Major Metabolic Pathways
Cellular respiration: Glycolysis
Anaerobic respiration
Kreb’s cycle / Citric acid cycle
Oxidative phosphorylation
Creation of energetic compounds from non-living matter:
Photosynthesis (plants, algae, cyanobacteria)
Chemosynthesis (some bacteria)
Other pathways occurring in (most or) all living organisms include:
Fatty acid oxidation (β-oxidation)
Gluconeogenesis
HMG-CoA reductase pathway (isoprene prenylation)
Pentose phosphate pathway (hexose monophosphate)
Porphyrin synthesis (or heme synthesis) pathway
Urea cycle
Metabolites: Metabolites are the intermediates and products of metabolism. The term metabolite
is usually restricted to small molecules. A primary metabolite is directly involved in the normal
growth, development, and reproduction. A secondary metabolite is not directly involved in those
processes, but usually has important ecological function.
Induction of secondary metabolism: The induction of secondary metabolism is linked to
particular environmental conditions or developmental stages. For example, when grown in a
nutrient-rich medium, most bacteria employ almost solely basic metabolism in order to grow and
reproduce. However, when nutrients are depleted, they start producing an array of secondary
metabolites in order to promote survival.
Plants produce secondary metabolites as a response to adverse environmental conditions or in
particular developmental stages. For example, exposure to UV radiation induces the biosynthesis
of UV-absorbing compounds
Secondary metabolites are those chemical compounds in organisms that are not directly involved
in the normal growth, development or reproduction of organisms. Typically primary metabolites
are found across all species within broad phylogenetic groupings, and are produced using the
same pathway (or nearly the same pathway) in all these species. Secondary metabolites, by
contrast, are often species-specific (or found in only a small set of species in a narrow
phylogenetic group), and without these compounds the organism suffers from only a mild
impairment, lowered survivability/fecundity, aesthetic differences, or else no change in
phenotype at all.
The function or importance of these compounds to the organism is usually of an ecological
nature as they are used as defenses against predators, parasites and diseases, for interspecies
competition, and to facilitate the reproductive processes (coloring agents, attractive smells, etc).
Since these compounds are usually restricted to a much more limited group of organisms, they
have long been of prime importance in taxonomic research.