) and oxidized, generating three molecules of NADH, one molecule of FADH2 (flavin adenine dinucleotide), and one molecule of ATP. The energy can be transported to the electron transport chain and used to produce more molecules of ATP. OAA is regenerated to get the next cycle going, and carbon dioxide produced during this cycle is exhaled from the lungs.
3. Electron transport chain
The electron transport chain is a series of energy compounds attached to the inner mitochondrial membrane. The electron molecules in the chain are called cytochromes. These electron-transferring proteins contain a heme, or iron, group. Hydrogen from oxidized food sources attaches to coenzymes that in turn combine with molecular oxygen. The energy released during these reactions is used to attach inorganic phosphate groups to ADP and form ATP molecules.
Pairs of electrons transferred to NAD+ go through the electron transport process and produce three molecules of ATP by oxidative phosphorylation. Pairs of electrons transferred to FAD enter the electron transport after the first phosphorylation and yield only two molecules of ATP. Oxidative phosphorylation is important because it makes energy available in a form the cells can use.
At the end of the chain, two positively charged hydrogen molecules combine with two electrons and an atom of oxygen to form water. The final molecule to which electrons are passed is oxygen. Electrons are transferred from one molecule to the next, producing ATP molecules.
Lipid metabolism only requires portions of the processes involved in carbohydrate metabolism. Lipids contain about 99 percent of the body’s stored energy and can be digested at mealtime, but as people who complain about fats going “straight to their hips” can attest, lipids are more inclined to be stored in adipose tissue — the stuff generally identified with body fat. When the body is ready to metabolize lipids, a series of catabolic reactions breaks apart two carbon atoms from the end of a fatty acid chain to form acetyl CoA, which then enters the Krebs cycle to produce ATP. Those reactions continue to strip two carbon atoms at a time until the entire fatty acid chain is converted into acetyl CoA.
Protein metabolism focuses on producing the amino acids needed for synthesis of protein molecules within the body. But in addition to the energy released into the electron transport chain during protein metabolism, the process also produces byproducts, such as ammonia and keto acid. Energy is released entering the electron transport chain. The liver converts the ammonia into urea, which the blood carries to the kidneys for elimination. The keto acid enters the Krebs cycle and is converted into pyruvic acids to produce ATP.
One last thing: That severe soreness and fatigue you feel in your muscles after strenuous exercise is the result of lactic acid buildup during anaerobic respiration. Glycolysis continues because it doesn’t need oxygen to take place. But glycolysis does need a steady supply of NAD+, which usually comes from the oxygen-dependent electron transport chain converting NADH back into NAD+. In its absence, the body begins a process called lactic acid fermentation, in which one molecule of pyruvate combines with one molecule of NADH to produce a molecule of NAD+ plus a molecule of the toxic byproduct lactic acid.