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In biochemistry
Biochemistry

Biochemistry is the study of the chemistry processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
, a metabolic pathway is a series of chemical
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 reactions occurring within a cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
. In each pathway, a principal chemical is modified by chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
s. Enzyme
Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
s catalyze these reactions, and often require dietary minerals, vitamins and other cofactors
Cofactor (biochemistry)

A cofactor is a non-protein chemical compound that is bound to an enzyme and is required for catalysis. They can be considered "helper molecules/ions" that assist in biochemical transformations....
 in order to function properly. Because of the many chemicals that may be involved, pathways can be quite elaborate. In addition, many pathways can exist within a cell. This collection of pathways is called the metabolic network
Metabolic network

A metabolic network is the complete set of metabolic and physical processes that determine the physiology and biochemistry properties of a cell....
.






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In biochemistry
Biochemistry

Biochemistry is the study of the chemistry processes in living organisms. It deals with the structure and function of cellular components such as proteins, carbohydrates, lipids, nucleic acids and other biomolecules....
, a metabolic pathway is a series of chemical
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
 reactions occurring within a cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
. In each pathway, a principal chemical is modified by chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
s. Enzyme
Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
s catalyze these reactions, and often require dietary minerals, vitamins and other cofactors
Cofactor (biochemistry)

A cofactor is a non-protein chemical compound that is bound to an enzyme and is required for catalysis. They can be considered "helper molecules/ions" that assist in biochemical transformations....
 in order to function properly. Because of the many chemicals that may be involved, pathways can be quite elaborate. In addition, many pathways can exist within a cell. This collection of pathways is called the metabolic network
Metabolic network

A metabolic network is the complete set of metabolic and physical processes that determine the physiology and biochemistry properties of a cell....
. Pathways are important to the maintenance of homeostasis
Homeostasis

Homeostasis is the property of a system, either open system or closed system, that regulates its internal environment and tends to maintain a stable, constant condition....
 within an organism
Organism

In biology, an organism is any life thing . In at least some form, all organisms are capable of response to stimulus , reproduction, growth and developmental biology, and maintenance of homeostasis as a stable whole....
.

Metabolism
Metabolism

Metabolism is the set of chemical reactions that occur in living organisms in order to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments....
 is a step by step modification of the initial molecule to shape it into another product. The result can be used in one of three ways.
  • Stored by the cell.
  • Be used immediately, as a metabolic product.
  • Initiate another metabolic pathway, called a flux generating step.


A molecule called a substrate
Substrate (biochemistry)

In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalysis chemical reactions involving the substrate. The substrate binds with the enzyme active site, and an enzyme-substrate complex is formed....
 enters a metabolic pathway depending on the needs of the cell and the availability of the substrate. An increase in concentration of anabolic
Anabolism

Anabolism is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy. One way of categorizing metabolic processes, whether at the cell ular, organ or organism level is as 'anabolic' or as 'catabolism', which is the opposite....
 and catabolic
Catabolism

Catabolism is the set of metabolic pathways which break down molecules into smaller units and release energy. In catabolism, large molecules such as polysaccharides, lipids, nucleic acids and proteins are broken down into smaller units such as monosaccharides, fatty acids, nucleotides and amino acids, respectively....
 end products would slow the metabolic rate for that particular pathway.

Overview


Each metabolic pathway is composed of a series of biochemical reactions that are connected by their intermediates: the reactants (or substrates) of one reaction are the products of the previous one, and so on. Metabolic pathways are usually considered in one direction (although all reactions are chemically reversible, conditions in the cell are such that it is thermodynamically more favorable for flux to be in one of the directions).

  • Glycolysis
    Glycolysis

    Glycolysis is the metabolic pathway that converts glucose, C6H12O6, into pyruvate, C3H5O3-....
     was the first metabolic pathway discovered:
  1. As glucose
    Glucose

    Glucose , a monosaccharide also known as grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology....
     enters a cell it is immediately phosphorylated by ATP
    Adenosine triphosphate

    This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
     to glucose 6-phosphate in the irreversible first step. This is to prevent the glucose leaving the cell.
  2. In times of excess lipid
    Lipid

    Lipids are broadly defined as any fat-soluble , naturally-occurring molecule, such as fats, oils, waxes, cholesterol, sterols, fat-soluble vitamins , monoglycerides, diglycerides, phospholipids, and others....
     or protein
    Protein

    Proteins are organic compounds made of amino acids arranged in a linear chain and joined together by peptide bonds between the carboxyl and amino groups of adjacent amino acid Residue ....
     energy sources glycolysis
    Glycolysis

    Glycolysis is the metabolic pathway that converts glucose, C6H12O6, into pyruvate, C3H5O3-....
     may run in reverse (gluconeogenesis
    Gluconeogenesis

    Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactic acid, glycerol, and glucogenic amino acids....
    ) in order to produce glucose 6-phosphate for storage as glycogen
    Glycogen

    Glycogen is a polysaccharide of glucose which functions as the secondary short term energy storage in animal cells. It is made primarily by the liver and the muscles, but can also be made by the brain and stomach....
     or starch
    Starch

    File:Amylose2.svgFile:Amylopektin Sessel.svgStarch or amylum is a polysaccharide carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds....
    .


