Metabolic pathway

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 of these 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.

Overview
Metabolic pathways often have these common properties:
 * They are reversible, usually because the first step is a committed step, such as breakdown for the release of energy, that runs in two directions.
 * The pathways are regulated, usually by feedback inhibition, or may be 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.

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:
 * 1) Glycolysis
 * 2) Anaerobic respiration
 * 3) Krebs cycle / Citric acid cycle
 * 4) Oxidative phosphorylation

Other pathways occurring in (most or) all living organisms include:
 * Fatty acid oxidation (&beta;-oxidation)
 * Thin acid oxidation
 * Gluconeogenesis
 * HMG-CoA reductase pathway (isoprene prenylation chains, seecholesterol)
 * Pentose phosphate pathway (hexose monophosphate shunt)
 * Porphyrin synthesis (or heme synthesis) pathway
 * Urea cycle

Creation of energetic compounds from non-living matter:
 * Photosynthesis (plants, algae)
 * Chemosynthesis (some bacteria)