In biochemistry, a metabolic pathway is a series of chemical reactions occurring within a cell, catalyzed by enzymes, resulting in either the formation of a metabolic product to be used or stored by the cell, or the initiation of another metabolic pathway (then called a flux generating step). Many pathways are elaborate, and involve a step by step modification of the initial substance to shape it into the product with the exact chemical structure desired. This article or section does not cite its references or sources. ... A few of the metabolic pathways in a cell. ... Chemistry (from the Greek word χημεία (chemeia) meaning cast together or pour together) is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms (such as molecules, crystals, and metals). ... Cells in culture, stained for keratin (red) and DNA (green). ... Generic graph showing the effect of a catalyst in an hypothetical exothermic chemical reaction. ... Ribbon diagram of the enzyme TIM. TIM is catalytically perfect, meaning its conversion rate is limited, or nearly limited to its substrate diffusion rate. ...

Overview

Metabolic pathways often have these properties:

• They contain many steps like a cascade. However, the first step is usually irreversible. The other in between steps need not be irreversible. In fact, many times, the pathway can go in opposite direction depending on the need of the cell.
• They are regulated, usually by feedback inhibition, or by a cycle where the end product starts the reaction again, such as the Krebs Cycle (see below).
• Anabolic and catabolic pathways in eukaryotes are separated by either compartmentation or by the use of different enzymes and cofactors.

In cybernetics and control theory, feedback is a process whereby some proportion or in general, function, of the output signal of a system is passed (fed back) to the input. ... The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. ... It has been suggested that Catabolisis be merged into this article or section. ... Anabolism is the aspect of metabolism that contributes to growth. ... Kingdoms Animalia - Animals Fungi Plantae - Plants Protista A eukaryote is an organism with a complex cell or cells, in which the genetic material is organized into a membrane-bound nucleus or nuclei. ... Schematic of typical animal cell, showing subcellular components. ...

Major metabolic pathways

Cellular respiration

Main article: Cellular respiration

Several distinct but linked metabolic pathways are used by cells to transfer the energy released by breakdown of fuel molecules to ATP. These occur within all living organisms in some forms: Cellular respiration is the process in which the chemical bonds of energy-rich molecules such as glucose are converted into energy usable for life processes. ... A fuel molecule is a molecule metabolized by a cell to generate ATP. Types Carbohydrate Fat Protein ... Adenosine 5-triphosphate (ATP), discovered in 1929 by Karl Lohmann, is a multifunctional nucleotide primarily known in biochemistry as the molecular currency of intracellular energy transfer. ...

1. Glycolysis
2. Anaerobic respiration
3. Krebs cycle / Citric acid cycle
4. Oxidative phosphorylation

Other pathways occurring in (most or) all living organisms include: Glycolysis is a series of biochemical reactions by which a molecule of glucose (Glc) is oxidized to two molecules of pyruvic acid (Pyr). ... Anaerobic respiration refers to the oxidation of molecules in the absence of oxygen to produce energy. ... The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. ... The citric acid cycle (also known as the tricarboxylic acid cycle, the TCA cycle, or the Krebs cycle) is a series of chemical reactions of central importance in all living cells that utilize oxygen as part of cellular respiration. ... Oxidative phosphorylation is a biochemical process in cells. ...

• Fatty acid oxidation (β-oxidation)
• Gluconeogenesis
• HMG-CoA reductase pathway (isoprene prenylation chains, see cholesterol)
• Pentose phosphate pathway (hexose monophosphate shunt)
• Porphyrin synthesis (or heme synthesis) pathway
• Urea cycle

Creation of energetic compounds from non-living matter: Fatty acids are an important source of energy for many organisms. ... Gluconeogenesis, ultimately, is the generation of glucose from noncarbohydrate sources like lactate, glycerol, and amino acids. ... The HMG-CoA reductase pathway, also known as MVA pathway or mevalonate-dependent (MAD) route, is an important cellular metabolic pathway present in virtually all organisms. ... Isoprene is a common synonym for the chemical compound 2-methyl-1,3-butadiene. ... Cholesterol is a sterol (a combination steroid and alcohol) and a lipid found in the cell membranes of all body tissues, and transported in the blood plasma of all animals. ... The pentose phosphate pathway (also called Phosphogluconate Pathway, or Hexose Monophosphate Shunt) is a process that serves to generate NADPH and the synthesis of pentose (5-carbon) sugars. ... A porphyrin is a heterocyclic macrocycle made from 3 pyrrole subunits and one pyrroline subunit, and linked on opposite sides through 4 methine bridges. ... Structure of Heme b 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. ... The Urea Cycle is a cycle of biochemical reactions occurring in many animal organisms that produces urea from ammonia. ...

• Photosynthesis (plants, algae)
• Chemosynthesis (some bacteria)

The leaf is the primary site of photosynthesis in plants. ... Chemosynthesis is the biological conversion of 1-carbon molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic molecules (e. ...

See also

• Cell metabolism

A few of the metabolic pathways in a cell. ...

External links

• Open Directory Project: Metabolic Pathways
• Metabolism, Cellular Respiration and Photosynthesis - The Virtual Library of Biochemistry and Cell Biology
• A metabolic pathway (Flash animation)
• KEGG: Kyoto Encyclopedia of Genes and Genomes
• MetaCyc: a database of nonredundant, experimentally elucidated metabolic pathways (700 pathways from more than 600 different organisms).
• aMAZE: a workbench for the representation, management, annotation and analysis of information on networks of cellular processes: genetic regulation, biochemical pathways, signal transductions.
• PathCase Pathways Database System
• Interactive Flow Chart of the Major Metabolic Pathways