Metabolic pathway

In biochemistry, a metabolic pathway is a series of chemical reactions occurring within a cell. In each pathway a principal chemical is modified by chemical reactions. These reactions are accelerated, more accurately catalyzed, by enzymes. Dietary minerals, vitamins and other cofactors are often needed by the enzyme to perform its task. Many pathways are elaborate. Various metabolic pathways within each cell form that cell's metabolic network. Pathways are needed by an organism to keep its homeostasis. Metabolism 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 enters a metabolic pathway depending on the needs of the cell and the availability of the substrate. An increase in concentration of anabolical and catabolical end products would slow the metabolic rate for that particular pathway.

Overview
Metabolic pathways often have these properties:
 * They contain many steps, like a cascade. The first step is usually irreversible.  The other steps need not be irreversible and in many cases, the pathway can go in opposite direction depending on the current need of the cell.
 * Glycolysis features excellent examples of these features:
 * As glucose enters a cell it is immediately phosphorylated by ATP to glucose 6-phosphate in the irreversible first step. This is to prevent the glucose leaving the cell.
 * In times of excess lipid or protein energy sources glycolysis may run in reverse (gluconeogenesis) in order to produce glucose 6-phosphate for storage as glycogen or starch.


 * They are regulated, usually 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 and catabolic pathways in eukaryotes are separated by either compartmentation or by the use of different enzymes and cofactors.

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 (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:
 * Photosynthesis (plants, algae, cyanobacteria)
 * Chemosynthesis (some bacteria)