  • Metabolic pathways are often regulated
    Control theory

    Control theory is an interdisciplinary branch of engineering and mathematics, that deals with the behavior of dynamical systems. The desired output of a system is called the reference....
     by feedback inhibition, or by a cycle where one of the products in the cycle starts the reaction again, such as the Krebs Cycle (see below).
  • Anabolic
    Anabolism

    Anabolism is the set of metabolic pathways that construct molecules from smaller units. These reactions require energy. One way of categorizing metabolic processes, whether at the cell ular, organ or organism level is as 'anabolic' or as 'catabolism', which is the opposite....
     and catabolic pathways in eukaryote
    Eukaryote

    Animals, plants, fungus, and protists are eukaryotes , organisms whose Cell are organized into complex structures enclosed within Cell membrane....
    s are separated by either compartmentation
    Organelle

    In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
     or by the use of different enzymes and cofactors.


Major metabolic pathways








Cellular respiration


Several distinct but linked metabolic pathways are used by cells to transfer the energy released by breakdown of fuel molecules to ATP
Adenosine triphosphate

This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
. These occur within all living organisms in some forms:
  1. Glycolysis
    Glycolysis

    Glycolysis is the metabolic pathway that converts glucose, C6H12O6, into pyruvate, C3H5O3-....
  2. Anaerobic respiration
    Anaerobic respiration

    Anaerobic respiration is the process of generating energy through cellular respiration , without the use of oxygen....
  3. Krebs cycle / Citric acid cycle
    Citric acid cycle

    The citric acid cycle ? also known as the tricarboxylic acid cycle ; the Krebs cycle; or, more rarely, the Szent-Gy?rgyi-Krebs cycle) ? is a series of enzyme-catalysed chemical reactions of central importance in all living cell s that use oxygen as part of cellular respiration....
  4. Oxidative phosphorylation
    Oxidative phosphorylation

    Oxidative phosphorylation is a metabolic pathway that uses energy released by the redox of nutrients to produce adenosine triphosphate . Although the many forms of life on earth use a range of different nutrients, almost all carry out oxidative phosphorylation to produce ATP, the molecule that supplies energy to metabolism....


Other pathways occurring in (most or) all living organisms include:
  • Fatty acid oxidation (ß-oxidation)
  • Gluconeogenesis
    Gluconeogenesis

    Gluconeogenesis is a metabolic pathway that results in the generation of glucose from non-carbohydrate carbon substrates such as lactic acid, glycerol, and glucogenic amino acids....
  • HMG-CoA reductase pathway
    HMG-CoA reductase pathway

    The mevalonate pathway or HMG-CoA reductase pathway or mevalonate-dependent route or isprenoid pathway, is an important cell metabolic pathway present in all higher eukaryotes and many bacteria....
     (isoprene
    Isoprene

    Isoprene is a common synonym for the chemical compound 2-methyl-1,3-butadiene. It is commonly used in industry, is an important biological material, and can be a harmful environmental pollutant and toxicant when present in excess quantities....
     prenylation chains, see cholesterol
    Cholesterol

    Cholesterol is a lipidic, waxy alcohol found in the cell membranes and transported in the blood plasma of all animals. It is an essential component of mammalian cell membranes where it is required to establish proper membrane permeability and membrane fluidity....
    )
  • Pentose phosphate pathway
    Pentose phosphate pathway

    The pentose phosphate pathway is a process that serves to generate NADPH and the synthesis of pentose sugars. There are two distinct phases in the pathway....
     (hexose monophosphate shunt)
  • Porphyrin synthesis
    Porphyrin

    Porphyrins are a group of chemical compounds of which many occur in nature, such as in green leaves and red blood cells, and in bio-inspired synthetic catalysts and devices....
     (or heme
    Heme

    A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin....
     synthesis) pathway
  • Urea cycle
    Urea cycle

    The urea cycle is a cycle of biochemistry reactions occurring in many animals that produces urea 2carbonoxygen from ammonia . This cycle was the first metabolic cycle discovered ....


Creation of energetic compounds from non-living matter:
  • Photosynthesis
    Photosynthesis

    File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
     (plants, algae, cyanobacteria
    Cyanobacteria

    Cyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis....
    )
  • Chemosynthesis
    Chemosynthesis

    Chemosynthesis is the biological conversion of one or more carbon molecules and nutrients into organic matter using the oxidation of inorganic molecules or methane as a source of energy, rather than sunlight, as in photosynthesis....
     (some bacteria)


See also

  • Metabolism
    Metabolism

    Metabolism is the set of chemical reactions that occur in living organisms in order to maintain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments....
  • Metabolic network
    Metabolic network

    A metabolic network is the complete set of metabolic and physical processes that determine the physiology and biochemistry properties of a cell....
  • Metabolic network modelling
    Metabolic network modelling

    Metabolic network reconstruction and simulation allows for an in depth insight into comprehending the molecular mechanisms of a particular organism, especially correlating the genome with molecular physiology ....


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